U.S. patent application number 08/908789 was filed with the patent office on 2002-02-28 for magnetic data-storage targets and methods for preparation.
Invention is credited to BARTHOLOMEUSZ, MICHAEL, CHAPPA, CARLOS.
Application Number | 20020023698 08/908789 |
Document ID | / |
Family ID | 26707844 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020023698 |
Kind Code |
A1 |
BARTHOLOMEUSZ, MICHAEL ; et
al. |
February 28, 2002 |
MAGNETIC DATA-STORAGE TARGETS AND METHODS FOR PREPARATION
Abstract
Cobalt-based Ta-containing magnetic target alloy materials are
produced in which homogeneity of the magnetic material is improved
by eliminating Ta-rich second phases in the microstructure by a
process comprising soaking ingots of said alloy from which targets
are to be produced at temperatures ranging from 1600.degree. to
2600.degree. F for periods of 10 minutes to 24 hours prior to
hot-rolling, preferably using multiple steps, then hot-rolling at
similar temperatures utilizing at least a 3% reduction for pass,
and optionally soaking the rolled plates from said rolling step at
temperatures ranging from 2000.degree. to 2600.degree.F. for
periods of 10 minutes to 24 hours.
Inventors: |
BARTHOLOMEUSZ, MICHAEL;
(PHOENIX, AZ) ; CHAPPA, CARLOS; (PHOENIX,
AZ) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Family ID: |
26707844 |
Appl. No.: |
08/908789 |
Filed: |
August 8, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60031983 |
Nov 29, 1996 |
|
|
|
Current U.S.
Class: |
148/313 ;
420/435; G9B/5.304 |
Current CPC
Class: |
H01F 41/183 20130101;
G11B 5/851 20130101; B82Y 25/00 20130101; B82Y 40/00 20130101; H01F
41/301 20130101; H01F 10/007 20130101 |
Class at
Publication: |
148/313 ;
420/435 |
International
Class: |
H01F 001/04 |
Claims
We claim:
1. A process for the production of Ta-containing Co-based magnetic
targets which possess chemically and mechanically uniform
microstructures and a lack of a Ta-rich second phase, the process
comprising the steps of: a. soaking ingots from which the target
are to be produced at temperatures ranging from 1600.degree. F. to
2600.degree. F. for a period of 10 minutes to 24 hours; prior to
hot-rolling or after casting of an as-cast target; b. hot-rolling
said soaked ingots at temperatures of 1600.degree. to 2600.degree.
F. utilizing at least a 3% reduction per pass to produce rolled
plates from which said targets are produced.
2. A process according to claim 1, wherein said rolled plates are
further soaked at temperatures ranging from 2000.degree. to
2600.degree. F. for periods of 10 minutes to 24 hours.
3. A process according to claim 1, wherein reduction per pass is
defined as (1-T.sub.0/T.sub.l) .times.100% where T.sub.l, is the
thickness of the ingot/plate input into the rolling mill and
T.sub.o is the thickness after rolling by one pass.
4. A process according to claim 1, wherein the rolled plate is then
cooled at a cooling rate equal to or faster than cooling by
air.
5. A process according to claim 4, wherein cooling is carried out
with a cold water quench.
6. A process according to claim 1, wherein soaking of said ingots
is carried out 1 to 5 times at the same or different temperatures
within the range of 1600.degree. to 2600.degree. F.
7. A process according to claim 1, wherein hot-rolling is carried
out 1 to 5 times utilizing at least a 3% reduction per pass.
8. A process according to claim 1, wherein the ingot is reheated
for periods of between 5 and 120 minutes during the hot-rolling
procedure to ensure that the temperature ranges remain in the
prescribed range.
9. A process according to claim 1, wherein hot-rolling is carried
out over a period of between 2 and 14 hours.
10. A process according to claim 1, wherein said target product is
a Ta-containing cobalt base magnetic target which contains less
than about 15 volume percent of Ta-secondary phase
particulates.
