U.S. patent application number 14/923878 was filed with the patent office on 2016-04-28 for apparatus and method for subsurface structural modification of materials at reduced temperatures.
The applicant listed for this patent is U.S. Army Research Laboratory ATTN: RDRL-LOC-I. Invention is credited to Kristopher A. Darling, Micah J. Gallagher, Laszlo J. Kecskes, Heather A. Murdoch, Anthony J. Roberts.
Application Number | 20160114462 14/923878 |
Document ID | / |
Family ID | 55791246 |
Filed Date | 2016-04-28 |
United States Patent
Application |
20160114462 |
Kind Code |
A1 |
Kecskes; Laszlo J. ; et
al. |
April 28, 2016 |
APPARATUS AND METHOD FOR SUBSURFACE STRUCTURAL MODIFICATION OF
MATERIALS AT REDUCED TEMPERATURES
Abstract
Nanostructured or ultra-fine grained metallic systems according
to embodiments of the invention may be formed of: pure Cu, pure Fe,
or pure Ti, with grain sizes of less than 140 nm, 348 nm, or 59 nm,
respectively. The metallic systems demonstrate a monotonically
increasing grain size dependence from a minimum value attained at
the surface; and a converse relation of microhardness, decreasing
from 160 kg/mm.sup.2, 265 kg/mm.sup.2, or 320 kg/mm.sup.2,
respectively. The grain refinement process at cryogenic conditions
relies on the suppression of room temperature dislocation-mediated
deformation mechanisms which facilitate grain restructuring,
relaxation, and reorientation. At the cryogenic conditions,
alternative mechanism for grain refinement, such as shear
localization or dynamic recrystallization may be more dominant.
Processes for refining the grain size of these metallic systems may
include: subjecting metal plates to a high-energy milling process
using a high-energy milling device to impart high impact energies
to its surface. Due to the high-efficiency of this attrition
process, these metallic systems are ideal candidates for improved
corrosion and wear resistance.
Inventors: |
Kecskes; Laszlo J.; (Havre
de Grace, MD) ; Gallagher; Micah J.; (Conestoga,
PA) ; Roberts; Anthony J.; (Chesapeake City, MD)
; Murdoch; Heather A.; (Baltimore, MD) ; Darling;
Kristopher A.; (Havre de Grace, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
U.S. Army Research Laboratory ATTN: RDRL-LOC-I |
Adelphi |
MD |
US |
|
|
Family ID: |
55791246 |
Appl. No.: |
14/923878 |
Filed: |
October 27, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62068994 |
Oct 27, 2014 |
|
|
|
Current U.S.
Class: |
72/53 |
Current CPC
Class: |
C21D 8/00 20130101; B24C
7/0007 20130101; B24C 1/10 20130101; C21D 7/06 20130101 |
International
Class: |
B24C 1/10 20060101
B24C001/10 |
Goverment Interests
GOVERNMENT INTEREST
[0002] The invention described herein may be manufactured, used,
and licensed by or for the United States Government without the
payment of royalties thereon.
Claims
1. A surface mechanical attrition treatment process for a metal
part comprising: providing at least one metal part that is formed
from a first metal composition; providing a plurality of metal
fragments that are formed from a second metal composition wherein
said metal fragments have a size that is significantly smaller than
the size of the at least one metal part; reducing the temperature
of the at least one metal part and the plurality of metal
fragments; impacting the at least one metal part with the plurality
of fragments at a reduced temperature (T.sub.r), for an impact
processing time (t.sub.i); wherein, the at least one metal part is
subjected to bombardment by the plurality of metal fragments at a
reduced temperature (T.sub.r), resulting in the as-received grain
size of the at least one metal part to be reduced, by several
orders of magnitude, and possessing a gradient structure from the
impact surface of the at least one metal part into the interior of
the bulk of the at least one metal part.
2. The surface mechanical attrition treatment process of claim 1,
wherein the reduced temperature (T.sub.r) is not greater than about
-50.degree. C.
3. The surface mechanical attrition treatment process of claim 1,
wherein the reduced temperature (T.sub.r) is not greater than about
-100.degree. C.
4. The surface mechanical attrition treatment process of claim 1,
wherein the reduced temperature (T.sub.r) is not greater than about
-150.degree. C.
5. The surface mechanical attrition treatment process of claim 1,
wherein the reduced temperature (T.sub.r) is not greater than about
-196.degree. C.
6. The surface mechanical attrition treatment process of claim 1,
wherein the refined grain size of the metallic system at its
surface is about 1000 nm or less.
7. The surface mechanical attrition treatment process of claim 3,
wherein the room temperature refined grain size of the metallic
system at its surface is about 355 nm or less.
8. The surface mechanical attrition treatment process of claim 3,
wherein the cryogenic refined grain size of the metallic system at
its surface is about 140 nm or less.
9. The surface mechanical attrition treatment process of claim 3,
wherein the room temperature processed metallic system has a
surface Vickers microhardness of about 110 kg/mm.sup.2 or more at
room temperature.
10. The surface mechanical attrition treatment process of claim 3,
wherein the cryogenic temperature processed metallic system has a
surface Vickers microhardness of about 150 kg/mm.sup.2 or more at
cryogenic temperature.
