U.S. patent application number 11/141407 was filed with the patent office on 2006-03-16 for composition of matter.
Invention is credited to Robert W. Broomfield, Robert A. Hobbs, Colin N. Jones, Sammy Tin.
Application Number | 20060057018 11/141407 |
Document ID | / |
Family ID | 32696723 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060057018 |
Kind Code |
A1 |
Hobbs; Robert A. ; et
al. |
March 16, 2006 |
Composition of matter
Abstract
A composition of matter comprising: 5 wt % to 8 wt % rhenium; 4
wt % to 8 wt % tantalum; 2 wt % to 5 wt % tungsten; 2 wt % to 5.5
wt % molybdenum; 2 wt % to 5 wt % chromium; 2 wt % to 6 wt %
ruthenium; 2 wt % to 8 wt % cobalt; 5 wt % to 7 wt % aluminium; 0
wt % to 2 wt % titanium; 0 wt % to 0.5 wt % hafnium; and the
balance nickel and incidental impurities.
Inventors: |
Hobbs; Robert A.;
(Cambridge, GB) ; Tin; Sammy; (Cambridge, GB)
; Broomfield; Robert W.; (Bristol, GB) ; Jones;
Colin N.; (Long Eaton, GB) |
Correspondence
Address: |
MANELLI DENISON & SELTER
2000 M STREET NW SUITE 700
WASHINGTON
DC
20036-3307
US
|
Family ID: |
32696723 |
Appl. No.: |
11/141407 |
Filed: |
June 1, 2005 |
Current U.S.
Class: |
420/444 ;
420/448 |
Current CPC
Class: |
F01D 25/005 20130101;
F01D 5/28 20130101; F05D 2300/607 20130101; F05D 2230/21 20130101;
C22C 19/057 20130101 |
Class at
Publication: |
420/444 ;
420/448 |
International
Class: |
C22C 19/05 20060101
C22C019/05 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 5, 2004 |
GB |
0412584.5 |
Claims
1. A composition of matter comprising: 5 wt % to 8 wt % rhenium; 4
wt % to 8 wt % tantalum; 2 wt % to 5 wt % tungsten; 2 wt % to 5.5
wt % molybdenum; 2 wt % to 5 wt % chromium; 2 wt % to 6 wt %
ruthenium; 2 wt % to 8 wt % cobalt; 5 wt % to 7 wt % aluminium; 0
wt % to 2 wt % titanium; 0 wt % to 0.5 wt % hafnium; and the
balance nickel and incidental impurities.
2. A composition of matter according to claim 1 comprising 5 wt %
to 7 wt % rhenium.
3. A composition of matter according to claim 1 comprising greater
than 6 wt % rhenium.
4. A composition of matter according to claim 1 comprising 3.5 wt %
to 5 wt % tungsten.
5. A composition of matter according to claim 1 comprising less
than 4 wt % tungsten.
6. A composition of matter according to claim 1 comprising 3.5 wt %
to 4.5 wt % molybdenum.
7. A composition of matter according to claim 1 comprising less
than 2.9 wt % molybdenum.
8. A composition of matter according to claim 1 comprising greater
than 4.5 wt % molybdenum.
9. A composition of matter according to claim 1 comprising 3 wt %
to 4 wt % chromium.
10. A composition of matter according to claim 1 comprising 3 wt %
to 5 wt % ruthenium.
11. A composition of matter according to claim 1 comprising greater
than 4 wt % ruthenium.
12. A composition of matter according to claim 1 comprising 3 wt %
to 8 wt % cobalt.
13. A composition of matter according to claim 1 comprising 5 wt %
to 6.5 wt % aluminium.
14. A composition of matter according to claim 1 comprising 0.05 wt
% to 0.5 wt % hafnium.
15. A composition of matter according to claim 1 comprising 0.l wt
% to 2 wt % titanium.
16. A single crystal article formed from a composition of matter
according to claim 1.
17. A single crystal according to claim 16 in the form of an
aerofoil.
18. A single crystal article according to claim 16 in the form of
aerofoil blade.
19. A single crystal article according to claim 17 in the form of a
turbine blade.
Description
FIELD OF THE INVENTION
[0001] This invention relates to compositions of matter. More
particularly, but not exclusively, this invention relates to nickel
based alloys, such as nickel based superalloys. Embodiments of the
invention relate to nickel-based single crystal superalloys.
DESCRIPTION OF THE PRIOR ART
[0002] To improve the performance and efficiency of gas turbine
engines, single crystal Ni-base superalloy turbine blades have been
increasingly alloyed with dense refractory elements to enhance high
temperature creep properties. As single crystal compositions have
become more heavily alloyed, the ease of manufacturing and
processing has decreased, primarily due to formation of
solidification related grain defects. Moreover, the recent trend of
adding ruthenium to single crystal Ni-base superalloys has led to
substantial increases in raw material costs, thus making
manufacturing yield improvements through the reduction of casting
defects of paramount importance. Hence, assessment of the
solidification characteristics of these complex multi-component
alloys and understanding the various elemental interactions is
critical during the development of advanced single crystal Ni-base
superalloys with improved high temperature properties and long term
stability.
[0003] In multi-component alloys such as nickel-base superalloys,
solidification involves solute redistribution of the alloying
elements during dendrite growth. This causes microsegregation i.e.
local variations in elemental concentrations from the dendrite
cores to the dendrite peripherals and interdendritic region of the
as-cast alloy. In single crystal Ni-base superalloys,
solidification begins with the formation of primary .gamma.-Ni
dendrites and typically terminates at a .gamma.+.gamma.' eutectic
reaction. The composition of the solid phase forming from the bulk
liquid during solidification varies from the initial bulk liquid
composition and it continually changes as the temperature
decreases.
[0004] The breakdown of single crystal solidification is often
attributed to the presence of elevated levels of dense refractory
elements that partition strongly to either the solid or liquid
phase during solidification and ultimately result in the formation
of freckle chains. Additions of Re and W partition strongly to the
dendritic regions during solidification, thus depleting the liquid
solute of these dense elements as solidification progresses. This
gives rise to large density imbalances between the bulk liquid and
the less dense solute contained within the dendritic mushy zone.
The compositional differences lead to the formation of convective
instabilities that create solute-rich plumes which solidify as
channels of equiaxed grains, or freckles. Research into
solid-liquid elemental partitioning during solidification has
demonstrated the importance of Ta, W and Re segregation in
promoting the formation of these grain defects.