11. A process according to claim 1, wherein said magnetic targets
are of the formula:
Co.sub.f-Cr.sub.a-Ni.sub.g-Pt.sub.c-B.sub.c,(Si, Zr, Fe, W, Mo,
Sm)d-Ta.sub.e wherein a=0 to60atomic % ; b=0 to 20 atomic % ; c=0
to 15 atomic % ; d =combination of one or more of these elements
not to exceed 30 atomic % ; e=0.5 to6 atomic % ; g=0 to 40 atomic %
; and f =remainder.
12. A process according to claim 1, wherein said Ta-containing
alloy is selected from the group consisting of: Co-10Cr-4Ta ;
Co-14Cr-6Ta-8Ni ; Co-18 Cr-10 Pt- 3Ta ; Co-13 Cr-4 Ta ; and Co-12
Cr-4 Ta-10 Ni.
13. A magnetic target comprising a Ta-containing Co-based alloy
which is substantially devoid of Ta-rich second phase segregation
in the matrix, and wherein said alloys have a Coercivity value
which is at least 1800 Oersteds.
14. A magnetic target according to claim 13, which comprises less
than about 15 volume percent of Ta-secondary phase
particulates.
15. A magnetic target produced by the process of claim 1.
16. A magnetic target according to claim 13 which comprises from 1
to 12 volume percent of Ta-secondary phase particulates.
17. A magnetic target according to claim 13 which comprises less
than about 2 volume percent of Ta-secondary phase particles.
18. A magnetic target according to claim 13, wherein said magnet
targets are of the formula:
Co.sub.f-Cr.sub.a-Ni.sub.g-Pt.sub.c-B.sub.c(Si, Zr, Fe, W, Mo,
Sm)d-Ta.sub.e wherein a =0 to 60 atomic % ; b =0 to 20 atomic % ; c
=0 to 15 atomic % ; d =combination of one or more of these elements
not to exceed 30 atomic %; e =0.5to6atomic % ; g =0 to 40 atomic %
; and f =remainder.
19. A magnetic target according to claim 13, wherein said
Ta-containing alloy is selected from the group consisting of: Co-10
Cr-4 Ta; Co-14 Cr-6 Ta-8Ni ; Co-18 Cr-10 Pt-3Ta; Co-13 Cr-4 Ta; and
Co-12 Cr-4 Ta-10 Ni.
Description
CROSS-REFERENCE to RELATED APPLICATION
[0001] This application claims the benefit of the filing date of
Provisional Patent Application Serial No. 60/031,983, filed
November 29, 1996.
FIELD of the INVENTION
[0002] This invention relates to
Co.sub.f--Cr.sub.a--Ni.sub.g--Pt.sub.b--B-
.sub.c--(Si,Zr,Fe,W,Mo,Sm).sub.d--Tae magnetic target materials and
more particularly relates to methods for production of magnetic
target materials with chemically homogeneous microstructures which
promote sputter deposited films with higher and more uniform
magnetic coercivities than films deposited using similar target
products known to the art.
[0003] a = 0 to 60 atomic %.
[0004] b = 0 to 20 atomic %.
[0005] c = 0 to 15 atomic %.
[0006] d = combination of one or more of these elements not to
exceed 30 atomic %.
[0007] e = 0.5 to 6 atomic %.
[0008] g = 0 to 40 atomic %.
[0009] f = remainder.
BACKGROUND ART
[0010] Data storage disks used in computer hard drives are
manufactured by magnetron or RF sputter deposition processes. The
disk itself comprises several different layers of material.
Typically, thin-film disk technology uses a Al blank as the base
substrate material with a hard amorphous Ni--P layer with thickness
of about 10 microns electrolessly plated onto it. Johnson et al.,
IBM J. Res. Develop., Vol. 40, Nos. 5, Sept. 1996, p. 511-536. The
Ni--P layer is scribed with fine texturizing grooves. An underlayer
of Cr with thickness between 20 to 100 nm is sputter deposited onto
the Ni--P layer to ensure nucleation of the magnetic film with the
easy axis of magnetization in-plane for longitudinal recording. A
Co-based magnetic film, with composition in the ranges described
above, is sputter deposited onto the underlayer with thickness'of
about 30 nm for magnetoresistive head applications. Finally, a
10-20 nm protective layer of hydrogenated carbon is reactively
sputtered on top of the magnetic layer. Alternate substrate (i.e.
glass) and underlayer materials (i.e. Cr--V, Ni--Al, Cr--Ti) are
commonly utilized in the data storage industry.