11. The surface mechanical attrition treatment process of claim 4,
wherein the room temperature refined grain size of the metallic
system at its surface is about 635 nm or less.
12. The surface mechanical attrition treatment process of claim 4,
wherein the cryogenic refined grain size of the metallic system at
its surface is about 350 nm or less.
13. The surface mechanical attrition treatment process of claim 4,
wherein the room temperature processed metallic system has a
surface Vickers microhardness of about 230 kg/mm.sup.2 or more at
room temperature.
14. The surface mechanical attrition treatment process of claim 4,
wherein the cryogenic temperature processed metallic system has a
surface Vickers microhardness of about 260 kg/mm.sup.2 or more at
room temperature.
15. The surface mechanical attrition treatment process of claim 5,
wherein the room temperature refined grain size of the metallic
system at its surface is about 155 nm or less.
16. The surface mechanical attrition treatment process of claim 5,
wherein the cryogenic refined grain size of the metallic system at
its surface is about 60 nm or less.
17. The surface mechanical attrition treatment process of claim 5,
wherein the room temperature processed metallic system has a
surface Vickers microhardness of about 280 kg/mm.sup.2 or more at
room temperature.
18. The surface mechanical attrition treatment process of claim 5,
wherein the cryogenic temperature processed metallic system has a
surface Vickers microhardness of about 320 kg/mm.sup.2 or more at
room temperature.
19. The surface mechanical attrition treatment process of claim 1,
wherein the impact processing time (t.sub.i) is at least about 5
minutes.
20. The surface mechanical attrition treatment process of claim 1,
wherein the impact processing time (t.sub.i) is at least about 10
minutes.
21. The surface mechanical attrition treatment process of claim 1,
wherein the impact processing time (t.sub.i) is at least about 20
minutes.
22. The surface mechanical attrition treatment process of claim 1,
wherein the impact processing time (t.sub.i) is at least about 40
minutes.
23. The surface mechanical attrition treatment process of claim 1,
wherein the impact processing time (t.sub.i) is at least about 60
minutes.
24. A method of modifying the surface of a metal part, the method
comprising the steps of: providing at least one metal part t;
providing a plurality of metal fragments wherein said metal
fragments have a size that is significantly smaller than the size
of the at least one metal part; reducing the temperature of the at
least one metal part and the plurality of metal fragments; and
impacting the at least one metal part with the plurality of
fragments at a reduced temperature (T.sub.r), for an impact
processing time (t.sub.i).
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application No. 62/068,994, titled "Apparatus and Method for
Subsurface Structural Modification of Materials at Cryogenic
Temperatures," filed on Oct. 27, 2014 which is hereby incorporated
by reference herein including all attachments and papers filed with
U.S. Provisional Application No. 62/068,994.
BACKGROUND
[0003] 1. Field of the Invention
[0004] The present disclosure relates to nanostructured surface
coatings and subsurface modifications, having uniform to gradient
microstructures, with single or multiple elemental constituents,
and methods of making the same.
[0005] 2. Background of the Invention
[0006] Nanostructured and nanocrystalline materials are well known
to have novel and/or enhanced properties compared to their
coarse-grained counterparts. Specifically, nanostructured metals
may have enhanced strength, hardness, and/or improved workability.
It has been shown that these nanostructures, when formed as
metallic surface layers, could result in a number of significant
improvements and new applications. For example, failure in metallic
materials, such as fatigue fracture, fretting fatigue, or wear and
corrosion often initiate and propagate from the surface of the
material. As such, optimization by nanostructuring of the surface
layer through surface modification would limit such detrimental
behavior and, in turn, not only enhance the surface, but also the
global or bulk properties of the material.
[0007] However, the abrupt transition in physical characteristics
from such a surface layer to the bulk would likely result in a
correspondingly different response to physical phenomena as well.
Therefore, it is hypothesized that creation of a gradual transition
region between the unaltered bulk and this surface coating, having
a range in properties, would be more effective in mitigating such
phenomena.
[0008] There are a number of techniques to generate nanostructured
surfaces by using traditional coating techniques, e.g., plasma
vapor deposition or chemical vapor deposition.
[0009] In the recent decade, similar to the well established method
of shot peening, a novel Surface Mechanical Attrition Treatment
(SMAT) technology has been successfully applied to many metal-based
material systems such as those based on Ni, Fe, Mg, Al, or Ti.
[0010] Surface mechanical attrition treatments have received much
attention due to their ability to enhance physical properties, such
as yield strength, hardness, wear, and fatigue resistance in
structural parts. Fundamentally, such treatments are carried out by
repeated impacts of the metal surface with some impingement media
which normally occurs at high strain rates inside a closed or
partially open container. Partially inelastic collisions during
processing transfer kinetic energy into the substrate in the form
of deformation strain energy and heat. In the initial stages of
SMATing it is expected that, at the surface, localized deformation
occurs through compressive stresses and shear banding followed by
heavy deformation, wherein the total strain energy increases due to
the creation of excess dislocations, stacking faults, and other
subgrain structures leading to an eventual attenuation and
reduction of the grain size. Finally, an equilibrium grain size is
reached, the specific length scale of which is determined by a
balance between the flux of induced defects, the evolution of the
generated defect cell structures, and their relaxation,
reorganization, and restructuring. As relaxation/restructuring is
thermally activated, the local as well as global temperature rise
during processing plays a significant role in determining the
resultant microstructure during SMAT processing.
