The invention produces superplastic deformation in a workpiece by altering the chemical composition of the workpiece material, while the workpiece is subjected to a biasing stress, in a manner that introduces a strain increment into the material that effects a change in a overall dimension of the workpiece without causing failure. In one approach, repeated cyclic alteration of chemical composition, so as to repeatedly alternately induce and reverse a phase transition that produces strain increment, allows accumulation of strain in an incremental fashion thereby achieving large overall superplastic deformations in the workpiece without applying large stresses.

FIELD OF THE INVENTION 
This invention relates to superplastic deformation. More particularly, this 
invention relates to a technique for inducing superplastic deformation by 
chemical means. 
BACKGROUND OF THE INVENTION 
Superplastic deformation is defined as the deformation of a workpiece to a 
very large strain by application of a small stress without disrupting the 
mechanical integrity of the workpiece. Although superplastic deformation 
is universally characterizable by the formula 
##EQU1## 
(in which .epsilon. is strain rate, A is a materials constant, .sigma.is 
stress, R is the gas constant, T is temperature and n is a stress exponent 
between one and two), this behavior can be produced by any of several 
different mechanisms. This phenomenon has been exploited in superplastic 
forming techniques. For example, titanium-based materials are desirable 
for their specific strength and stiffness at ambient and elevated 
temperatures but have high resistance to deformation at temperatures 
appropriate for traditional hot-working operations. However, titanium 
alloys having a fine, stable grain structure deforms superplastically, a 
phenomenon known as "fine-grain superplasticity". Titanium-forming 
techniques based on fine-grain superplasticity only operate successfully 
within a restricted window of process parameter values. For example, only 
small strain rates can be imposed, so the process output rate is limited. 
The deformation mechanism requires that grain size be maintained within 
certain limits throughout the deformation process. 
In another superplastic mechanism, called "transformation superplasticity" 
(described, e.g., in U.S. Pat. No. 5,413,649, the entire disclosure of 
which is incorporated herein by reference), the workpiece is cycled 
through a phase transformation by changing the temperature. The technique 
is advantageous compared to earlier approaches in that it is not limited 
to a workpiece material with a fine-grain microstructure and the grain 
growth limitation is relaxed. Also, the higher strain rates achievable 
result in more efficient process output. However, prolonged residence at 
high temperatures as required for some thermal cycling procedures can 
promote grain growth to sizes deleterious to the mechanical properties of 
the finished product. Implementing the required temperature cycling 
capability can be costly and difficult. Also, repeated thermal cycling can 
promote fatigue of the treatment apparatus. 
DESCRIPTION OF THE INVENTION 
OBJECTS OF THE INVENTION 
An object of the invention is, accordingly, to provide a technique for 
inducing superplasticity that is applicable to a wide range of workpiece 
materials, including titanium-based materials. 
Another object of the invention is to provide a technique for inducing 
superplasticity that is not limited to any specific workpiece 
microstructure or composition. 
Another object of the invention is to provide a technique for forming 
composites. 
Another object of the invention is to provide a method of inducing 
transformation superplasticity without thermal cycling. 
Another object of the invention is to provide a method of inducing 
superplasticity that allows fast deformation of the workpiece. 
Still another object of the invention is to provide a method of inducing 
superplasticity that may be applied repeatedly to a workpiece with 
accumulation of deformation from each repetition. 
BRIEF SUMMARY OF THE INVENTION 
The method of the invention produces superplastic deformation in a 
workpiece by altering the chemical composition of the workpiece material, 
while the workpiece is subjected to a biasing stress, in a manner that 
introduces a strain increment into the material and thereby effects a 
change in a overall dimension of the workpiece, without causing failure. 
Depending on the material, the strain increment can be greater than 0.5% 
or 1%, even as much as 1.5% and greater. Known apparatus for fine-grain 
superplastic forming can be modified in a straightforward manner to 
incorporate the method of the invention by adding a mechanism for 
introducing and/or withdrawing a chemical component to effect the desired 
chemical composition change. The present invention may also be used for 
compacting a workpiece initially comprising several distinct bodies (e.g., 
powder, wires, foils) to form a dense article or for foaming a workpiece 
by the expansion of internal cavities. 