[0011] Magnetron sputtering to form the magnetic film on the data
storage device (disk), involves the arrangement of permanent or
electromagnets behind the magnetic target material (cathode). The
applied magnetic field transmits through the target and focuses a
discharge plasma onto the front of the target. The front of the
target surface is atomized with subsequent deposition of the
magnetic target atoms on top of the underlayer of the evolving
disk. Typically, sputtering of the various non-magnetic and
magnetic layers, comprising the architecture of the disk, is
conducted on both sides of the disk.
[0012] A potential problem which results during the sputtering of
Co-based alloys containing Ta additions is the effect of the
homogeneity of the multi-phase target microstructure on the
resultant magnetic properties of the deposited film. The maximum
solid solubility of Ta in Co is 4 atomic % at 1280 Celsius
(Massalski et al. "Binary Phase Diagrams", ASM International, Vol.
2,1990, p.1245) as depicted in FIG. 1. As other alloying additions
are added to Co, the maximum solid solubility of Ta in the matrix
is further decreased. Therefore, the driving force for the
formation of eutectic Ta-rich particulates in the microstructure of
Co-based magnetic alloys is very large. The eutectic Ta-rich
particulates have been identified as possessing a Co.sub.2Ta
stoichiometry, (Schlott et al., IEEE Transactions on Magnetics,
Vol. 31, No. 6, Nov.1995, p. 2818-2820; Massalski et al. supra).
Even in the case of alloys possessing Ta contents less than the
maximum solid solubility limit, the propensity for Ta-rich
second-phase formation is very large since these alloys are
typically not thermomechanically processed at the eutectic
temperature.
[0013] Furthermore, even if the Co-based alloys containing Ta are
processed within proximity of the eutectic temperature, Ta-rich
second-phase formation will occur if sufficient cooling rates are
not employed since the solid solubility limit diminishes rapidly
with decreasing alloy temperature.
[0014] It is fairly typical that a pair of magnetic alloy targets
can be used to fabricate in excess of 10,000 individual
data-storage disks. Since the magnetic target alloy is continually
losing surface atomic layers during the sputtering process, through
thickness and in-plane target microstructural homogeneity is
essential to ensure film property homogeneity on the many thousands
of disks fabricated from each target and the many thousand more
fabricated from the numerous targets constituting a production lot
or originating from several individual production lots. A
production lot represents all the targets that are exposed to
exactly the same thermomechanical history (i.e. originating from
one melted ingot or one hot-isostatic-press container).
[0015] Ta-rich second-phase segregation in the matrix of Co-based
magnetic target alloys has been shown to impact deposited film
magnetic properties such as Coercivity. When tens of thousands of
data storage devices are being made from several targets, it is
necessary that the Coercivity response be consistent on all the
disks, i.e., quality control, and not be a function of the specific
target utilized. Therefore, there is a substantial need in the art
for Ta containing Co-based magnetic targets which exhibit
consistent performance, both within a target and from target to
target.
[0016] Standard production practices for magnetic target alloys
involve the following thermomechanical steps. See Schlott et al.,
supra; U.S. Pat. No. 5,468,305; U.S. Pat. No. 5,334,267; and U.S.
Pat. No. 5,282,946. Ingots are fabricated by either casting or
Hot-Isostatic-Pressing (HIPping) of elemental powders. Hot-rolling
is then conducted primarily to heal any residual porosity in the
ingots and form plates from which the magnetic targets can be
obtained. The ingot and plate sizes are pre-determined to enable
extraction of the particular target geometry required (i.e.
rectangular targets and circular targets possessing a variety of
different dimensions). Single-step hot-rolling practices at
temperatures between 700F to 2200F are typically employed. After
hot-rolling, heat-treatment and cold-texture deformation processes
are utilized to reduce the bulk magnetic permeability of the target
product. A reduction in bulk magnetic permeability is utilized to
improve the efficiency of the sputtering process by facilitating
optimum passage of magnetic flux through the bulk of the target.