[0011] The prior art teaches the conventional application of SMAT,
wherein, plastic deformation, at high strains and strain rates, is
imposed on the surface layers of bulk samples. Typically, spherical
steel (or ceramic) balls of a few millimeters in diameter with
smooth surfaces are placed in the bottom of a chamber that is
sonically vibrated (usually with a frequency of 50 Hz.about.20
kHz). When the balls are resonated, the sample surface is impacted
by a large number of flying balls over a short period of time. Each
impact of a flying ball (with a velocity of 1.about.20 m/s) induces
plastic deformation at a high strain rate in the surface layer.
Repeated multidirectional impacts result in repeated plastic
deformation in the top surface layer and the creation of bulk
structural defects (dislocations, subgrain and stacking fault
formation, etc.) followed by a process of grain restructuring,
relaxation, and reorientation, which, in turn, induces progressive
grain refinement down to the nanometer regime. What differentiates
this form of SMAT from shot peening is that in the latter case, the
balls are directed onto the surface using a nozzle. As such, the
kinetic energy and directionality of each ball is roughly the same
as another ejected from the nozzle, rendering no major variation in
the total quanta of deformation energy being imparted to the
surface.
[0012] Surface mechanical attrition treatment offers some unique
advantages over coating and deposition methods. For one, there is
no change in chemical composition from the surface to the bulk or
matrix. Another advantage is the relative ease in the ability to
create a continuous, gradient variation from the top surface, which
is nanocrystalline, less than 100 nanometers, to the sub-surface,
which is ultra-fine grained, less than one micrometer, to the bulk,
which is coarse-grained, greater than one micrometer. With SMAT,
bonding between the surface and matrix is not an issue.
[0013] The observed surface attrition process presents similar to
that of mechanical alloying/milling during active communication
where the imparted kinetic energy depends on the frequency and the
amplitude of the specific mill used; and, like in mechanical
milling, a similar analogous temperature rise (50-150.degree. C.)
in the substrate is expected to occur for higher energy processes.
This is primarily due to the deformation induced heat generated
within the substrate, ball to ball, ball to wall collisions, and
frictional heating effects during the repeated attrition process.
While generally this temperature rise is considered small, it can
have a noticeable effect on certain metals and alloys (e.g., high
purity, face centered cubic [FCC] nanocrystalline metals which have
been shown to undergo grain growth at room temperature).
[0014] Here we describe the procedures and advantageous effects of
cryogenic SMAT processing at sub-ambient temperatures, preferably
below the ductile-brittle transition temperature of the material
being treated, more preferably, below -50.degree. C., most
preferably, at liquid nitrogen temperature for commercially
available oxygen free high conductivity copper (Cu) (OFHC), pure
iron (Fe), and pure titanium (Ti), each representative of a
different crystallographic system. In the spirit of this invention,
these exemplary systems are used as illustrations for what is
possible with our concept. However, it is noted and emphasized that
the applicability of the processes described herein have greater
utility to a much broader range of material systems, including, but
not limited to, other pure metals and alloys with commercialization
potential. In turn, to demonstrate the unique, non-obvious, and
salient features of this invention, we will differentiate the low
temperature structural evolution of these metals from that
occurring during room temperature processing.
[0015] Note, while SMAT processes have been applied to a number of
systems including, essentially pure metals such as Cu, Ni, Ti, Fe,
and alloys such as stainless steels, currently, there is no or very
limited data on decoupling the thermomechanical effect during the
processing of such materials, especially at low temperatures where
dynamic recovery and recrystallization are dramatically
suppressed.
[0016] This invention utilizes equipment, developed for high energy
cryogenic mechanical alloying/milling, which presents a unique,
non-obvious opportunity when applied to low temperature or
cryogenic SMATing.
BRIEF SUMMARY OF THE INVENTION
[0017] Various unary, binary, and higher order metallic systems,
and methods of making the same, are presented herein according to
embodiments of the invention.
[0018] According to various embodiments, these metallic systems may
include: pure copper (Cu), pure iron (Fe), or pure titanium (Ti),
each representative of a specific crystallographic class, face
centered cubic (FCC), body centered cubic (BCC), and hexagonal
close packed (HCP), respectively. The untreated coarse-grained
metal, upon being subjected to cryogenic SMAT, will possess a
gradient microstructure from the surface to the bulk. That is, the
as-processed metal will consist of a nano- to submicrometer length
scale grain structure at its surface, monotonically reaching, some
distance away below the surface, the initial, untreated grain size
of the bulk. The ability to control the depth below the surface and
the effectiveness of this grain size reduction is one the novel
aspects of this invention.
[0019] According to various embodiments, these metallic systems may
also include: multiple elements, wherein upon SMAT, the resultant
component elements may be in a discrete or disordered or
alternatively ordered, layered, in a stratified arrangement,
consisting of a single to multiple layers, each with its own
distinct features, displaying various levels of intermixing between
the initial constituents. Alternatively, adjustment of the
conditions may generate a cellular structure on a multiscale level,
with varying dimensions of the cell size.