SUMMARY OF THE INVENTION
[0005] According to a first aspect of this invention, there is
provided a composition of matter comprising: [0006] 5 wt % to 8 wt
% rhenium; [0007] 4 wt % to 8 wt % tantalum; [0008] 2 wt % to 5 wt
% tungsten; [0009] 2 wt % to 5.5 wt % molybdenum; [0010] 2 wt % to
5 wt % chromium; [0011] 1 wt % to 6 wt % ruthenium; [0012] 2 wt %
to 8 wt % cobalt; [0013] 5 wt % to 7 wt % aluminium; [0014] 0 wt %
to 2 wt % titanium; [0015] 0 wt % to 0.5 wt % hafnium; [0016] and
the balance comprising nickel.
[0017] The composition of matter may comprise 5 wt % to 7 wt %
rhenium. The composition of matter may comprise greater than 6 wt %
rhenium.
[0018] The composition of matter may comprise 3.5 wt % to 5 wt %
tungsten. The composition of matter preferably comprises less than
4 wt % tungsten.
[0019] The composition of matter may comprise 3.5 wt % to 4.5 wt %
molybdenum. The composition of matter may comprise greater than 4.5
wt % molybdenum. The composition of matter may comprise less than
2.9 wt % molybdenum.
[0020] The composition of matter may comprise 3 wt % to 4 wt %
chromium.
[0021] The composition of matter may comprise 2 wt % to 6 wt %
ruthenium, preferably 3 wt % to 5 wt % ruthenium. The composition
of matter may comprise greater than 4 wt % ruthenium.
[0022] The composition of matter may comprise 3 wt % to 8 wt %
cobalt.
[0023] The composition of matter may comprise 5 wt % to 6.5 wt %
aluminium.
[0024] The composition of matter may comprise 0.05 wt % to 0.5 wt %
hafnium.
[0025] The composition of matter may comprise 0.1 wt % to 2 wt %
titanium.
[0026] In one embodiment the composition of matter may comprise:
[0027] 5 wt % to 7 wt % rhenium; [0028] 4 wt % to 8 wt % tantalum;
[0029] 3.5 wt % to 5 wt % tungsten; [0030] 3.5 wt % to 4.5 wt %
molybdenum; [0031] 3 wt % to 4 wt % chromium; [0032] 3 wt % to 8 wt
% cobalt; [0033] 5 wt % to 6.5 wt % aluminium; [0034] 0 wt % to 0.5
wt % hafnium; [0035] 0 wt % to 2 wt % titanium; [0036] 3 wt % to 5
wt % ruthenium; [0037] 0.1 wt % to 2 wt % titanium; [0038] 0.05 wt
% to 0.5 wt % hafnium; [0039] and the balance comprising
nickel.
[0040] One or more formulations of this embodiment may comprise
less than 4 wt % tungsten. One or more formulations of this
embodiment may comprise greater than 4 wt % tungsten.
[0041] One or more formulations of this embodiment may comprise
greater than 4 wt % ruthenium. One or more formulations of this
embodiment may comprise less than 4 wt % ruthenium.
[0042] The composition of matter is preferably a superalloy,
desirably a nickel based superalloy.
[0043] According to a second aspect of this invention, there is
provided a single crystal article formed from a composition of
matter as described above.
[0044] Preferably the article is an aerofoil. The article may be an
aerofoil blade, preferably a turbine blade.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] Embodiments of the invention will now be described by way of
example only with reference to the accompanying drawings, in
which:
[0046] FIG. 1 are graphs of the effect of Cr and Mo additions on
the solidification paths of W and Re in C17;
[0047] FIG. 2 is a photo of an as-cast turbine blade illustrating
the four areas for documentation of solidification related defects
arising during the casting trials;
[0048] FIG. 3 are graphs illustrating the effects of Cr and Mo
additions by comparing the solidification paths of W and Re an
UCSX6 and UCSX7;
[0049] FIG. 4 shows photos of the macroetched platform of the
as-cast turbine blades of (a) UCSX6 (b) UCSX7 and (c) UCSX8 showing
a decrease in the number of freckle defects from left to right;
[0050] FIG. 5 shows optical micrographs of the as-cast
microstructures of (a) C17 (b) C17+Mo (c) C17 and (d) C17+Cr+Mo
showing decreasing levels of interdendritic eutectic as Mo and Cr
are added;
[0051] FIG. 6 is a graph of quantitative analysis showing a
decreasing volume fraction of eutectic with increasing Mo and Cr
contents in C17; and
[0052] FIG. 7 is a graph of the effect of Ir additions on the
partitioning behaviour of Ta in C17.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0053] One feature of the preferred embodiment of the present
invention is the specific combination of high Cr and Mo contents
complemented by a low (W+Re)/Ta ratio to reduce the susceptibility
of the nickel-based superalloy to solidification related defects
during single crystal solidification.
[0054] It is known from the prior art that some elements, such as W
and Re, are detrimental to the ease of single crystal
solidification. The beneficial effect of Cr and Mo additions in
decreasing the severity of the solid-liquid partitioning of
elements such as W and Re, was noted in the analysis of the as-cast
structures of forty-seven Ni-base superalloy compositions to assess
the influence of the constituent elements on their solidification
characteristics. The compositions in wt % of selected alloys
investigated are listed in Table 1.
[0055] A Cameca SX-100 electron microprobe with five wavelength
dispersive spectrometers (WDS) was used to quantify the degree to
which the constituent elements of all thirty-nine alloys segregate
during solidification. A 15 by 15 point grid was used over a 1
mm.sup.2 area of the polished sample surface with a 10 second
collection time per peak for each element in addition to a 5 second
background measurement either side of the peak. This provided
representative compositional information from the dendrite cores to
the interdendritic regions of the non-equilibrium solidified
alloys. The solid-liquid partition coefficients, or k values (where
k=X.sub.S/X.sub.L)i for each element were then quantified using a
modified Scheil-analysis. The Scheil equation is:
Xs=kX.sub.0(1-f.sub.s).sup.(k-1) Where X.sub.s is the mole fraction
of solute in the solid, f.sub.s is the volume fraction solid,
X.sub.0 is the nominal composition, or in this instance the average
composition of each element as determined by the electron
microprobe. The degree of segregation is related to the magnitude
of the partition coefficient. No segregation occurs when k=1 while
coefficients greater and less than unity indicate that the
corresponding element is partitioned preferentially to the solid
and liquid respectively during solidification. The compositional
data for the elements in each alloy measured at each point in the
electron probe microanalysis was arranged into ascending or
descending order depending on whether the element partitioned
preferentially to the dendrite core or to the interdendritic
regions. This was then plotted as a function of the volume fraction
solid. To determine the solid-liquid partition coefficients, k, for
each of the constituent elements, the Scheil equation was fitted to
the experimental data and then the value of k was adjusted until
the best fit was achieved.