Shunting of magnetic flux within the bulk of the target adversely
affects the stability of the sputtering process, material yield of
the target and thickness uniformity of the deposited film.
Alternate fabrication practices involving water-cooled as-cast
target fabrication with no further down-stream thermomechanical
processing are also known in the prior art. A review of the prior
art reveals that controlling hot-rolling temperatures, or
utilization of individual homogenization practices, have not been
employed to minimize the formation of Ta-rich second-phase
particulates in the target microstructure.
[0017] The present invention will focus on the addition of specific
homogenization treatments to the fabrication of magnetic target
materials in order to minimize and homogenize the presence of
Ta-rich second-phase particulates in the microstructure.
SUMMARY of the INVENTION
[0018] It is accordingly one object of the present invention to
provide a method for the production of Ta containing Co-based
magnetic targets which promote consistent deposited film magnetic
property performance.
[0019] A further object of the invention is to provide a process
for the production of magnetic targets wherein the potential for
formation of a course Ta-rich second phase in the microstructure is
reduced.
[0020] An even further object of the invention is to provide
magnetic target alloys wherein the presence of a Ta-rich second
phase in the microstructure has been substantially eliminated.
[0021] Other objects and advantages of the present invention will
become apparent as the description thereof proceeds.
[0022] In satisfaction of the foregoing objects and advantages, the
present invention provides a process for the production of magnetic
target alloy materials which possess chemically and mechanically
uniform microstructures both within a target and from target to
target, wherein a homogenization and hot-rolling practice is
utilized to dissolve the Ta-rich second-phase back into solution.
The process involves soaking the ingots from which the targets are
produced at temperatures ranging from 1600.degree. F. to
2600.degree. F. for periods of 10 minutes to 24 hours prior to
hot-rolling, optionally using multiple steps, then hot rolling at
similar temperatures utilizing at least a 3% reduction per pass,
and finally, optionally soaking the rolled plates from which the
targets are produced at temperatures ranging from 2000.degree. F.
to 2600.degree. F. for periods of 10 minutes to 24 hours. Note,
reduction per pass is defined as: (1- T.sub.o/T.sub.i) .times.100%,
where T.sub.i is the thickness of the ingot/plate input into the
rolling mill and T.sub.o is the thickness after rolling by one
pass. Directly following the thermomechanical processing, the
cooling rate of the plate must be equal to or faster than an air
cool. A cold water quench is preferable. It has been discovered
that this practice will retard the formation of Ta-rich
second-phase and result in a Co-based magnetic target alloy wherein
the tantalum second-phase is substantially minimized or completely
eliminated. Also provided by the present invention are target
alloys depicting the beneficial effect of a homogenized magnetic
target alloy microstructure on the magnetic Coercivity of the
resulting sputter deposited thin film.
BRIEF DESCRIPTION of DRAWINGS
[0023] Reference is now made to the drawings accompanying the
application wherein:
[0024] FIG. 1 is a Co--Ta binary phase diagram;
[0025] FIGS. 2a and 2b are comparative SEM-BEI images of standard
and homogenized fabrication practices, respectively, for
Co-10Cr-4Ta;
[0026] FIGS. 3a and 3b are comparative SEM-BEI images of standard
and homogenized fabrication practices, respectively, for
Co-14Cr-6Ta-8Ni.sub.j;
[0027] FIGS. 4a and 4b are comparative SEM-BEI images of standard
and homogenized fabrication practices, respectively, for
Co-18Cr-10Pt-3Ta;
[0028] FIGS. 5a and 5b are graphs showing SEM-EDS of the matrix and
second phase particles, respectively, of Co-10Cr-4Ta;
[0029] FIGS. 6a and 6b are graphs showing SEM-EDS of matrix phase