[0020] In the various embodiments of the metallic systems, the
initial metal may have an untreated grain size greater than 20 to
50 .mu.m. The various exemplary metal systems, subjected to room
temperature SMAT, may have grain sizes as fine as 150 nm up to 360
nm. However, the grain size reduction, only after one hour
treatment, across the three exemplary metallic systems, is
considerably more effective at cryogenic temperatures: 60 nm up to
140 nm, or 45% to 62%, respectively.
[0021] For the purposes of this invention, various ASTM measurement
standards or their accepted equivalents were used. For example, for
the determination of microhardness, ASTM E384-11e1, Standard Test
Method for Knoop and Vickers Hardness of Materials, ASTM
International, West Conshohocken, Pa., 2011 was used. Likewise, for
the determination of grain size, the linear intercept method, quite
common in microstructural analysis practices was used as well as
the ASTM E112-13, Standard Test Methods for Determining Average
Grain Size, ASTM International, West Conshohocken, Pa., 2013 was
used as reference.
[0022] Furthermore, in these various embodiments, the depth of
nanostructuring, as measured from the surface, can vary from 250 to
1000 .mu.m. With the use of cryogenic temperatures, the depth
changes, by as much as a factor of two, as determined by the
crystallographic class of and the ease of deformation within the
material. As such, for the BCC and HCP cases, the depth of the
affected region is significantly narrower, whereas, for the FCC
case, it is wider. While it is preferred that a low temperature is
used, more preferably the lowest attainable with the cooling
system, various embodiments may necessitate altering and increasing
the processing temperature to prevent the processed material from
prematurely fracturing or failing, due to it being too brittle or
below its ductile-brittle transition temperature.
[0023] These embodiments thus provide a new class of nanostructured
and nanocrystalline metals and alloys which have significantly
improved properties compared to their coarse grained counterparts.
Moreover, the exemplary copper (Cu) metallic system subjected to
room temperature SMAT may have a Vickers microhardness (HV) of
about 110 kg/mm.sup.2 at its surface, while an equivalent sample,
processed at a cryogenic temperature, may have an HV of about 155
kg/mm.sup.2; a significant increase. Similarly, the exemplary iron
(Fe) metallic system subjected to room temperature SMAT may have an
HV of about 220 to 250 kg/mm.sup.2; for iron (Fe) the resultant
hardness difference between room and cryogenic temperature
processed samples is not notable. Lastly, the exemplary titanium
(Ti) metallic system, processed with room temperature SMAT may have
an HV of about 280 kg/mm.sup.2; whereas, the cryogenically treated
sample may have an HV of about 320 kg/mm.sup.2. Again, in the
spirit of this invention, these aforementioned microhardness values
are illustrations of typical properties for a specific processing
condition, one hour of SMAT. It is conceivable that longer SMAT
times will lead to further hardness increases, especially at the
surface. It is also believed that shorter SMAT times, i.e. impact
times, may be sufficient to achieve the desirable improvement in
final properties. Thus, in certain embodiments, the processing
time, i.e. the total amount of time that the metal parts or parts
are impacted with metal fragments (t.sub.i) at reduced temperature
(T.sub.r) is at least about 5 minutes, more preferably at least
about 10 minutes, still more preferably at least about 20 minutes,
still even more preferably at least about 40 minutes and in some
instances at least about 1 hour or 60 minutes.
[0024] According to further embodiments, the SMAT process for
forming the exemplary nanostructured Cu, Fe, or Ti metallic
systems, comprised of a gradient microstructure from nano- to
micro- to macroscales, imparts these metals with a preferred grain
orientation or texture. For each system, the texture evolves, as
determined by the ease of deformation and grain reorientation when
subjected to high strain and high stress conditions.
[0025] During SMAT, using a high-energy milling device may utilize
a mixing vial to contain the metallic plate in addition to or
replacing the vial's lid and, in some circumstances, may utilize a
plurality of milling balls for inclusion within the mixing vial. In
other embodiments, especially for those when surface alloying is
desired, in addition to the plurality of milling balls, the vial
may contain the powder(s) to be alloyed therein. In certain
circumstances, the vial and steel milling media may be precoated
with a specific metal to preferentially augment or reduce the
presence of a specific element or species. The ball-to-powder mass
ratio utilized by the high-energy milling device may be 1:1, more
specifically, 10:1, or more. Furthermore, the milling balls may be
comprised of high strength steel or ceramic. During the high-energy
milling process, the vial and its contents may be cooled to
sub-ambient temperatures, more preferably, a cryogenic temperature.
This may be accomplished by cooling the milling device with any
type of cryogenic liquid, more specifically, liquid nitrogen.
Alternatively, the high-energy milling process may be performed at
ambient or room temperature. The high-energy milling process may be
further improved using an additive or a surfactant. In some
instances, the vial and its contents may be continuously or
semi-continuously cooled during the high-energy milling process. In
further embodiments, at the conclusion of the milling process, the
metallic powder may be subjected to annealing by exposing it to
elevated temperature in the range of about 300 to 800.degree.
C.
[0026] These embodiments thus provide a methodology for forming a
new class of nanostructured and nanocrystalline metals or
composites which possess enhanced mechanical properties with a
gradient grain size microstructure.