[0056] For the majority of the SRR300 series of alloys, levels of
Co, A1 and Hf were held constant while levels of Cr, Mo, W, Re and
Ta were systematically varied to investigate the effects of these
additions on the resulting solidification characteristics. For
example, to identify the effect of increasing Re and W additions,
SRR300A was doped with 1.2 wt % Re (SRR300B), 1.2 wt % W (SRR300C)
and 0.6 wt % Re+0.6 wt % W (SRR300D). The influence of Co on the
solid-liquid partition coefficients of the other major alloying
elements was investigated with alloys SRR300J, SRR300D and SRR300K
where Ni was substituted by increasing Co contents ranging from 2,
to 8, to 12 wt % respectively. Four of the alloys, RR3010, SRR300B,
SRR300C and SRR300D were doped with 1 and 3 wt % (0.6 and 1.9 at %)
ruthenium. Prior art has demonstrated Ru to be a potentially
beneficial alloying element that is capable of stabilising the
microstructure against the formation of topologically close-packed
phases at elevated temperatures. It was therefore considered
important to observe whether such additions would be detrimental to
the solidification characteristics of the alloy. Included within
these experimental alloys are nine high refractory Ru-containing
single crystal alloys with the UCSX prefix. UCSX2 includes three
variants of increasing Ru content ranging from 2 to 5 wt % at the
expense of Ni to negate dilution effects. UCSX6, UCSX7 and UCSX8
were designed for the purposes of casting trials and are detailed
later in the document. A simplified Ni-base superalloy, C17, with
constant levels of Co, W, Re, and Ta was used to isolate the
effects of A1, Cr, Mo and Ir additions on the segregation behaviour
of the constituent elements. While prior art has also demonstrated
that Ir is a potent microstructural stabilising element, in this
research Ir was added to investigate how the presence of another
element, which also partitions preferentially to the growing solid
during solidification, influences the relative severity of
segregation of W and Re. Finally eight alloys, with the LDSX
prefix, were investigated to explore the benefits of low W, with a
combination of high Cr, Mo and Ru on castability.
[0057] The results from the electron probe microanalysis are
summarised in Table 2. Statistical fluctuations associated with the
modified-Scheil analysis are all within an average deviation of
0.05 pf the partition coefficients reported. Generally, the more
strongly segregating the element, the greater the average
deviation. Consistent with other investigations, the high-density
refractory elements, Ta, W and Re were found to segregate most
severely during solidification i.e. their k values were furthest
from unity. The degree to which each of these elements partitioned
however, varied significantly with composition over the range of
experimental alloys analysed. Relative changes in both the Mo and
Cr alloying levels were found to have the most significant effect.
The presence of Mo and Cr in the SRR300 series of alloys was found
to decrease the extent of segregation for the dense refractory
elements know to promote freckle defects. For example, alloy
SRR3001, which has the highest overall content of Cr+Mo, exhibits a
substantially lower degree of segregation when compared to alloys
SRR300L and SRR300H, which have similar levels of refractory
alloying additions by lower Cr+Mo levels. With an intermediate
level of Cr+Mo, the measured segregation of Re, W and Ta in SRR300B
is moderate when compared to SRR300I, SRR300L and SRR300H.
[0058] It is difficult to isolate the effects of Cr and Mo
additions within the SRR300 series of alloys, since changes in
their concentrations throughout the alloy compositions investigated
are companied by significant changes in other important alloying
elements. Hence, in an attempt to isolate their effects, additions
of 4.5 wt % Cr and 2.2 wt % Mo were systematically made to the
experimental Ni-based single crystal alloy containing no Cr and Mo
additions, TABLE-US-00001 TABLE 1 Compositions (in wt. %) of
selected alloys analysed. Alloy Ni Al Cr Co Mo Ti Nb Ta W Re Ru Ir
Hf RR3010 Bal. 5.9 1.7 3.1 0.5 0.1 0.1 8.5 5.5 6.8 -- -- -- RR3010
+ 1 Ru 5.8 1.7 3.1 0.5 0.1 0.1 8.4 5.4 6.7 1.0 -- -- RR3010 + 3 Ru
Bal. 5.7 1.7 3.0 0.5 0.1 0.1 8.3 5.3 6.6 3.0 -- -- SRR300A Bal. 5.8
4.0 8.0 2.2 -- -- 7.5 4.6 4.1 -- -- 0.1 SRR300B Bal. 5.8 4.0 8.0
2.2 -- -- 7.5 4.6 5.2 -- -- 0.1 SRR300B + 1 Ru Bal. 5.7 4.0 7.9 2.2
-- -- 7.4 4.6 5.2 1.0 -- 0.1 SRR300B + 3 Ru Bal. 5.6 3.9 7.8 2.1 --
-- 7.3 4.5 5.1 3.0 -- 0.1 SRR300C Bal. 5.8 4.0 8.0 2.2 -- -- 7.5
5.8 4.1 -- -- 0.1 SRR300C + 1 Ru Bal. 5.7 4.0 7.9 2.2 -- -- 7.4 5.7
4.1 1.0 -- 0.1 SRR300C + 3 Ru Bal. 5.6 3.9 7.8 2.1 -- -- 7.3 5.6