and second phase particles, respectively, in Co-14Cr-6Ta-8Ni;
[0030] FIGS. 7a and 7b are graphs showing SEM-EDS of matrix phase
and second phase particles, respectively, of Co-18Cr-10Pt-3Ta;
[0031] FIGS. 8a and 8b show the as-cast air-cooled and
water-cooled, respectively, microstructure of Co-10Cr-4Ta;
[0032] FIG. 9 is a DTA trace of Co-14Cr-6Ta-8Ni;
[0033] FIG. 10 is a DTA trace of Co-18Cr-1OPt-4Ta;
[0034] FIG. 11 is a DTA trace of Co-2OCr-8Pt;
[0035] FIG. 12 is an SEM-BEI of the same area as FIG. 4b taken at
lower magnification;
[0036] FIG. 13a is an optical micrograph of Co-10Cr-4Ta fabricated
using standard practice (600X);
[0037] FIG. 13b is an optical micrograph of Co-10Cr-4Ta fabricated
using the homogenized practice (150);
[0038] FIGS. 14a and 14b are comparative optical micrographs of
Co-14Cr-6Ta-8Ni, fabricated using standard and homogenized
practices, respectively, (270X);
[0039] FIG. 15 a is an optical micrograph of Co-18Cr-10Pt-3Ta
without using the homogenization practice (213X);
[0040] FIG. 15b is an optical micrograph of Co-18Cr-10PT-3Ta using
homogenization practice (185X);
[0041] FIG. 16a is a SEM-BEI image of a Target A fabricated using
standard practice;
[0042] FIG. 16b is a SEM-BEI of a Target B fabricated using the
homogenization practice of the invention; and
[0043] FIG. 17 is a sputter track cross-section of Target A.
DESCRIPTION of the INVENTION
[0044] This invention involves improvements in the performance of
magnetic material sputtering targets which are used in the
production of data storage disks, in particular, the magnetic film
layer comprising the architecture of the data storage disk. The
present invention is especially concerned with the homogeneity of
magnetic material sputtering targets because the lack of
homogeneity can adversely impact the absolute values and uniformity
of the deposited film's magnetic properties (i.e. Coercivity). As
noted above, a current problem with such targets is inhomogeneity
caused by the formation of a Ta-rich second-phase in the
microstructure of Co-based alloys. The present invention provides a
solution to this problem.
[0045] The magnetic target alloys with which this invention is
concerned can be produced from any of the well-known magnetic
target alloys:
Co.sub.fCr.sub.a-Ni.sub.g-Pt.sub.b-Bc-(Si,Zr,Fe,W,MO,Sm)d-Tae where
a =0 to 60 atomic %, b =0 to 20 atomic %, c =0 to 15 atomic %, d
=combination of one or more of these elements not to exceed 30
atomic %, e =0.5 to 6 atomic %, g =0 to 40 atomic %, and f
=remainder. Many different alloys are used in the production of
magnetic target alloys although certain representative alloys are
preferred in this invention. The alloys selected to demonstrate the
homogenization practices presently discussed are Co-10Cr-4TA,
Co-20Cr-8Pt, Co-13Cr-4Ta, Co-12Cr-4Ta-1 Ni, Co-14Cr-6Ta-8Ni and
Co-18Cr-10Pt-3Ta (all numbers refer to elemental concentrations in
atomic Irrespective of the alloy from which the target is produced,
it is necessary that the resulting targets exhibit substantially
identical coercivities and remain homogeneous. Unfortunately, the
element tantalum (Ta) has a very low solid solubility in cobalt
based alloys, the maximum solid solubility of Ta in Co is 4 atomic
% at 1280 Celsius (Massalski et al. supra). As other alloying
additions are added to Co, the maximum solid solubility of Ta in
the matrix is further decreased. Therefore, the driving force for
the formation of incoherent Ta-rich particulates in the
microstructure of Co-based magnetic alloys is very large. Even in
the case of alloys possessing Ta contents less than the maximum
solid solubility limit, the propensity for Ta-rich second-phase
formation is very large since these alloys are typically not
thermomechanically processed at the eutectic temperature.
Furthermore, even if the Co-based alloys containing Ta are
processed within proximity of the eutectic temperature, Ta-rich
second-phase formation will occur if sufficient cooling rates are
not employed since the solid solubility limit diminishes rapidly
with decreasing alloy temperature.