[0027] These and other, further embodiments of the invention are
described in more detail, below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] So that the manner in which the above recited features of
the present invention can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments, including less effective but also less expensive
embodiments which for some applications may be preferred when funds
are limited. These embodiments are intended to be included within
the following description and protected by the accompanying
claims.
[0029] FIG. 1 shows scanning electron micrographs of the gradient
nature of the subsurface grain size reduction in pure copper (Cu),
comparing samples processed at ambient and cryogenic temperatures,
respectively.
[0030] FIG. 2 shows the x-ray diffraction patterns of the
as-received, ambient temperature processed, and cryogenically
processed copper (Cu) samples, respectively.
[0031] FIG. 3 shows scanning electron micrographs of the gradient
nature of the subsurface grain size reduction in pure iron (Fe),
comparing samples processed at ambient and cryogenic temperatures,
respectively.
[0032] FIG. 4 shows the x-ray diffraction patterns of the
as-received, ambient temperature processed, and cryogenically
processed iron (Fe) samples, respectively.
[0033] FIG. 5 shows scanning electron micrographs of the gradient
nature of the subsurface grain size reduction in pure titanium
(Ti), comparing samples processed at ambient and cryogenic
temperatures, respectively.
[0034] FIG. 6 shows a high magnification scanning electron
micrographs for copper (Cu), iron (Fe), and titanium (Ti) metal
types revealing the dramatic decrease in grain size with the use of
cryogenic processing conditions versus those at ambient
conditions.
[0035] FIG. 7 shows the x-ray diffraction patterns of the
as-received, ambient temperature processed, and cryogenically
processed titanium (Ti) samples, respectively.
[0036] FIG. 8 shows the Vickers microhardness values of the copper
(Cu), iron (Fe), and titanium (Ti) samples, as a function of depth
away from the surface for both the ambient and cryogenic
temperature processed samples.
DETAILED DESCRIPTION OF INVENTION
[0037] Unlike that in the prior art, we utilize for the first time
a novel low temperature or cryogenic SMAT process on commercially
available, oxygen-free high-conductivity (OFHC) copper (Cu), pure
iron (Fe), and pure titanium (Ti) and show the advantages in terms
of microstructure refinement as compared to conventional ambient or
room temperature (RT) SMAT processing. While SMAT processes in a
conventional configuration have been applied to a number of
metallic systems, including Cu, Ni, Ti, Fe, and stainless steels,
there currently is very limited data demonstrating decoupling of
thermomechanical effects during the processing, especially at
cryogenic temperatures (e.g., below -150.degree. C.) where dynamic
recovery and recrystallization are dramatically suppressed. By
utilizing equipment developed for high-energy cryogenic mechanical
alloying or milling, which is not typically used, it is possible to
show the substantial microstructure refinement of the cryogenic
SMAT process in OFHC Cu, pure Fe, or pure Ti over what is
attainable at RT.
[0038] Thus, in certain preferred embodiments the present invention
provides a method of modifying the surface of a metal part that
includes: providing at least one metal part that is formed from a
first metal composition; providing a plurality of metal fragments
that are formed from a second metal composition wherein said metal
fragments have a size that is significantly smaller than the size
of the at least one metal part; reducing the temperature of the at
least one metal part and the plurality of metal fragments;
impacting the at least one metal part with the plurality of
fragments at a reduced temperature (T.sub.r), for an impact
processing time (t.sub.i); wherein, the at least one metal part is
subjected to bombardment by the plurality of metal fragments at a
reduced temperature (T.sub.r), resulting in the as-received grain
size of the at least one metal part to be reduced, by several
orders of magnitude, and possessing a gradient structure from the
impact surface of the at least one metal part into the interior of
the bulk of the at least one metal part.
[0039] In the exemplary embodiments provided below, the metal
fragments were steel balls or milling balls. However, ceramic
milling balls may be used or some other material exhibiting
properties similar in nature. Suggested steel milling balls will
have a diameter of at least about 0.015 mm and more preferably at
least about 1.5 mm and may have a diameter as large as about 2.5
cm. In certain embodiments, the milling balls are formed from
tungsten. Thus, the mass of an individual milling ball may range
from about 1.01 gram to about 160 grams. Desirably, the properties
of the milling balls, including but not limited to hardness, should
be similar to or greater than the properties of the material being
milled or SMATed. However, in certain embodiments the milling balls
may be abraded during the process to produce an alloy or composite
structure at the surface of the part or material that is being
milled or SMATed. In certain other embodiments a powder of is
included in the milling container to produce an alloyed surface or
to introduce a second phase at the SMATed surface.
[0040] In the exemplary embodiments provided below, the at least
one metal part included a plurality of metal plates. However, in
commercial applications it is suggested that the at least one metal
part is a finished part or an almost finished part, for example a
head of a golf club, a gear or gear tooth or a part of a gun or an
engine component prior to assembly or packaging. The composition of
the metal part(s) may be same or different from the composition of
the metal fragments. In the exemplary embodiments provided below,
the composition of the metal parts, e.g. metal plates, and the
composition of the fragments were different. However, in some
applications it is may be desirable that the composition of the
metal part(s) may be same as the composition of the metal fragments
or substantially the same as the composition of the metal
fragments. For example, both the part(s) and the impact fragments
may be formed from high-conductivity (OFHC) copper. In certain
embodiments the surface at least one part that is being SMATed at
reduced temperature is cut away and then used as a sub part or
component. Additionally, the SMATed surface may be further
processed. For example, the high density of grain boundaries in a
nanocrystallline microstructure also increases the available
diffusion pathways at the surface for secondary processing such as
carburization or nitriding, enabling decreases in process
temperature of possibly improved species absorption.