4.0 3.0 -- 0.1 SRR300D Bal. 5.8 4.0 8.0 2.2 -- -- 7.5 5.2 4.7 -- --
0.1 SRR300D + 1 Ru Bal. 5.7 4.0 7.9 2.2 -- -- 7.4 5.1 4.7 1.0 --
0.1 SRR300D + 3 Ru Bal. 5.6 3.9 7.8 2.1 -- -- 7.3 5.0 4.6 3.0 --
0.1 SRR300E Bal. 5.8 4.0 8.0 2.2 -- 0.6 6.5 5.2 4.7 -- -- 0.1
SRR300G Bal. 5.8 4.0 8.0 3.5 -- 0.6 6.5 4.0 4.0 -- -- 0.1 SRR300H
Bal. 5.8 4.0 8.0 -- -- -- 7.5 7.0 5.3 -- -- 0.1 SRR300I Bal. 5.8
5.5 8.0 2.0 -- -- 7.5 4.0 5.3 -- -- 0.1 SRR300J Bal. 5.8 4.0 2.0
2.2 -- -- 7.5 5.2 4.7 -- -- 0.1 SRR300K Bal. 5.8 4.0 12.0 2.2 -- --
7.5 5.2 4.7 -- -- 0.1 SRR300L Bal. 6.0 2.5 8.0 2.2 -- -- 7.5 4.6
5.3 -- -- 0.1 C17 Bal. 6.0 -- 12.0 -- -- -- 6.0 9.3 6.0 -- -- --
C17 + A1 Bal. 6.5 -- 11.9 -- -- -- 6.0 9.3 6.0 -- -- -- C17 + Cr
Bal 5.7 4.5 11.5 -- -- -- 5.7 8.9 5.7 -- -- -- C17 + Mo Bal. 5.9 --
11.7 2.2 -- -- 5.9 9.1 5.9 -- -- -- C17 + Cr + Mo Bal. 5.6 4.5 11.7
2.2 -- -- 5.6 8.7 5.6 -- -- -- C17 +1 at. % Ir Bal. 5.8 -- 11.7 --
-- -- 5.8 9.0 5.8 -- 3.0 -- C17 +3 at. % Ir Bal. 5.5 -- 11.0 -- --
-- 5.5 8.6 5.5 -- 8.6 -- UCSX2 + 2 Ru Bal. 5.4 3.0 8.0 1.0 -- --
8.0 8.0 6.5 2.0 -- 0.1 UCSX2 + 3 Ru Bal. 5.4 3.0 8.0 1.0 -- -- 8.0
8.0 6.5 3.0 -- 0.1 UCSX2 + 5 Ru Bal. 5.4 3.0 8.0 1.0 -- -- 8.0 8.0
6.5 5.0 -- 0.1 UCSX6 Bal 6.3 -- 4.0 -- -- -- 6.0 8.0 6.8 3.0 -- --
UCSX7 Bal 6.0 1.5 4.0 3.0 -- -- 6.0 8.0 6.8 3.0 -- -- UCSX8 Bal 5.7
1.5 6.0 3.0 -- -- 8.0 6.0 6.0 3.0 -- -- LDSX1 Bal. 6.0 3.0 3.0 2.5
0.25 -- 6.5 2.9 6.2 3.5 -- 0.1 LDSX2 Bal. 6.0 3.0 8.0 5.0 0.25 --
6.5 2.9 6.2 3.5 -- 0.1 LDSX3 Bal. 6.0 3.0 3.0 5.0 0.25 -- 6.5 4.8
6.2 3.5 -- 0.1 LDSX4 Bal. 6.0 3.0 8.0 2.5 0.25 -- 6.5 4.8 6.2 3.5
-- 0.1 LDSX5 Bal. 6.0 3.0 8.0 2.5 0.25 -- 6.5 2.9 6.2 5.0 -- 0.1
LDSX6 Bal. 6.0 3.0 3.0 2.5 0.25 -- 6.5 4.8 6.2 5.0 -- 0.1 LDSX7
Bal. 6.0 3.0 3.0 5.0 0.25 -- 6.5 2.9 6.2 5.0 -- 0.1 LDSX8 Bal. 6.0
3.0 8.0 5.0 0.25 -- 6.5 4.8 6.2 5.0 -- 0.1
C17. The partition coefficients of the dense refractory elements in
the base C17 alloy are far from unity and are comparable to RR3010
(Table 2), which is also a low Cr, low Mo content alloy. While the
improvements in the solid-liquid partition coefficients of W and Re
upon addition of Cr and Mo to C17 are not as dramatic as observed
in the SRR300 alloy series, the trends are nonetheless consistent
with the prior findings, emphasising the decrease in segregation
associated with the overall Cr and Mo content present within a
given alloy. Thee effects are illustrated more clearly in FIG. 1,
where the segregation characteristics are clearly being influenced
by the presence of Cr and Mo. In this particular set of alloys, Mo
additions appear more potent than Cr in suppressing the segregation
behaviour of W and Re. An addition of 2.2 wt % (1.4 at %) Mo
decreases the microsegregation of W and Re to a greater extend than
an addition of 4.5 wt % (5.2 at %) Cr. The largest decrease
however, was achieved when both Cr and Mo were added to C17. No
significant improvements to the partitioning of Ta were noted upon
addition of Cr and Mo.
[0059] Ruthenium was found to be largely neutral in its influence
on the solidification characteristics of the alloys. Slightly lower
levels of Cr, Mo and Re segregation were measured in alloys
containing 1 wt % Ru, however little to no additional improvement
accompanied an increase in Ru content to 3 wt %. This is consistent
with the results for the Ru-variants of UCSX2, where any changes in
the partition coefficients of the constituent elements associated
with increasing Ru contents are insignificant. Ru itself partitions
only slightly to the dendrite core, having a k value close to
unity.
[0060] No significant alteration to the solidification paths of the
constituent elements was noted with large changes in Co contents
from 2, to 8, to 12 wt % in alloys SRR300J, SRR300D and SRR300K
respectively. The same was true for an increase of 0.5 wt % A1 to
the base C17 alloy. Excluding Cr and Mo additions, no other
elemental additions were revealed to significantly influence the
partitioning of W and Re. However, increasing the overall
concentrations of W and Re leads to more severe partitioning of the
elements to the initial fraction solid (compare SRR300A to SRR300B
and SRR300C in Table 2). Comparison of the solid-liquid partition
coefficients of W and Re in SRR300E to those in SRR300G in Table 2
shows the benefit of combining a high Mo concentration with lowered
W and Re concentrations. The LDSX series of alloys support this
finding. The severity of partitioning of the constituent elements
is lessened through the combination of high Cr and Mo contents with
a low W content despite maintaining the high Re and Ru contents
necessary for enhanced creep properties and microstructural
stability at elevated temperatures.
