Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-12-23
2002-03-12
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C419S066000
Reexamination Certificate
active
06355120
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to mismatch plastic deformation. More particularly, this invention relates to a technique for inducing mismatch plastic deformation, including transformation-mismatch 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
ϵ
.
=
A
σ
n
exp
(
-
Q
R
T
)
(in which {dot over (&egr;)} is strain rate, A is a materials constant, &sgr; 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 repeatedly through a phase transformation by changing the temperature. At the end of each thermal cycle, a strain increment is produced, a phenomenon which can be termed “transformation-mismatch plasticity.” The strain can be accumulated upon multiple cycles up to large overall values, at which point the material is considered to have deformed by transformation-mismatch superplasticity (or “transformation-superplasticity”). It is important to note that transformation-mismatch plasticity is produced by the biasing of internal mismatch stresses and is thus distinct from “transformation-induced plasticity” (TRIP) observed in many steels, in which the strain is produced by a biasing of the martensitic shape-change and not by internal mismatch stresses, and thus cannot be accumulated through multiple cycles up to large strain values.
Transformation superplasticity is advantageous compared to earlier approaches to superplasticity 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 transformation-mismatch plasticity, including superplasticity, without thermal cycling .
Another object of the invention is to provide a technique for inducing transformation-mismatch plasticity, including superplasticity, that is not limited to any specific workpiece microstructure or composition and is applicable to a wide range of workpiece materials, including titanium-based materials.
Another object of the invention is to provide a powder compaction technique.
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 mismatch plasticity, including superplasticity, without causing the workpiece material to undergo a phase transition.
Another object of the invention is to provide a method of inducing transformation-mismatch plasticity, including superplasticity, that allows fast deformation of the workpiece.
Still another object of the invention is to provide a method of inducing transformation-mismatch plasticity, including 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 e invention produces deformation by mismatch plasticity in a workpiece by altering the chemical composition of the workpiece material—by increasing and/or decreasing the concentration of a chemical component—while the workpiece is subjected to a biasing stress, in a manner that introduces a strain increment into the material, deforming the workpiece without causing, failure. Known apparatus for fine-grain superplastic forming or forging 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 accomplish the desired chemical composition change. The operation of the method of the invention on the workpiece is such that the mass of the workpiece is not increased thereby except in cases in which chemical component is provided to the workpiece to effect the strain increment and remains in the workpiece at the end of the operation. The method of the invention adds no bulk material to the workpiece, such as would be done by a bonding operation.
The method of the invention is not limited with respect to the deformation geometry it effects. The workpiece may be simply stretched, under uniaxial tension. Or, more elaborate shaping may be performed, for example corrugation of a sheet by superplastic forming into a die or foaming by expansion of internal cavities. The wide range of deformation arrangements achievable by the method of the invention are conveniently described with reference to the effective von Mises strain e
eff
, well known in the art to be defined as
e
eff
=⅔((e
1
−e
2
)
2
+(e
1
−e
3
)
2
+(e
2
−e
3
)
2
)
½
ê,
for a strain state characterizable by principal strains e
1
, e
2
and e
3
. In a preferred embodiment, the deformation causes the effective von Mises strain to change by a minimum amount at some location in the workpiece. The method of the invention effects a change in the von Mises strain on the order of 0.5% or even greater. Depending on the material, the mismatch plastic strain increment may generate a change in von Mises strain of 5%, even as much as 10% and greater.
The alteration in composition under the biasing stress may be monotonic, resulting in a permanent change in the concentration of the component. Or, after completion of the deformation process the concentration change may be fully reversed, restoring the preprocess mass of the workpiece to within 0.01%, or partially reversed. In one embodiment the concentration alteration under stress is cyclic, comprising an initial change of the concentration of the chemical component in a first direction, followed by change in the opposite direction to accomplish a partial or total reversal of the initial change while the workpiece remains subject to the biasing stress. As used herein, reversing the concentration change encompasses a partial reversal as well as a full reversal, and the stipulation that the composition of the workpiece be altered while the workpiece is subject to the biasing stress denotes that a nonzero biasing stress is in effect at some time during the concentration changing operation. The biasing stress need not be constant during a unidirectional concentration change or follow the same profile during a part of the procedure adding chemical component to the workpiece
Dunand David C.
Zwigl Peter
Cesari and McKenna LLP
Massachusetts Institue of Technology
Wyszomierski George
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