[0041] Samples were prepared by cutting commercially available OFHC
Cu, pure Fe, or pure Ti into disks 6.35 cm in diameter and 0.6 cm
thick. These samples were then polished to a mirror finish.
Nominally supplied high energy SPEX.RTM. SamplePrep Corporation
mill vial lids were then replaced by the samples and sealed in a
high-purity argon glovebox. In each case, the hardened steel vials
were loaded with 17 5/16'' diameter and 161/4'' diameter 440 C
stainless steel ball bearings to constitute a total mass of 50 g.
For the Cu and Ti samples, the vial was precoated with a thin layer
of Cu or Ti, respectively. This was to limit potential
contamination of the as SMATed surface from the Fe constituent of
the milling vial. The precoating was accomplished by placing
approximately 0.5 g of each respective powder into the vial and
operating the SPEX mill for 10 min at room temperature. The actual
SMAT samples were either processed at cryogenic or RT for a period
of 1 hour. For the purposes of the invention, low or cryogenic
temperatures are defined as temperatures well below ambient room
temperature conditions, preferably below about -50.degree. C.
(223K), not greater than about -100.degree. C. (173K), more
preferably not greater than about -150.degree. C. (123K), and most
preferably not greater than about -196.degree. C. (77K). RT milling
was accomplished by loading the vials into a commercially available
single vial SPEX SamplePrep Model 8000M mill, while cryogenic
milling was performed in a modified SPEX SamplePrep Model 8000M
mill. The modified mill was equipped with a Teflon sleeve into
which the sealed steel vials could be inserted. The Teflon sleeve
was fitted to allow the inflow and outflow of liquid nitrogen at a
temperature of -196.degree. C. (77K) to envelope the outside of the
steel vial. After the cryogenic and RT SMAT processing, the samples
were mounted and polished for microstructure analysis.
[0042] The microstructure was analyzed using optical microscopy,
X-ray diffraction for grain size and strain analysis, and scanning
electron microscopy (SEM) for chemical, grain size and orientation
analysis. Electron backscatter imaging in the SEM was used to
analyze the grain size. Optical imaging of etched samples was used
to assess the specific grain size and defect morphology. A
dual-beam FEI Nano600 FIB was used to prepare samples for electron
backscatter diffraction (EBSD) imaging and produce ion-channeling
contrast images to highlight the active deformation mechanisms
during cryogenic SMAT processing.
[0043] FIGS. 1a and 1b are SEM images of the etched sample
cross-sections of two copper (Cu) samples prepared by RT and
cryogenic SMAT processing for 1 hour, respectively. For the
purposes of the invention, the duration of processing is defined as
times being at least 5 minutes, preferably at least 10 minutes,
more preferably at least 30 minutes, and most preferably at least
60 minutes. Note, however, the duration of processing is a strong
function of the ductile-brittle temperature or malleability of the
metal being treated, its crystallographic class, and the
temperature dependence of its malleability. As such, for the
purposes of the invention, there is no upper bound in processing
times, except that determined by the failure of the metal plate due
to void formation, fracture, spallation, and subsequent
fragmentation. As such, under certain conditions, processing times
as long as two to three hours may be feasible. The exemplary and
representative micrographs illustrate the change in grain structure
as a function of depth from the SMAT surface, which is analogous to
how the microstructure evolves as a function of time for the SMAT
process. The SMAT surface had up to 20 .mu.m non-continuous layer
with some Fe contamination from the use of stainless steel ball
bearings; this is present in both RT and cryogenic samples.
[0044] There is a noticeable transition from nanostructured or
ultrafine-grained grain structure to a region of banded structures
to the bulk microstructure within the first 500 .mu.m. The back
surface of the SMAT specimens (images to the left, denoted by a
square) is 0.6 cm from the SMAT surface. The microstructure near
the back surface represents a pristine microstructure unaffected by
the SMAT process; the average equiaxed grain size in this region is
150 .mu.m in diameter with some twinning present. The average grain
size for the cryogenic sample (140 nm mean diameter) is
approximately 60% smaller than the grain size achieved through RT
processing (355 nm mean diameter).
[0045] The cryogenic SMAT sample maintains a constant finer grain
size to a much deeper penetration depth. That is, in contrast to
the cryogenic sample, the RT processed sample appears to have a
continuous monotonic increase in grain size as a function of depth
into the plate. Furthermore, the overall region of grain
refinement, defined here as the volume containing grains smaller
than 10 .mu.m in diameter, is found to vary with processing
conditions. This region is approximately 150 .mu.m deep for the
cryogenically processed sample and 300 .mu.m deep for the RT
processed sample, respectively.
[0046] As shown in FIG. 2, X-ray diffraction patterns were
collected from the as-received OFHC copper (Cu) plate, RT, and
cryogenic SMAT samples. Upon inspection for the fundamental Bragg
crystallographic reflections, the full widths at half-maximum
increased and the amplitude decreased in the following order:
as-received, 1 hour RT condition and 1 hour cryogenic condition.