[0061] Having determined the partition coefficients for each
composition, multiple linear regression analysis was performed on
the experimental data in Table 2 to obtain formulae for the
prediction of the solid-liquid partition coefficients of the major
constituent elements. The magnitude of the coefficient associated
with each of the elements in the linear regression analysis
provides an indication of the relative influences of the other
elements on the partitioning of the element in question. The
regression equations corresponding to the elements know to be most
important in the promotion of grain defects, namely W and Re, show
the potential benefits of Cr and Mo additions in decreasing the
intrinsic susceptibility of an alloy to single crystal breakdown
during solidification (note that any coefficients of order
10.sup.-4 and less have been omitted): Kw=0.281+0.0988[wt %
A1]-0.00316[wt % Cr]+0.0101[wt % Co]-0.0063[wt % Mo]+0.0289[wt %
Ta]-0.00325[wt % W]+0.0258[wt % Re]+0.00418[wt % Ru]
kR.sub.e=1.37+0.0205[wt % A1]-0.0168[wt % Cr]-0.00586[wt %
Co]-0.0416[wt % Mo]-0.0035[wt % Ta]+0.0055[wt % W]+0.0192[wt %
Re]-0.00461[wt % Ru]
[0062] TABLE-US-00002 TABLE 2 Measured and calculated solid-liquid
partition coefficients of selected alloys. Alloy Ni Al Cr Co Mo Ta
W Re Ru Ir RR3010 0.97 0.88 1.15 1.08 -- 0.77 1.26 1.57 -- --
RR3010 + 1 Ru 0.98 0.87 1.10 1.08 -- 0.76 1.26 1.53 1.04 -- RR3010
+3 Ru 0.98 0.87 1.10 1.08 -- 0.76 1.27 1.54 1.04 -- SRR300A 0.98
0.94 1.04 1.04 1.06 0.80 1.20 1.36 -- -- SRR300B 0.97 0.92 1.11
1.06 1.09 0.78 1.21 1.39 -- -- SRR300B + 1 Ru 0.98 0.91 1.06 1.05
1.08 0.76 1.22 1.35 1.04 - SRR300B + 3 Ru 0.98 0.91 1.05 1.05 1.07
0.76 1.21 1.36 1.04 -- SRR300C 0.97 0.92 1.11 1.06 1.10 0.78 1.23
1.36 -- -- SRR300C + 1 Ru 0.98 0.91 1.05 1.06 1.08 0.76 1.23 1.32
1.03 -- SRR300C + 3 Ru 0.98 0.91 1.04 1.05 1.07 0.77 1.23 1.32 1.03
-- SRR300D 0.97 0.91 1.11 1.07 1.09 0.76 1.24 1.43 -- -- SRR300D +
1 Ru 0.98 0.91 1.06 1.05 1.07 0.76 1.24 1.40 1.04 -- SRR300D + 3 Ru
0.98 0.91 1.05 1.05 1.07 0.77 1.23 1.39 1.04 -- SRR300E 0.98 0.90
1.08 1.06 1.09 0.77 1.23 1.42 -- -- SRR300G 0.98 0.95 1.06 1.04
1.08 0.83 1.19 1.27 -- -- SRR300H 0.97 0.89 1.06 1.07 -- 0.75 1.27
1.47 -- -- SRR300I 0.98 0.95 1.08 1.04 1.08 0.87 1.12 1.23 -- --
SRR300J 0.98 0.91 1.07 1.07 1.08 0.77 1.25 1.43 -- -- SRR300K 0.97
0.91 1.06 1.06 1.07 0.76 1.24 1.43 -- -- SRR300L 0.97 0.90 1.07
1.06 1.08 0.74 1.27 1.48 -- -- C17 0.95 0.85 -- 1.05 -- 0.65 1.30
1.55 -- -- C17 + Al 0.95 0.85 -- 1.06 -- 0.66 1.31 1.55 -- -- C17 +
Cr 0.96 0.85 1.03 1.05 -- 0.64 1.28 1.53 -- -- C17 + Mo 0.96 0.87
-- 1.05 1.10 0.67 1.25 1.44 -- -- C17 + Cr + Mo 0.97 0.87 1.03 1.05
1.10 0.67 1.22 1.40 -- -- C17 + 1 at. % Ir 0.95 0.85 -- 1.05 --
0.68 1.29 1.55 -- 1.12 C17 + 3 at. % Ir 0.95 0.85 -- 1.04 -- 0.76
1.28 1.53 -- 1.13 UCSX2 + 2 Ru 0.96 0.86 1.07 1.06 1.08 0.72 1.26
1.48 1.05 -- UCSX2 + 3 Ru 0.96 0.86 1.07 1.06 1.08 0.72 1.26 1.47
1.05 -- UCSX2 + 5 Ru 0.96 0.86 1.07 1.06 1.07 0.72 1.25 1.47 1.06
-- UCSX6 0.95 0.84 -- 1.08 -- 0.72 1.31 1.63 1.06 -- UCSX7 0.96
0.86 1.09 1.08 1.09 0.73 1.25 1.47 1.06 -- UCSX8 0.97 0.88 1.07
1.07 1.06 0.74 1.23 1.42 1.05 -- LDSX1 0.98 0.91 1.09 1.06 1.09
0.81 1.23 1.42 1.05 -- LDSX2 0.98 0.93 1.06 1.05 1.06 0.80 1.27
1.28 1.07 -- LDSX3 0.98 0.91 1.09 1.07 1.06 0.79 1.21 1.32 1.06 --
LDSX4 0.97 0.90 1.07 1.05 1.09 0.76 1.28 1.40 1.06 -- LDSX5 0.98
0.91 1.06 1.05 1.08 0.79 1.29 1.38 1.06 -- LDSX6 0.97 0.89 1.09
1.07 1.08 0.78 1.23 1.42 1.05 -- LDSX7 0.98 0.92 1.08 1.06 1.06
0.82 1.22 1.31 1.06 -- LDSX8 0.97 0.91 1.06 1.05 1.05 0.77 1.27
1.29 1.07 --
[0063] The coefficients for Cr and Mo in the determination of
k.sub.w and k.sub.R.sub.e exert the largest influence in the
minimisation of both towards unity. The coefficient for Mo is
greater than that of Cr in both instances demonstrating the greater
potency of Mo additions in minimising the severity of W and Re
segregation.
[0064] Casting trials were performed on three alloy compositions
UCSX6, UCSX7 and UCSX8 (Table 1) specially devised to validate the
importance of Cr and Mo additions and a low (W+Re)/Ta ratio in
minimising the formation of solidification related defects.
[0065] The elemental concentrations in each alloy are typical of
advanced single crystal superalloy compositions and were
intentionally designed to investigate whether such alloys could be
made more amenable to single crystal solidification whilst
maintaining the high refractory contents necessary to enhance creep
resistance. UCSX6, a Cr- and Mo-free alloy with an undesirable
(W+Re)/Ta ratio was designed to the most prone to solidification
defects while UCSX8, with reduced amounts of Re and W, which
partition preferentially to the growing solid during
solidification, and complementing this with increased Ta contents,
which further decreases the density inversion by partitioning
preferentially to the bulk liquid, was designated to be the least
prone. UCSX7 was designed to experimentally verify the potential
benefits of Cr and Mo additions on the severity of the solid-liquid
partitioning of Re and W. The sole difference between UCSX6 and
UCSX7 is the addition of 1.5 wt % Cr and 3.0 wt % Mo and the amount
of Al was adjusted to ensure the same .gamma.' volume fraction as
predicted by the JMatPro software for UCSX6.
[0066] A production scale Bridgman furnace was used to
simultaneously solidify five solid turbine blades in a ceramic
cluster mould at constant processing conditions using a withdrawal
rate of 230 mm per hour. The as-cast crystals were subsequently
macroetched to reveal the presence, location and number of
macroscopic grain defects, such as freckle chains and misoriented
grains, on the surface of the castings. To document the location of
any defects, the blade was separated into four areas: the tip,
blade, platform and root (FIG. 2). For every blade of each
composition the number of defects in each area were counted and
averaged over the five blades for subsequent comparison.