The alteration in composition may be monotonic, either resulting in a 
permanent change in the concentration of the component or reversed after 
completion of the superplastic deformation process. Or the alteration may 
be cyclic, comprising an initial increase or decrease in the concentration 
of the chemical component, followed by a partial or total reversal of the 
initial change while the workpiece remains subject to the biasing stress. 
In one approach, the composition changes within a single-phase stability 
field, a concomitant change in lattice strain producing the strain 
increment without phase transformation. In another approach, the 
alteration in composition induces a phase transition that gives rise to 
the strain increment. 
Such a change in composition in the material may affect all of the 
workpiece material or only a part of it. The overall deformation is 
usually proportional to the fraction of the workpiece involved in the 
alteration. As used in this document, the term "segment" refers to the 
portion of the workpiece material undergoing a composition change and/or a 
phase transformation, whether it corresponds to the entire workpiece or 
not. The segment may, for example, form a continuous layer surrounding an 
unaltered core or be a collection of distinct isolated regions, each 
surrounded by unaltered material. In the case of phase transition, each 
forward or reverse transformation changes the transformed segment with 
respect to some aspect--its specific volume or, in some instances, some 
geometric aspect such as lattice type, lattice orientation or shape--so 
that the transformation generates an internal transformation stress in the 
material. In the case of chemical composition cycling it is usually 
desirable that the segment transformed by the reverse transformation 
correspond to that transformed by the forward transformation, so that the 
original phase constitution of the material is completely restored. 
However, the invention does not require such a correspondence; some of the 
material may remain in the forward-transformed state at the end of a 
cycle. 
The transformations may occur along a macroscopic transformation front 
between an original phase in the material and a new phase in the material, 
originating in the reaction where the chemical composition change is 
introduced and advancing into the material in an organized fashion; or 
they may arise simultaneously at several discrete sites, having phase 
boundaries that move in random directions during transformation. 
The scope of the invention is not limited with respect to type of phase 
transition or workpiece material. The phase transition may involve 
precipitation of a compound due to solute saturation or be, for example, 
allotropic, martensitic, peritectoid or eutectoid in nature. The method of 
the invention is compatible with, but not limited to, metallic ionic and 
covalent materials including pure metals and alloys, such as 
intermetallics, ceramic, polymeric or geologic workpiece materials. 
The biasing stress influences the orientation of the strain increment to 
produce the desired superplastic deformation. The biasing stress may 
originate in a source either internal to or external to the sample; or, 
both internal and external sources may contribute to the bias. Residual 
internal stress in the workpiece may provide the biasing stress or, the 
transformation stress of the phase transition may itself give rise to the 
bias. In a preferred embodiment, the bias is provided by an externally 
applied stress, the magnitude of which is chosen according to the strength 
of the material. Depending on the deformation desired, the externally 
applied biasing stress may be hydrostatic or nonhydrostatic, such as a 
uniaxial or multiaxial stress. Such stresses may include tensile, 
compressive, noncompressive, torsional or bending stresses as are 
conventionally used to effect, for example, drawing, punching, stamping, 
extruding, rolling, pulling, bending, and twisting. 
In a preferred embodiment, chemical composition cycling is applied to the 
workpiece repeatedly, each repetition introducing a strain increment. 
Repetitive cycling in a manner that causes the alternate induction or 
reversal of a phase transition to repeat is especially beneficial. The 
strain increment per cycle may be as much as 1.5%, or greater. The 
accumulation of strain in this incremental fashion allows achievement of 
large overall superplastic deformations in the workpiece without applying 
large stresses, which would risk disruption of the mechanical integrity of 
the workpiece. The invention does not require that a chemical composition 
change applied or segment affected in any given cycle correspond exactly 
to that transformed in any other cycle of a repetitive series. 