These trends are consistent with a decrease in grain size and/or an
increase in local strain with processing conditions. The Scherrer
estimates of grain size indicate that the average grain size for
the cryogenic SMAT samples is lower than that calculated for the RT
SMAT samples. However, the grain size estimates using the Scherrer
formula were significantly lower than the previously measured grain
size using SEM micrographs.
[0047] The texture change during SMAT can also be measured by the
change in the relative intensity of the fundamental
crystallographic reflections of Cu. For the cryogenic SMAT sample,
the relative intensities of the fundamental reflections of pure Cu,
as given by the JCPDS index, are: (111) 100%, (200) 46%, (220) 20%,
(311) 17% and (222) 5%. For the unprocessed plate, the second and
third reflections have higher percent relative intensities: (200)
58% and (220) 63%. This is in contrast to the texture generated
during the SMAT process. Hence, in general, SMAT induces a texture
wherein the (111) orientation is favored relative to the other
reflections. Furthermore, cryogenic SMAT processing enhances this
type of texturing over RT SMAT processing. For the cryogenically
processed sample, the ratio of intensities of the (200) and (111)
reflections increases to a slightly higher value than that with RT
processing.
[0048] FIGS. 3a and b show the equivalent SEM images of the etched
iron (Fe) sample cross-sections processed for 1 hour, respectively.
In these images the contrast in grain morphology between the RT and
cryogenic processed samples is rather striking. For iron (Fe)
processed at RT, the deformed and reduced grains show a significant
texturing effect, wherein the grains appear in layers and are
stratified perpendicular to the plate normal. Whereas, the iron
(Fe) processed at a cryogenic temperature, the as-deformed region
is less stratified and more equiaxed. Comparing the affected depths
between conditions, the RT processed specimen is significantly
deeper than that processed at cryogenic temperature. For the
former, the highly refined grain size region is about 200 .mu.m
deep; for the latter, the corresponding region is only about 50
.mu.m. An intermediate region between the unaltered bulk is also
wider for the sample processed at RT.
[0049] Unlike the FCC copper (Cu) samples which showed a dramatic
increase in the (111) crystallographic orientation relative to all
of the others, the BCC iron (Fe) samples show a considerably lesser
effect. As shown in FIG. 4, are the X-ray diffraction patterns of
the as-received iron (Fe) plate, RT, and cryogenic SMAT samples.
Inspection of the fundamental Bragg reflections show that while the
full widths at half-maximum did significantly change, the
amplitudes did not decrease between the conditions presented. This
effect is directly related to peak broadening due to grain size
reduction, however, without, a significant change in
crystallographic orientations. That is, aside from a reduction in
grain size and pancaking of the grains, SMAT, in either RT or
cryogenic conditions, does not introduce grain reorientation.
[0050] As was seen for both copper (Cu) and iron (Fe), there is a
corresponding decrease in the size of region of nano to
ultrafine-scale grains for HCP titanium (Ti) as well. The grains
appear to be mostly equiaxed; absent is the layering or
stratification of the grains. In FIGS. 5a and 5b, SEM images of the
etched sample cross-sections of two titanium (Ti) samples prepared
by RT and cryogenic SMAT processing for 1 hour, respectively, show
that submicrometer grains persist to a depth of about 75 .mu.m in
the RT processed sample, whereas, this depth is only about 40 .mu.m
for the cryogenic sample. Again, the transition region from the
nano-scale to macroscale grains is deeper for the cryogenic
condition. However, overall, the depths of the affected regions are
about the same; 450 .mu.m. This effect is similar to the case of
copper (Cu), but different from that of iron (Fe).
[0051] X-ray diffraction patterns of the as-received titanium (Ti)
plate, RT, and cryogenic SMAT samples are shown in FIG. 6. The full
widths at half-maximum of the Bragg peaks illustrate significant
line broadening for both SMAT conditions. In fact, the two primary
reflections of (002) and (101), partially and completely overlap
for the RT and cryogenic SMAT conditions, respectively. Note, with
the exception of the growth of the (101) peak, the there is little
change in the relative peak heights of the other peaks between the
processing conditions. Given the fact that HCP titanium (Ti) has a
limited number of operative deformation slip systems it is most
likely, that recrystallization is the primary grain refinement
mechanism in this material. This is consistent with the observed
grain morphology and lack of texturing as indicated by the changes
in relative peak heights.
[0052] For the purposes of the invention and the preferred
embodiments described herein, it is important to realize that,
whereas, the one hour processing time may have been closer to
optimum conditions for FCC copper (Cu), however, this processing
time may not have been the case for latter, namely the BCC iron
(Fe) and HCP titanium (Ti) embodiments. That is, it is implied from
a comparison of results that the resultant texturing in the latter
systems have not yet fully evolved. In other words, different
crystallographic systems will develop differently due to their
intrinsic properties and underlying deformation mechanisms. Thus,
it is likely that longer processing times would have resulted in
the evolution of a stronger texture in the other exemplary
metals.