[0067] The results were listed in Table 2 and Table 3
experimentally verify both the beneficial effect of Cr and Mo
additions and the importance of maintaining a low (W+Re)/Ta ratio.
TABLE-US-00003 TABLE 3 Number, location and total number of
solidification related defects in each alloy tested in the casting
trails. Location UCSX6 UCSX7 UCSX8 Tip 7.5 .+-. 1.0 0 0 Blade 1.5
.+-. 0.5 0 0 Platform 16.3 .+-. 1.5 10.8 .+-. 1.7 3.0 .+-. 1.4 Root
18.8 .+-. 1.3 9.3 .+-. 1.9 0.8 .+-. 0.5 Total No. of Defects 44.0
.+-. 1.4 20.0 .+-. 1.2 3.8 .+-. 1.3
[0068] The Cr- and Mo-free UCSX6 alloy with the most severe
solid-liquid partitioning of the constituent elements was found to
be the most prone to freckle formation. The total number of freckle
defects in UCSX6 was more than halved in UCSX7 solely due to the
addition of 1.5 wt. % (1.9 at. %) Cr and 3.0 wt. % (2.0 at. %) Mo.
The change in the solidification paths of W and Re as a result of
these additions is illustrated in FIG. 3. Manipulation of the
(W+Re)/Ta ratio in UCSX8 resulted in further reductions in the
solid-liquid partitioning coefficients and consequently
substantially fewer defects.
[0069] In all three alloys, freckle defects were concentrated in
the platform and root of the casting while only in UCSX6, designed
to be the most susceptible of the three alloys to single crystal
breakdown, were any freckle defects observed in the tip and blade.
The macroetched platforms of the as-cast blades of UCSX6, UCSX7 and
UCSX8 in FIG. 4 show a decrease in the number of freckle defects
from UCSX6 to UCSX8.
[0070] Freckle formation occurs when the driving force for fluid
flow, as described by the destabilising buoyancy forces
corresponding to the solute-induced density inversion term
.DELTA..rho./.rho.o exceeds the surrounding frictional forces.
Hence, by reducing the amount by which W and Re are depleted from
the interdentritic solute during solidification, Cr and Mo
additions decrease the potential for density inversion and, in so
doing, lessen the susceptibility of the alloy to the formation of
localised convective instabilities.
[0071] To explain the effect of Cr and Mo on the solid-liquid
partitioning of Re and W the principal factors which control the
solidification characteristics of an alloy need to be considered,
namely the phases present in the as-cast microstructure, the
freezing range and the overall thermodynamics of the system. During
directional solidification under steady state conditions, the mushy
zone is comprised of single phase y dendrites and liquid solute.
Since the alloying additions in Ni-base superalloys tend to
partition preferentially into either the .gamma. or .gamma.'
phrases, limited solubility of .gamma.' forming elements exists
within the single phase dendrites during solidification. Hence,
elements such as Ta, A1 and Hf become enriched into the liquid
solute. The other alloying additions, Re, W, Cr, Co, Mo and Ru, are
soluble in the .gamma. phase and tend to partition preferentially
to different degrees into the .gamma. dendrites during
solidification. A reduction in the degree of microsegregation could
occur if the respective alloying additions shifted the overall
composition of the alloy closer to that of the .gamma./.gamma.'
eutectic. An initial composition further from the eutectic
composition would enable segregation over a greater freezing range
prior to attainment of the eutectic composition, at which point the
remaining liquid would solidify as eutectic and no further
solid-liquid partitioning could take place. However, both Cr and Mo
additions were shown to decrease the degree of segregation (FIG. 3)
and the volume fraction of eutectic in the as-cast condition (FIG.
5 and FIG. 6). FIG. 5 reveals the dendritic as-cast structures for
the C17 alloy series. Pools of .gamma./.gamma.' eutectic dispersed
in the interdendritic regions are clearly distinguishable within
the dendritic structure. Qualitative examination of the as-cast
microstructures of each alloy set indicated that the
.gamma./.gamma.' eutectic content decreased with increasing Cr and
Mo contents (FIG. 5(a)-(d)). In the base alloy a continuous
distribution of eutectic around the dendrites is evident whereas
the eutectic pools become more isolated and dispersed as the
overall content of alloying additions increases. This trend was
confirmed quantitatively (FIG. 6), where the volume fraction
eutectic decreased at a rate comparable to the overall amount of
alloying addition. For example, an addition of 4.5 wt % Cr
decreased the eutectic content to a greater extend than an addition
of 2.2 wt % Mo, while the least amount of eutectic was present in
the alloy containing 6.7 wt % (4.5 wt % Cr+2.2 wt % Mo) of alloying
additions. The decrease in eutectic volume fraction in the C17
alloys was not unexpected since doping of the alloys with Cr and Mo
effectively diluted the system, thus drawing the composition of the
alloy further from that of the eutectic. In addition, Cr and Mo are
primarily .gamma. rather than .gamma.' formers so it would be
unlikely that they would promote eutectic formation.
[0072] The effect of Cr and Mo on the temperature range over which
segregation could occur was also investigated by measuring the
solidus and liquidus temperatures of the C17 base alloy and the Cr-
and Mo-containing counterparts using Differential Scanning
Calorimetry (DSC). Since the magnitude of the freezing range
(T.sub.S-T.sub.L) governs the extent of the mushy zone during
directional solidification, minor changes could also influence the
segregation characteristics of the alloy. Alloys that solidify over
a relatively small freezing range may exhibit minimal levels of W
and Re segregation since the thermal fields are likely to have a
larger influence than the solute fields during solidification. The
results however, show that the freezing range is narrowest for the
undoped C17 base alloy (Table 4). Increases of .about.7.degree. C.
were observed with 2.2 wt % Mo additions while Cr additions to C17
increased the freezing range by .about.20.degree. C. The alloy
containing both elemental additions exhibited the largest freezing
range. Coupled with the microstructural observations regarding the
volume fraction of eutectic, results from this study indicate that
the solidification characteristics are strongly dependent upon
alloy composition. TABLE-US-00004 TABLE 4 DSC results showing the
effect of Cr and Mo additions on the freezing range of C17. Solidus
Liquidus Freezing Range Alloy (.degree. C.) (.degree. C.) (.degree.
