Atul H. Chokshi

Superplasticity in advanced materials

Abstract The ability to achieve a high tensile ductility in a polycrystalline material is of interest both from a scientific point of view and also because of potential applications in the materials forming industry. The superplasticity of conventional metallic alloys is now well-documented and understood reasonably well. However, the field of superplasticity has expanded recently beyond the traditional metallic alloys to include evidence of superplastic-like behavior in a very wide range of new and advanced materials. To date, superplasticity has been reported in mechanically alloyed metals, metal matrix composites, ceramics, ceramic matrix composites and intermetallic compounds. This review presents an overview of these new developments using the established behavior of conventional metallic alloys as a standard for comparison with the mechanical properties of these new materials. As well be demonstrated, the new materials often exhibit significant differences in their flow characteristics in comparison with the traditional superplastic metallic alloys. The successful utilization of superplastic materials in forming applications requires an understanding of the failure processes occurring in the materials in terms of both the localization of external flow and the accumulation of internal damage through the nucleation and growth of cavities. These problems are also addressed in this review.

DYNAMIC RECRYSTALLIZATION IN HIGH-STRAIN, HIGH-STRAIN-RATE PLASTIC DEFORMATION OF COPPER

When copper is deformed to high plastic strain (y ~ 34) at high strain rates (~ ~ I04 s -1) a microstructure with grain sizes of ~0.1 am can be produced. It is proposed that this microstructure develops by dynamic recrystallization, which is enabled by the adiabatic temperature rise. By shock-load- ing the material, and thereby increasing its flow stress, the propensity for dynamic recrystallization can be enhanced. The grain size-flow stress relationship observed after cessation of plastic deformation is consistent with the general formulation proposed by Derby (Acta metall, mater. 39, 955 (1991)). The temperatures reached by the specimens during dynamic deformation are calculated from a constitutive equation and are found to be, for the shock-loaded material, in the 500-800 K range; these temperatures are consistent with static annealing experiments on shock-loaded specimens, that show the onset of static recrystallization at 523 K. A possible recrystallization mechanism is described and its effect on the mechanical response of copper is discussed.

The Effect of Grain Size on the High-Strain, High-Strain-Rate Behavior of Copper

Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117- and 315-μm grain-size specimens and virtually absent for the 9.5-μm grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shockinduced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges.

The high temperature mechanical characteristics of superplastic 3 mol% yttria stabilized zirconia

A detailed study was undertaken to characterize the deformation behavior of a superplastic 3 mol% yttria-stabilized tetragonal zirconia (3YTZ) over a wide range of strain rates, temperatures and grain sizes. The experimental data were analyzed in terms of the following equation for high temperature deformation: Image Full-size image ∞ σn d−pexp(−Q/RT), where Image Full-size image is the strain rate, σ is the flow stress, d is the grain size, Q is the activation energy, R is the gas constant, T is the absolute temperature, and n and p are constants termed the stress exponent and the inverse grain size exponent, respectively. The experimental data over a wide range of stresses revealed a transition in stress exponent. Deformation in the low and high stress regions was associated with n not, vert, similar 3 and p not, vert, similar 1, and n not, vert, similar 2 and p not, vert, similar 3, respectively. The transition stress between the two regions decreased with increasing grain size. The activation energy was similar for both regions with a value of not, vert, similar 550 kJ mol−1. Microstructural measurements revealed that grains remained essentially equiaxed after the accumulation of large strains, and very limited concurrent grain growths occurred in most experiments. Assessment of possible rate controlling creep mechanisms and comparison with previous studied indicate that in the n not, vert, similar 2 region, deformation occurs by a grain boundary sliding process whose rate is independent of impurity content. Deformation in the n not, vert, similar 3 region is controlled by an interface reaction that is highly sensitive to impurity content. It is concluded that an increase in impurity content increases yttrium segregation to grain boundaries, which enhances the rate of the interface reaction, thereby decreasing the apparent transition stress between the n not, vert, similar 2 and n not, vert, similar 3 regions. This unified approach incorporating two sequential mechanisms can rationalize many of the apparently dissimilar results that have been reported previously for deformation of 3YTZ.