Although the invention is applicable to a wide range of workpiece 
materials, alterable by a commensurately broad range of compositional 
changes, superplastic deformation is most efficiently accomplished if the 
compositional change is imposed by varying the concentration of a chemical 
component that has a high diffusivity in the workpiece material before and 
after the ensuing phase transformation. As a practical matter, it is 
desirable that the component be easily transportable to and removable from 
the surface of the workpiece. It is thus preferable that the chemical 
component be transported in the gas phase or produced by reaction at the 
workpiece of a species delivered in the gas phase. Such a component can 
then be removed by exposing the workpiece to vacuum or to another gas with 
zero or reduced pressure of the component, or by providing a getter to 
absorb the gaseous species. It is further preferable that small changes in 
the concentration of the chemical component produce a significant strain 
increment. 
Using the method of the invention, it is possible to obtain the 
superplastic effects of phase transformations previously exploited by 
thermal cycling--such as allotropic phase transformation between a 
lower-temperature phase and a higher-temperature phase--without deliberate 
imposition of heating or cooling operations. This approach simplifies the 
control equipment required to operate the treatment apparatus and 
decreases its energy consumption. Related benefits are reduced risk of 
thermal fatigue of the treatment apparatus and reduced risk of undesirable 
grain growth in the workpiece material. 
Introduction of an alloying element that shifts the composition of the 
original material sufficiently so that at least some of the original 
material converts to a different allotropic form is one way to induce a 
phase transition in accordance with the invention. For example, hydrogen, 
vanadium and niobium are known to be beta-phase stabilizers for titanium. 
Adding such a stabilizer to titanium produces an alloy having a lower 
transus temperature between the lower-temperature alpha phase and the 
higher-temperature beta phase than the transus for pure titanium (about 
882.degree. C.). Consequently, adding a sufficient amount of beta 
stabilizer to alpha-phase titanium causes at least some of the alpha-phase 
material to transform to the beta phase, with an attendant change in 
specific volume of the transformed material. Removing the beta-stabilizer 
from the material reverses the transformation. In a preferred embodiment, 
superplasticity is induced in a titanium-based workpiece material by 
changing the concentration of hydrogen therein. 
The invention is not limited to transformations accessible by the thermal 
pathways of the prior art but also enables superplastic behavior to be 
induced by other transformations, not accessible through temperature 
change alone. The method of the invention is generally applicable to 
materials susceptible upon change in chemical composition to a phase 
transformation that generates the strain increment. Some such cycles may 
be executable isothermally. However, the method of the invention also 
encompasses process pathways that include temperature change in addition 
to the chemical cycling, whether the temperature change occurs 
simultaneously with or sequentially to the chemical change. The thermal 
variation may be actively imposed on the workpiece or originate within the 
workpiece due to the imposed change in chemical concentration. 
In a preferred embodiment, hydrogen concentration is changed in a 
titanium-based material to cause alternate precipitation and dissolution 
of a second, titanium hydride phase. Titanium hydride precipitates when 
hydrogen is added to titanium in excess of the hydrogen solid solubility 
limit. The relatively high specific volume of the hydride phase translates 
into a molar volume mismatch on the order of 17% with respect to the 
original titanium. The volume mismatch generates sufficient internal 
stress to produce very large superplastic deformation. In one embodiment, 
the original workpiece material comprises a single phase. In an 
alternative embodiment, the workpiece material is a multiphase composite 
including a matrix of one or more phases and one or more additional 
phases. In a preferred embodiment, the change in chemical composition of 
the workpiece material alternately induces and reverses a phase transition 
in one or more transformable phases, which may be an additional phase or 
part of the matrix. The composite may also include one or more phases not 
subject to phase transformation upon the change in chemical composition. 