[0053] Another factor in these embodiments is the evolution of
steady state equilibrium conditions during SMAT processing,
wherein, the heat generation due to deformation is offset by active
cooling. That is, in all likelihood, the processing temperature,
while fixed at -196.degree. C. (77K), primarily, for convenience,
delivery, and availability of liquid nitrogen will have a
significant effect on the effectiveness of the grain size
refinement process. Thus, for the purposes of this invention, it is
hypothesized that it is highly likely that at lower or higher
temperatures, corresponding to alternate equilibrium conditions,
would result in a different more favorable outcome, i.e.,
potentially finer grain size reduction for the latter metal
systems.
[0054] FIG. 7 reveals higher magnification SEM images of regions
located at greater depths below the SMAT surface. There are major
microstructure differences with respect to the processing
conditions. First, the cryogenically processed samples maintain a
near-equiaxed grain morphology, whereas the RT processed samples
have larger regions where the grain morphology is distorted from
its initial equiaxed shape. Second, a high density of etch pits,
mostly likely associated with dislocations intersecting the
polished surface, are present in the RT sample. When metals undergo
severe plastic deformation at or near RT, dislocation slip and
deformation twinning are the principal modes of deformation. While
both FCC and BCC metals have adequate numbers of operational slip
systems, there is a considerable limitation of such slip systems in
HCP metals. Regardless, dislocation tangles and their specific
subgrain structures (dislocation cells, walls, geometrically
necessary boundaries and incidental dislocation boundaries) are
generated and equilibrated by thermally activated processes. As
such, these are the means for achieving grain refinement during
SMAT processing.
[0055] The cryogenic SMAT samples, however, show very different
microstructure evolution to that of the RT SMAT samples. At lower
temperatures, the dislocation-based processes are suppressed, as
indicated by the lack of etch pits in the cryogenically processed
samples. Moreover, in contrast to the RT SMAT samples, a large
number of banded structures were observed within the large grains
of the cryogenic SMAT samples. In many cases, these banded
structures initiate from the grain boundaries and either terminate
at the inside wall of the same grain or intersect the opposite side
boundary. The spacing between many of the bands is less than 10
.mu.m. In general, these bands contain small ultrafine equiaxed
grain structures or structures that are elongated and parallel to
each other across the width of the band. These observations are
consistent with observations of twin/matrix bundles, bamboo
nanograins, and shear bands that evolve as a function of strain
during dynamic plastic deformation under liquid nitrogen
temperatures. These observations indicate a shift in the dominant
deformation mechanism during SMAT from dislocation-mediated
behavior at room temperature to twinning/shear band-mediated at
cryogenic temperatures.
[0056] A time evolution of the microstructure can be constructed
for the cryogenic SMAT process. First, due to the high-strain-rate
impacts at cryogenic temperatures, twin/matrix bundles and/or shear
bands originate at the surface or at internal stress concentration
sites (grain boundaries, triple junctions, etc.). The repeated
impacts generate overlapping twin/matrix bundles and/or shear
bands, which effectively refine the large micrometer-sized grains.
Additional grain refinement may occur within any shear band present
due to dynamic recrystallization. As the internal stress continues
to accumulate, the microstructure evolves by producing ultrafine
grains with long aspect ratios. The accumulated shear strain leads
to fragmentation and rotation of these elongated grains, resulting
in an equiaxed nanocrystalline microstructure.
[0057] FIG. 8 displays the results of microhardness measurements
taken from the three metal specimen types both after being
subjected to RT or cryogenic SMAT. The three embodiment exhibit a
monotonic fall off in microhardness that strongly depends on the
local grain size at the relative depth measured from the surface.
Whereas, the RT induced hardness change is gradual from the surface
to the interior, for samples processed at cryogenic temperatures,
this hardness change is more rapid, presumably due to the shallower
nature of the structurally modified region. The most significant
change occurs for pure copper (Cu), then pure titanium (Ti), and
lastly pure iron (Fe). The figure clearly illustrates the
variations between the three embodiments. The advantage for the use
of cryogenic conditions is most beneficial in the case of the
softer copper (Cu), however, if a sharper hardness profile and
grain size restructuring is desired, this could be achieved by
altering the SMAT conditions, with lengthening the duration and
potentially lowering the temperature further. The embodiment using
titanium (Ti) illustrates that quite well. Note, in the case of
pure iron (Fe), there is little to no benefit to the use of
cryogenic processing conditions, below the surface. However, this
does not preclude the use and the resultant benefits that derive
from lower temperatures or longer SMAT times.
[0058] Supporting and/or additional details may be found in a
journal article titled "Enhancing grain refinement in
polycrystalline materials using surface mechanical attrition
treatment at cryogenic temperatures" Scripta Materialia 69 (2013)
461-464 by Dr. Kristopher A. Darling et al. which is hereby
incorporated by reference herein.
[0059] The foregoing description, for the purposes of providing an
explanation, has been described with reference to specific
embodiments. However, the illustrative discussions above are not
intended to be exhaustive or to limit the invention to the precise
forms and conditions disclosed. Many modifications and variations
are possible in view of the above teachings. The embodiments were
chosen and described in order to best explain the operating
principles of the present disclosure and its practical applications
to thereby enable others skilled in the art to best utilize the
invention and its various embodiments with various modifications as
may be suited to the particular use contemplated.
[0060] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow.
* * * * *