A model for diffusional cavity growth in superplasticity

There is an enhancement in the diffusional growth of cavities in superplastic materials if the cavity size exceeds the grain size so that vacancies diffuse into the cavity along a number of grain boundary paths. A model for this process is developed, and it is shown that the rate of change of cavity radius with strain due to superplastic diffusion growth is given by drdϵ ∼- 45ΩδDgbd2kT(σe) where r is the cavity radius, e is the total strain, Ω is the atomic volume, δ is the grain boundary width, Dgb is the coefficient for grain boundary diffusion, d is the spatial grain size, k is Boltzmanns constant, T is the absolute temperature, σ is the applied stress and e is the strain rate. Superplastic diffusion growth is therefore independent of the instantaneous cavity radius and inversely proportional to the square of the grain size; in practice, this process becomes important at the lower testing strain rates and when the specimen grain size is typically less than ~5 μm. It is demonstrated that the occurrence of superplastic diffusion growth is capable of providing an explanation for the experimental observations of large rounded cavities in several superplastic materials.

Superplasticity in fine grained ceramics and ceramic composites: current understanding and future prospects

Superplasticity in ceramics has now been reported in a wide range of materials with elongations to failure of more than 100%. Although the experimental observations of large deformation are in some ways similar to those reported in numerous metallic alloys, there are significant differences in the mechanical properties and cavitation failure characteristics of superplastic ceramics. This paper provides an overview of superplastic deformation and failure in ceramics, with specific emphasis on a 3 mol.% yttria stabilized zirconia and a zirconia-20wt.%alumina composite. It is demonstrated that there is a transition in the deformation behavior of zirconia which is dependent on the grain size and the impurity content of the material. Many of these materials fail by the nucleation, growth and interlinkage of cavities, so that the ductility is governed by the imposed stress and the grain size. Potential areas for additional research on superplastic ceramics are highlighted.

An investigation of grain boundary sliding in superplasticity at high elongations

Experiments were performed on the superplastic Zn-22% Al eutectoid alloy to determine the contribution of grain boundary sliding at both low (35%) and high (∼235%) elongations. The tests were conducted at two different strain rates in the superplastic Region II, and the results show that, within the accuracy of the measurements, there is a large sliding contribution at both elongations. By taking detailed measurements of both the magnitude of the sliding offset and the type of interface, it is shown that the average offsets are generally a maximum at the Zn-Zn boundaries, there is less sliding at the Zn-Al interfaces, and the offsets are a minimum at the Al-Al boundaries. In addition, the distributions of the magnitudes of the sliding offsets are similar at both the low and high elongations. It is concluded that grain boundary sliding is an important deformation process in the superplastic Region II and that it remains important even when the elongation is very high. The nature of the results indicates also that experimental observations of the deformation behaviour in superplastic materials at low elongations (up to 50%) provide meaningful information on the behaviour at much higher (superplastic) elongations.

Microstructural aspects of superplastic tensile deformation and cavitation failure in a fine-grained yttria stabilized tetragonal zirconia

A fine-grained yttria stabilized tetragonal zirconia exhibits an optimum superplastic elongation to failure of ∼ 700% at 1823 K and a strain rate of 8.3 × 10−5s−1. A detailed microstructural investigation of the superplastically deformed specimens reveals the occurrence of extensive concurrent grain growth and internal cavitation. An expression is developed to characterize the extent of deformation enhanced concurrent grain growth, as influenced by experimental factors such as true strain, strain rate and temperature. The variation in the level of concurrent cavitation with strain rate conforms closely to the variation in elongation to failure with strain rate. It is demonstrated that the tendency towards cavity interlinkage in a direction perpendicular to the tensile axis is an important factor influencing the total elongation to failure obtained in superplastic materials.

High temperature mechanical properties of single phase alumina

The creep properties of a commercial, single phase aIurnina have been determined in the temperature range of 1623 to 1723 K. The stress exponent,n, in the relationship ɛ ∝ σn was determined to be 1.9 and the true activation energy was found to be 635 kJ mol−1. Normal primary stage creep transients were observed up to strains of 1%. At low stresses, steady-state conditions were not obtained due to the occurrence of concurrent grain growth. It is shown that the steady state creep results are consistent with the occurrence of an interface controlled diffusionaI creep mechanism.

Diffusion, diffusion creep and grain growth characteristics of nanocrystalline and fine-grained monoclinic, tetragonal and cubic zirconia

The experimentally measured grain size compensated diffusion creep rates are essentially identical in cubic, tetragonal and monoclinic zirconia, suggesting a similarity in the absolute magnitudes of their grain boundary diffusion coefficients. However, grain growth is substantially slower in tetragonal zirconia due to significant grain boundary segregation.