C.) C17 1399 1420 21 C17 + Mo 1392 1420 28 C17 + Cr 1381 1422 41
C17 + Cr + Mo 1374 1420 46
[0073] As Ni-base superalloys become more heavily alloyed,
supersaturation of the .gamma. phase with Re, W, Co, Cr, Mo and Ru
during solidification could occur as the solubility limits are
exceeded. Elements that tend to increase the liquidus temperature
of Ni (Re and W), also tend to segregate most strongly to the
single phase .gamma. dendrites during solidification. Ru is an
unusual alloying addition as it slightly increases the liquidus
temperature, but segregates only mildly to the .gamma. phase. Other
.gamma. forming elements, Co, Cr and Mo, tend to slightly lower the
solidus temperature and segregate only moderately to the .gamma.
phase during solidification. Based on the observed changes in
microstructure and associated changes in freezing range,
compositional changes associated with the Cr and Mo additions
appeared to be altering the solid-solution solubility limits that
govern segregation during solidification. In general, higher levels
of refractory element segregation were measured in alloys
containing low overall levels of the potent .gamma. forming
elements. Based on atomic percentages, the lowest combined levels
of Re, W, Cr and Mo from the initial study were found in RR3010 and
C17, 6.6% and 5.25% respectively. These alloys also exhibited the
largest degree of segregation, with K.sub.R.sub.e of 1.57 and 1.55
and k.sub.w of 1.26 and 1.30 for RR3010 and C17 respectively.
Alloys, such as SRR300G and SRR3001, which contain significantly
higher levels (9.6 and 10.9 at % respectively) of these potent
.gamma. forming elements tend to result in less segregation during
solidification (K.sub.R.sub.e is 1.28 and 1.23 and k.sub.w is 1.19
and 1.12 for SRR300G and SRR300I respectively). This was
investigated further using 1 and 3 at % (3 and 9.6 wt %) iridium
additions to C17. Despite Ir partitioning to the solid more
strongly than either Cr or Mo in the same alloy system (Table 2)
and the doping concentrations being greater than the amount to
which Mo was added to C17, no significant changes in the
solidification paths of either W or Re was noted. Interestingly
however, Ir greatly reduced the solid-liquid partitioning of Ta
(FIG. 7). The Ir--Ta binary phase diagram indicates extensive
interactions between the two elements, including the formation of a
phase over a wide composition range, the same intermetallic phase
observed in the binary phase diagrams of Cr--Re, Mo--Re and W--Re.
No such interactions are observed for the elements (Co, A1 and Ru)
which exhibited a negligible influence on the solidification paths
of W and Re. While it is an over simplification to compare binary
with multi-component systems, the fact that Cr, Mo, W and Re all
combine to form thermodynamically stable intermetallic
topologically close-packed (TCP) phases in multi-component alloys
at elevated temperatures suggest that these strong interactions
extend to multi-component systems.
[0074] The electron configurations of Cr, Mo, W and Re are listed
in Table 5. TABLE-US-00005 Atomic Element Number Electron
Configuration Cr 24 1s.sup.2 2s.sup.2 2p.sup.6 3s.sup.2 3p.sup.6
3d.sup.5 4s.sup.1 Mo 42 1s.sup.2 2s.sup.2 2p.sup.6 3s.sup.2
3p.sup.6 3d.sup.10 4s.sup.2 4p.sup.6 4d.sup.5 5s.sup.1 W 74
1s.sup.2 2s.sup.2 2p.sup.6 3s.sup.2 3p.sup.6 3d.sup.10 4s.sup.2
4p.sup.6 4d.sup.10 4f.sup.14 5s.sup.2 5p.sup.6 5d.sup.4 6s.sup.2 Re
75 1s.sup.2 2s.sup.2 2p.sup.6 3s.sup.2 3p.sup.6 3d.sup.10 4s.sup.2
4p.sup.6 4d.sup.10 4f.sup.14 5s.sup.2 5p.sup.6 5d.sup.5
6s.sup.2
[0075] While not wishing to be limited to a particular theory, it
is thought to be the electron vacancies in the d-shell orbitals
(highlighted in bold) which provide the high potential for the
formation of strong TCP phase bonds between these atoms. The d
electrons are loosely bound and become delocalised together with s
electrons resulting in stronger attractions due to the involvement
of more electrons. The inherent stability of TCPs indicates there
may be extensive covalent bonding interactions via the d electrons
and orbitals of the atoms of the TCP forming elements supplementing
their metallic bonding. While no long range ordering exists in the
liquid state, a limited degree of short range ordering may exist
between these elements. Hence, it is thought that the affinity
these elements have for one another in the solid state persists in
the melt and consequently the solidification paths of W and Re are
altered through their interactions with Cr and Mo atoms. The
greater potency of Mo as compared to Cr additions in reducing the
partitioning of W and Re can also be explained through these d
shell interactions. The 4d orbitals of Mo atoms are much more
extended than the 3d orbitals of Cr relative to the filled s and p
orbitals of the same shell. This is because the nuclear charge is
increased from Cr to Mo, meaning the s and p filled subshells,
which feel the increased nuclear charge more strongly due to their
orbits running much closer to the nucleus, cannot expand as much as
the 4d orbital thus allowing for greater overlap of the 4d orbitals
with the d orbitals of neighbouring atoms resulting in greater
interaction. This is exemplified by the wider composition range
over which .sigma. phase is stable in the Mo--Re binary phase
diagrams compared to that of Cr--Re. The exact mechanisms by which
these interactions decrease the extent of microsegregation of W and
Re during solidification is unclear. The effect may be both
thermodynamic and kinetic in nature. It is possible that these
interactions cause Cr and Mo atoms to form metastable clusters with
W and Re within the melt which increase the thermodynamic stability
of the liquid with respect to the solid, particularly in the
vicinity of the crystallisation temperature where volume
differences between the solid and liquid are small and the atomic
arrangements in the liquid are consequently more or less similar to
their arrangements in the corresponding solid bodies. Kinetically,
these same clusters could effectively lower the liquid
diffusivities of W and Re, thus reducing the rate at which W and Re
atoms can diffuse towards the solid-liquid interact. The result is
a compromise of the solid-liquid partition coefficients of each
participant element in the cluster; that is the coefficients of W
and Re are reduced and those of Cr and Mo are increased. This is
best illustrated by comparison of the solidification
characteristics of UCSX6 to UCSX7 and SRR300A to SRR300B in Table
2. The addition of Cr and Mo to UCSX6 results in the reduction of
the k values of W and RE closer to those of Cr and Mo whereas the
increase in partitioning of Re in SRR300B, associated with a higher
Re content, results in a corresponding increase in the partitioning
of Cr and Mo to the growing solid K.sub.C.sub.r is 1.04 and 1.11
and K.sub.M.sub.o is 1.06 and 1.09 in SRR300A and SRR300B
respectively). This compromise is beneficial to the overall
castability of the alloy due to the higher density of W and Re
compared to Cr and Mo.