[0046] FIGS. 2a, 3a and 4a are Scanning-Electron-Microscopy (SEM)
Back-Scattered-Electron-lmages (BEI) of the microstructures of
Co-10Cr-4Ta, Co-14Cr-6Ta-8Ni and Co-18Cr-10Pt-3Ta, respectively,
fabricated using standard fabrication practices. The dark matrix
phase and white second-phase for the three microstructures were
identified using SEM Energy-Dispersive-Spectroscopy (EDS). FIGS. 5
, 6 and 7 represent the SEM-EDS spectra for the matrix phase and
second-phase in the microstructures of the Co-10Cr-4Ta,
Co-14Cr-6Ta-8Ni and Co-18Cr-10Pt-3Ta alloys, respectively,
fabricated using standard fabrication practices. FIGS. 5 to 7
demonstrate that the second-phase particles in Ta containing
Co-based alloys are always predominantly Co-Ta, regardless of the
other alloying additions. FIG. 8a depicts the as-cast air-cooled
microstructure of the Co-10Cr-4Ta alloy. The white appearing phase,
as in the case of FIGS. 2 to 4, has been identified using SEM-EDS
as the Ta-rich second-phase. This figure demonstrates that
second-phase formation occurs during casting of the alloy. FIG. 8b
depicts the as-cast water-cooled microstructure of the Co10Cr-4Ta
alloy. FIG. 8b demonstrates that even very rapid cooling after
casting is not sufficient to prevent the formation of deleterious
second-phase networks in the microstructure of Ta-containing
Co-based alloys. It was this result that provided the impetus for
the development of post-casting homogenization practices for
Ta-containing Co-based alloys of this invention.
[0047] This analysis and the results of extensive investigations by
the inventors on such alloy systems indicate that Ta-rich second
phase formation is detrimental to product microstructural and
property homogeneity and adversely affects the performance and
consistency of the deposited magnetic film. As a result, a
homogenization/hot-rolling practice has been developed for
treatment of such alloys to dissolve the Ta-rich second phase back
into solution. The process of the invention involves soaking the
ingots from which the targets are produced at temperatures ranging
from 1600.degree.F. to 2600.degree. F. for periods of 10 minutes to
24 hours prior to hot-rolling or after casting in the case of
as-cast product. Various steps can be used within this temperature
range using either the lower or higher temperature as the initial
step. Thereafter, the ingot is hot-rolled at temperatures ranging
from 1600.degree. F. to 2600.degree.F., utilizing at least a 3%
reduction per pass. Ingot re-heating, between 5 to 120 minutes, can
be employed during the hot-rolling procedure to ensure that the
temperature range remains in the prescribed range. Maintaining a
minimum reduction per pass of 3% ensures mechanical homogenization
of the microstructure in addition to the chemical homogenization.
The minimum 3% reduction per pass is especially important in the
last 5 passes of the rolling campaign. This requirement is
particularly important in the case of alloys where some remnant
Ta-rich second-phase is present in the microstructure, because the
deformation processing promotes uniform dispersion of the remnant
second-phase and neutralizes its propensity to form coarse
inhomogeneously distributed aggregates.
[0048] Various hot-rolling steps can be used within the temperature
range of 1600.degree. F. to 2600.degree. F. using either the lower
or higher temperature as the initial step. Either directly after
hot-rolling or after further downstream thermomechanical processing
to optimize on the bulk properties of the target material. (See
Schlott et al. supra; Weigert et al., Mat. Sci and Engineering,
A139, 1991, p. 359-363) An optional soaking of the rolled plates
from which the targets are produced at temperatures ranging from
2000.degree. F. to 2600.degree. F. for periods of 10 minutes to 24
hours can be employed as a final homogenization step. Directly
following hot-rolling or the final optional homogenization step,
the cooling rate of the plate must be equal to or greater than an
air cool. A cold water quench is preferable.
[0049] The homogenization practice of this invention was developed
using Differential Thermal analysis (DTA) on a whole spectrum of
Co-based alloys. DTA traces enable identification of solid/liquid,
solid/solid and order/disorder phase transformation temperatures
and the corresponding enthalpies of transformation. DTA involves
heating and cooling samples at fixed temperature rates and
monitoring the power of the temperature controller. Exothermic or
endothermic phase transformation temperatures can be very
accurately determined by voltage deviations in the power signal.
