D. W. Livesey

Bio: D. W. Livesey is an academic researcher from University of Manchester. The author has contributed to research in topics: Superplasticity & Strain rate. The author has an hindex of 5, co-authored 5 publications receiving 147 citations.

Superplastic deformation, cavitation, and fracture of microduplex Cu-Ni-Zn alloys

Abstract: A study has been made of the superplastic behavior during tensile straining of two α/β Cu-Ni-Zn alloys (nickel silvers). Cavitation occurred during deformation and has been studied using metallographic and density techniques. Cavities nucleated at α/β boundaries and triple points involving two phases, and cavity growth and interlinkage led to brittle superplastic fracture. Density studies showed that the volume of cavities increased with increasing strain, but was relatively independent of strain rate and temperature. The results were consistent with a high rate of cavity nucleation in the early stages of deformation, followed by a grain boundary sliding mechanism of growth.

Cavitation and cavity growth during superplastic flow in microduplex Cu-Zn-Ni alloys

Abstract: A study has been made of cavity growth during superplastic tensile deformation of two microduplex α/β nickel-silvers, one a Cu-Zn-Ni alloy and the other a Cu-Zn-Ni-Mn alloy. For cavities with radii of >0.5 /gmm, measured growth rates were found to be in good agreement with values calculated on the assumption that cavity growth was controlled by viscous flow of the matrix. For smaller cavity sizes a diffusional growth mechanism could predominate. Metallography revealed that cavity morphology changed with strain in a manner consistent with diffusion-controlled growth at small sizes, and matrix deformation controlled growth at intermediate and large cavity sizes. Density studies showed that the overall level of cavitation was independent of both strain rate and temperature, and was influenced only by strain.

Effect of cavitation on post-deformation tensile properties of a superplastic copper-base alloy

Abstract: The effects of cavitation, introduced during superplastic tensile flow, on post-deformation tensile properties have been studied for a microduplex Cu-Zn-Ni alloy. Increasing levels of cavitation of up to 4 percent by volume caused a progressive reduction in tensile strength, while ductility was decreased to a substantially greater extent. Increasing the volume fraction of cavities also changed the appearance of the fracture surface from cup and cone to macroscopically brittle. A post deformation annealing treatment reduced the level of cavitation and caused an increase in room temperature ductility, but the proportion of cavities removed during annealing became progressively less as cavitation was increased.

Effect of strain, strain rate and temperature on cavity size distribution in a superplastic copper-base alloy

Abstract: The distribution of cavity sizes in a microduplex α/β copper-zinc-nickel-manganese alloy (a nickel-silver) subjected to superplastic tensile straining has been examined as a function of strain, temperature and strain rate using quantitative optical metallography. The number of cavities that became optically visible increased throughout straining, but the rate at which they became visible decreased at higher strains. The distributions of cavity sizes in specimens deformed to the same strain at different temperatures or strain rates were essentially identical. The size distribution data were fully consistent with the observations that for a given strain the overall volume of cavities formed in the alloy was independent of temperature and strain rate. Growth for all cavity sizes is dominated by matrix plastic flow. The insensitivity of void volume and cavity size distribution to strain rate and temperature, and hence stress, reflects the resolution of the density and metallographic techniques. While higher stresses will lead to a wider range of initial cavity sizes by lowering the critical nucleus size, cavities at the lower end of the size range will not grow sufficiently to become optically resolvable or to produce a density differential compared with a specimen deformed to the same extent at a lower stress.

Activation energies for superplastic tensile and compressive flow in microduplex α/ß copper alloys

Abstract: Logarithmic stress against strain rate curves have been determined at various temperatures for a superplastic commercialα/β nickel-silver alloy strained in tension, and a laboratory prepared microduplex alloy of nominally similar composition strained in compression. The shapes of the curves were found to be affected by grain growth at high temperatures and strain softening at low temperatures. After taking these factors into account, it was apparent that with decreasing strain rate in both alloys a change in deformation mechanism occurred giving rise to a Region I of low strain-rate sensitivity. By confining activation energy (Q) measurements to temperatures at which steady-state deformation occurred, it was found thatQ for Region II was very similar to that measured for grain boundary diffusion in theα phase of a nickel-silver alloy of similar composition, whileQ for Region I was substantially higher than that for lattice diffusion. Values of strain-rate sensitivity andQ were found to be similar for each direction of applied stress.

The mechanical properties of superplastic materials

Abstract: The relationship between stress and strain rate is often sigmoidal in superplastic materials, with a low strain rate sensitivity at low and high strain rates (regions I and III, respectively) and a high strain rate sensitivity at intermediate strain rates (region II) where the material exhibits optimal superplasticity This relationship is examined in detail, with reference both to the conflicting results reported for the Zn-22 pct Al eutectoid alloy and to the significance of the three regions of flow

Hot workability of titanium and titanium aluminide alloys—an overview

Abstract: The hot workability of conventional titanium alloys and titanium aluminides is reviewed. For both alloy classes, the influence of hot working variables and microstructure on failure via fracture or flow-localization controlled processes is summarized. The occurrence of wedge cracking and cavitation during bulk forming of α / β alloys with Widmanstatten microstructures or γ titanium aluminides with lamellar or equiaxed structures, is examined. In particular, the effects of grain size, grain boundary second phases and process variables on failure are presented. Observations and models of flow localization and cavitation processes which lead to failure during low strain rate, superplastic, tensile-type deformation of titanium and titanium aluminide alloys with fine equiaxed structures, are also described. In the area of flow-localization-controlled failure during bulk forming, the occurrence of shear bands and other flow nonuniformities during both conventional and isothermal hot working processes is reviewed. The influence of material properties, such as flow softening rate and strain rate sensitivity and process variables, which lead to temperature and hence flow nonuniformities, is examined. The flow localization concepts are illustrated for several hot working processes.

Fracture processes in superplastic flow

Abstract: There are four distinct types of fracture in superplastic materials: failure by quasistable plastic flow, failure by necking, cavitation failure, and quasibrittle failure. The characteristics of these four types are described with reference to experimental examples. Maximum elongation occurs in a superplastic material when it pulls out to a fine wire in quasi stable flow. It is demonstrated that there are two basic requirements for this type of flow: (a) a suppression of localized (but not diffuse) necking, and (b) a suppression of significant cavity interlinkage (but not necessarily of cavity nucleation and growth).

Microstructural aspects of superplasticity

Abstract: The microstructural aspects of the superplastic phenomenon are reviewed. The experimental results of a very large number of investigations are critically analysed in the context of: grain shape and size; grain growth; grain boundary sliding and migration, grain rotation and rearrangement; diffusion and dislocation activity. It is shown, that in spite of often conflicting evidence in the literature, a common pattern of microstructural behaviour emerges for all the materials and conditions investigated to date.