[0076] Despite quantification of the mechanisms by which Cr and Mo
influence the solid-liquid partitioning of W and Re, the observed
effects of Mo and Cr are interesting particularly considering the
recent trends towards developing low chromium, low molybdenum
content superalloys, where for example in RR3010 the Mo and Cr
contents are now at 0.5 and 1.7 wt % respectively. This trend is
primarily associated with the destabilising effect that Cr and Mo
have on the .gamma.-.gamma.' microstructure after prolonged
exposures at elevated temperatures. Minimisation of these elements
have enabled advanced Ni-based single crystals to be alloyed with
increasing levels of potent solid solution strengtheners, such as W
and Re, to enhance the high temperature creep properties. Cr is
typically maintained at sufficient concentrations to provide a
certain level of hot corrosion and oxidation resistance. These
changes however, appear to be detrimental to the manufacture and
production of advanced single crystal components because of the
increased likelihood for grain defect formation during directional
solidification. As the overall content of Re has increased the
price of bar stock, improvements in yield are of greater
importance. The results from this investigation demonstrate that
the intrinsic tendency of high refractory Ni-based single crystal
alloys to form grain defects during solidification is decreased by
increasing the overall level of Cr and Mo in the alloy. Both of
these elements reduce the extent of microsegregation of the dense
refractory elements W and Re during solidification known to cause
single crystal breakdown. In fact, this beneficial effect appears
as though it is due to the very fact that both Cr and Mo are TCP
forming elements. As a result, increasing the levels of both Cr and
Mo to improve the solidification characteristics of single crystal
Ni-base superalloys will further destabilise the microstructure.
However, the resulting microstructural instabilities could
potentially be controlled through the addition of ruthenium.
Preliminary studies suggest that Ru can be added in the
concentration necessary to improve microstructural stability
without significant detriment to the solidification characteristics
of the alloy. Moreover, the decreased level of microsegregation
accompanying increases in Mo and Cr contents would reduce the
extent of local Re supersaturation in the as-cast crystals and
enable homogenisation to occur more rapidly during solution-heat
treatment.
[0077] Any practical single crystal alloy has to have a combination
of useful properties. These properties include alloy density,
creep-rupture properties, high temperature strength and fatigue
resistance, microstructural stability and oxidation and hot
corrosion resistance, together with acceptable raw material and
processing costs. Increasing the mechanical strength nearly always
involves additions which are both costly and dense, so in order to
maintain an acceptable component cost, the processing costs must be
kept as low as possible.
[0078] This invention is linked to one important aspect of
processing cost, the yield of acceptable castings, commonly called
castibility. The novel aspect of this invention is the
identification of relatively high chromium and molybdenum contents
as beneficial, in combination with the well-known damaging effects
of rhenium and tungsten, and beneficial effect of tantalum. These
considerations lead to the claimed composition ranges in the
following way.
[0079] The chromium content, preferably 3 to 4 weight percent,
should not be less than about 2 weight percent nor more than about
5 weight percent (all compositional percentages herein are by
weight). The chromium content is desirably high due to its benefit
on both hot corrosion and oxidation resistance and castability by
minimising the formation of solidification related grain defects.
However it desirably does not exceed 5% because chromium
contributes to microstructural instability with respect to the
formation of deleterious topologically close-packed (TCP) phases
following prolonged exposure at elevated temperatures. Below 2%
chromium the hot corrosion and oxidation resistance become
unacceptable in the preferred embodiment.
[0080] The molybdenum content ranges from 2 to 5.5%, and preferably
3.5 to 4.5%. Molybdenum is preferably present in concentrations
greater than 2% because it improves the castability of the alloy.
It is also an effective strengthening element in the gamma phase
and has a lower density than the alternative strengtheners tungsten
and rhenium. An upper limit of 5.5% is desirable in the preferred
embodiment because molybdenum destabilises the microstructure
leading to precipitation of damaging TCP precipitates.
[0081] Ruthenium is present in the preferred embodiment in an
amount of at least 1%, conveniently, from about 1 to about 6%,
desirably 2 to about 6%, and preferably from about 3 to about 5%.
Ruthenium provides strength and stabilises the microstructure with
respect to the formation of TCP phases, so counteracting the effect
of the necessary elevated chromium and molybdenum contents for
improved castability. In this work ruthenium was found to be
largely neutral in its effect on castability; high levels of
ruthenium can therefore be added for strength and stability without
compromising casting yield. A preferred upper limit is about 6%,
due to the expense of ruthenium additions.
[0082] Tungsten contents range in the preferred embodiment, from
about 2 to about 5%, preferably 3.5 to 5%. Tungsten partitions to
both the gamma and gamma prime phases and is also an effective
strengthener. Concentrations greater than 2% are desirable to
provide sufficient strength to the superalloy but its density
undesirably increases the density of the alloy and greatly hinders
the ease of single crystal solidification. Its content must
therefore by maintained below about 5% to ensure a good casting
yield by minimising the (W+Re)/Ta ratio. These lower levels of
tungsten will also improve the microstructural stability and
oxidation and hot corrosion resistance of the alloy.
[0083] Rhenium is present in an amount of from about 5 to about 8%,
preferably from about 5 to about 7%. Concentrations of rhenium
above 5% are desirable to achieve the high temperature strength,
particularly when coupled with a relatively low tungsten content,
since it is a potent solid-solution strengthening element of the
gamma phase. Rhenium should not be added in amounts greater than
about 8% to the preferred embodiments because it is a dense and
expensive element, is detrimental to castability and promotes the
formation of TCP phases.
[0084] Tantalum is present in an amount of from about 4 to 8%,
preferably 5.5 to 7%. Tantalum is desirable in concentrations
greater than 4% because it strengthens the gamma prime phase,
provides resistance to hot corrosion but, most notably in this
invention, reduces the formation of solidification related grain
defects by minimising the (W+Re)/Ta ratio. If the tantalum content
is above 8% however, then density of the alloy is undesirably
increased. The novel benefits of chromium and molybdenum, coupled
with low tungsten mean that tantalum can be reduced to achieve a
reduced alloy density.
[0085] The preferred embodiments of the present invention provide a
nickel-based single crystal superalloy which have the advantage of
exhibiting improved castability, i.e. less susceptibility to the
formation of solidification related grain defects during single
crystal solidification, by increasing the Cr and Mo contents and
minimising the (W+Re)/Ta ratio by decreasing the W content relative
to typical nickel-based single crystal superalloy compositions.
[0086] The preferred embodiment comprises a nickel-based single
crystal superalloy where the compositions consists of 2-8 of Co,
2-5 of Cr, 2-5.5 of Mo, 2-5 of W, 5-7 of A1, 4-8 of Ta, 5-8 of Re,
2-6 of Ru, 0-2 of Ti and 0-0.5 of Hf in terms of % by weight and
residual part substantially consists of Ni wherein said alloy may
contain unavoidable impurities.
* * * * *