The integrated area of the voltage signals can be utilized to
calculate the actual energy of transformation. FIGS. 9, 10 and 11
are DTA traces of Co-14Cr-6Ta-8Ni, Co-18Cr-10Pt-3Ta and Co-2OCr-8Pt
alloys, respectively. In these figures, reading from left to right,
the first downward (endothermic) pointing peak represents the alloy
solidus temperature (solid/liquid transformation temperature during
heating). The first, and largest, upward (exothermic) pointing peak
represents the liquidus temperature (liquid/solid transformation
temperature during cooling). Comparison of the Ta containing alloys
(FIGS. 9 and 10) with the non-Ta containing alloy (FIG. 11) reveals
the presence of the second exothermic Ta-rich second-phase
formation peak, at a temperature below the liquidus temperature.
This phase is most likely the .lambda..sub.3 Co.sub.2Ta phase. FIG.
11 demonstrates Co--Cr--Pt alloys possess chemically homogeneous
microstructures due to the lack of Ta as an alloying addition.
Knowledge of the solidus, liquidus and second-phase formation
temperature enables determination of a homogenization practice at
temperatures near the maximum solid solubility of Ta--containing
alloys in order to ensure maximum dissolution of the Ta atoms into
the matrix phase.
[0050] Utilizing the information from the DTA traces, a
homogenization practice, prior to hot-rolling, at temperatures
controlled between 2200 F and 2600F. for a period of time between 2
to 6 hours was selected for the alloys presented in FIGS. 2b to 4b.
After the homogenization treatment, the alloys were hot-rolled in
the same temperature range as that utilized in the homogenization
treatments. Water quenching was employed after hot-rolling and the
alloys were given the same downstream thermomechanical treatments
and machined into the same final product configurations as the
alloys depicted in FIGS. 2a to 4a.
[0051] FIGS. 2b, 3b and 4b are Scanning-Electron-Microscopy (SEM)
Back-Scattered-Electron-lmages (BEI) of the microstructures of
Co-1OCr-4Ta, Co-14Cr-6Ta-8Ni and Co-18Cr-10Pt-3Ta, respectively,
fabricated using 3 the new homogenized fabrication practice. As in
FIGS. 2a to 4a, the dark matrix phase and white Ta-rich
second-phase for the three microstructures were identified using
SEM Energy-Dispersive-Spectro- scopy (EDS). Comparison of FIGS. 2b
to 4b with FIGS. 2a to 4a reveals that the homogenization practice
employed successfully minimized the presence of Ta-rich
second-phase in the microstructure. FIG. 12 is a lower
magnification picture than FIG. 4b conclusively demonstrating the
successful dissolution of the Ta-rich second-phase particulates.
Table 1 compares the Volume Percent Second Phase (VPSP) prior to
and after homogenization calculated using a standard point counting
method.
1TABLE 1 VPSP VPSP Grain Size Grain Size (before (after (before
(after Alloy homog.) homog.) homog.) homog.) Co-10Cr-4Ta 13% 1% 20
microns 280 microns Co-14Cr-6Ta-8Ni 45% 12% 30 microns 33 microns
Co-18Cr-10Pt-3Ta 15% 2% 81 microns 150 microns
[0052] The results in Table 1 demonstrate that the homogenization
practice developed is very effective in minimizing the chemical
inhomogeneity in the microstructure of Ta containing Co-based
alloys. In the case of Co-14Cr-6Ta-8Ni, the VPSP after chemical
homogenization is higher than in the other alloys due to the higher
Ta content of this alloy. In cases like this, where the alloy Ta
content significantly exceeds the solid-solubility limit, it is
thermodynamically impossible to get rid of all the second-phase
particles. However, by utilizing rolling reductions greater than 3%
per pass, the remnant Ta-rich second-phase particles can be
mechanically homogenized. Mechanical homogenization is employed to
shear the Ta-rich particles to render them as small as possible and
introduce enough deformation in the microstructure to minimize
through-thickness second-phase morphology gradients.