The phase distribution is selected so as to allow forward and reverse 
phase transformation without interfacial decohesion. Bonding between the 
composite's phases may contribute to the internal stress caused by the 
phase transition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The use of hydrogen concentration cycling to induce superplasticity in a 
titanium-based material is demonstrated with reference to FIG. 1. In pure, 
hydrogen-free titanium the alpha and beta phases exist over mutually 
distinct temperature ranges separated by the transus temperature of 
882.degree. C. At nonzero concentrations of hydrogen in titanium, the 
stability field 10 of the higher-temperature beta phase extends to 
temperatures lower than 882.degree. C. and overlaps the temperature range 
of the stability field 20 of the lower-temperature alpha phase. Increasing 
the hydrogen concentration of a volume of alpha-phase material initially 
in a state 24--either along direction A or isothermally along direction 
B--into the two-phase field 30 causes conversion of a segment of the 
alpha-phase volume to the beta phase. The extent of the segment increases 
with the overall hydrogen content of the volume until its composition lies 
in the beta phase field 10. 
A nominally pure titanium sample was superplastically deformed by 
chemically induced alpha-beta transformation. The sample was brought to 
808.degree. C. by radiative heating. The sample was held in an argon 
environment and, within the alpha stability field 20 well below the 
transus temperature, subjected to a uniaxial tensile stress of 2.5 MPa. 
Hydrogen was then provided to the heated sample in tension by adding 4% 
hydrogen gas in the argon stream. This gas-phase hydrogen concentration 
was maintained for 600 seconds, after which hydrogen was withdrawn from 
the sample by restoring the pure argon stream for 600 seconds. FIG. 2 
shows the strain increment present after this 1200 second cycle due to the 
difference in specific volume between the alpha and beta phases. A total 
of nine hydrogen concentration cycles were applied to the sample in 
tension which was maintained at a constant temperature throughout the 
cycling. As illustrated in FIG. 2, additional strain accumulates with each 
cycle. The total strain was over 12%, corresponding to about 1.4% per 
cycle, which is much greater than deformations seen in identical samples 
maintained under the same conditions in an argon or argon-hydrogen 
atmosphere without chemical cycling. 
Many variations of this process are within the scope of the invention. For 
titanium, a tensile stress up to about 10 MPa or even higher may be used, 
the strain introduced per cycle increasing with applied stress. Additional 
steps may be included. For example, when the desired deformation has been 
achieved, residual hydrogen may be removed by vacuum annealing if desired. 
Chemically induced superplasticity using hydrogen is also appropriate for 
workpiece materials other than pure titanium. For example, hydrogen 
similarly affects phase relationships in titanium-based materials, for 
example titanium alloys such as Ti6Al4V. Other allotropic metals such as 
zirconium, neodymium, lanthanum, strontium, and uranium and their alloys 
also show phase relationships that allow chemical induction of 
superplasticity by cycling hydrogen concentration. Allotropic and 
nonallotropic metals that form hydrides with mismatch with respect to the 
host metal matrix--such as titanium, zirconium, niobium, tantalum and 
vanadium--are deformable through chemically induced superplasticity by 
addition of hydrogen under hydride-forming conditions. In the case of 
titanium, such a process converts a segment of the workpiece to the delta 
phase, which, with reference again to FIG. 1, has single-phase stability 
field 40. (This approach is easily combined with microstructure refinement 
of titanium, by cyclic hydriding and dehydriding, for improving its 
room-temperature properties.) 
Chemical composition may also be changed reversibly using nitrogen or 
oxygen in materials based on, respectively, nitride or oxide ceramics, or 
based on allotropic metals such as iron, titanium, zirconium, and yttrium. 
Carbon may be delivered to an iron-based workpiece material by a gas such 
as methane and then removed by reaction with a gas such as hydrogen or 
oxygen. 
It will therefore be seen that the foregoing represents a highly 
advantageous approach to inducing superplastic deformation. The terms and 
expressions employed herein are used as terms of description and not of 
limitation, and there is no intention, in the use of such terms and 
expressions, of excluding any equivalents of the features shown and 
described or portions thereof, but it is recognized that various 
modifications are possible within the scope of the invention claimed.