[0053] FIGS. 13,14 and 15 are optical micrographs of the
microstructures of Co-10Cr-4Ta, Co-14Cr-6Ta-8Ni and
Co-18Cr-10Pt-3Ta fabricated using the standard and
new-homogenization fabrication practices. These figures, and the
average grain-size data summarized in Table 1, demonstrate that the
homogenization practice can result in an increase in average
product grain size. The higher processing temperatures, soaking or
rolling, typically associated with the homogenization practice
described in the present document result in a greater propensity
for grain growth in the alloy microstructures. This is especially
true in the lower Ta containing alloys, where the almost complete
dissolution of second-phase particles removes particulate pinning
points that retard microstructural grain growth.
[0054] The larger alloy grain sizes depicted in FIGS. 13b to 15b
are typical of one-step homogenizing or roll/homogenizing
treatments. More involved multi-step roll/homogenize processing
practices, within the temperature ranges specified in this patent,
can be employed to minimize homogenized product grain size.
[0055] The numerous "scratches" on the surface of the optical
metallographs are most likely a combination of three things: (1)
microcracks introduced during polishing of the samples, (2)
preferential etching of deformation-twin boundaries intersecting
the polished surface and (3) preferential etching of dislocation
sub-boundaries intersecting the polished surface. The morphology of
the "scratches"is very reminiscent of a dislocation polygonization
reaction. Co-based alloys are allotropic in nature and can possess
FCC, HCP or combination FCC/HCP crystallography. Dislocation
polygonization occurs during elevated temperature deformation of
FCC and HCP materials and is a manifestation of dynamic recovery
and work-hardening processes. Bartholomeusz et al., Metallurgical
Transactions A. Vol 25, 1994, p. 2161-2171; Bartholomeusz et al.
Material Science and Engineering, A 201, 1995, p. 24-31.
[0056] To illustrate the effect of Ta-rich second-phase particles
in the target microstructure on the sputter deposited film
properties consider the following example: Two targets of a
Co-13Cr-4Ta alloy were fabricated. Target A was fabricated using
standard techniques and possesses a coarse Ta-rich second-phase
microstructural morphology. Target B was fabricated using the new
homogenization practice described in the present invention. FIGS.
16a and 16b are SEM-BEI micrographs of Targets A and B,
respectively, depicting the morphological difference of the target
microstructures.
[0057] Sputter process trials were conducted using an Intevac
sputtering machine equipped with a CM-Gun cathode. The two targets,
A and B, were placed on either side of the sputtering chamber so
that they would be used for material deposition on the opposite
sides of the same disk. These precautions were taken to ensure that
exactly the same sputter conditions and testing conditions were
applied for films deposited using the two differently fabricated
targets. Furthermore, targets A and B were interchanged in the
sputtering chamber to ensure that no anomalies associated with
location in the chamber were obscuring the results of the
investigation. The results of the analysis revealed that the
magnetic films on disks fabricated using target A exhibited
coercivities that ranged from 1580 Oersteds to 1780 Oersteds. In
contrast, the films on disks sputter deposited with magnetic
material using target B exhibited coercivities that ranged between
1920 to 2000 Oersteds. The film Coercivity was ascertained using
conventional VSM testing techniques, widely employed in the disk
manufacturing industry. There are several noteworthy points
resulting from this analysis. First, target A, possessing a
chemically inhomogeneous microstructure, resulted in films with a
significantly lower Coercivity response than films deposited using
target B which possessed a chemically homogeneous microstructure.
Second, the actual film coercivities obtained from target A
(overall range =200 Oersteds) were much less consistent than the
film coercivities obtained from target B (overall range =80
Oersteds). These results demonstrate that if Ta-rich second-phase
particles are not minimized in the microstructure of Co-based
alloys and if any remnant particles are not adequately mechanically
homogenized through the thickness of the target, the resulting
Coercivity response of the sputtered film can be diminished and the
disk-to-disk Coercivity consistency can be adversely effected.
[0058] FIG. 17 is a cross section of the target A sputter track and
reveals preferential second-phase coalescence towards the center of
the target. This figure graphically illustrates the type of through
thickness microstructural inhomogeneity that occurs in Ta
containing Co-based alloys if homogenization practices are not
employed.
[0059] The invention has been described with reference to certain
preferred embodiments. However, as obvious variations thereon will
become apparent to those skilled in the art, the invention is not
to be considered as limited thereto.
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