C. W. Humphries

Bio: C. W. Humphries is an academic researcher from University of Manchester. The author has contributed to research in topics: Superplasticity & Cavitation. The author has an hindex of 3, co-authored 3 publications receiving 106 citations.

Cavitation in alloy steels during superplastic deformation

Abstract: A study of cavitation during superplastic tensile straining of two microduplex steels has been made using density measurements and quantitative optical metallography. The steels were of basically similar composition with the exception of a trace addition of boron made to one alloy. During deformation cavities formedα/γ boundaries and matrix-carbide interfaces; the growth and coalescence of these cavities led to failure. Density measurements showed that the extent of cavitation increased with increasing strain and decreasing strain-rate, but the level of cavitation was reduced by the presence of boron. A time dependence of overall void volume of 1.4 to 2.0 was observed. Quantitative metallographic studies of the nucleation and growth contributions to the overall rate of void formation showed that boron inhibited each of these processeS. However, both the nucleation rate and the magnitude of the time exponent of void volume increase suggested that a substantial number of voids grew from pre-existing nuclei which were probably present as non-coherent carbide-matrix interfaces.

Cavitation during the superplastic deformation of an α/β brass

Abstract: Cavitation during superplastic tensile flow has been studied in anα/β brass using metallography and precision density measurements. Cavities nucleated primarily at triple points and were sometimes associated with small second phase particles. The level of cavitation increased as strain, strain rate and grain size were increased and as the temperature was decreased. The influence of these variables can be interpreted in terms of their effects on cavity nucleation and/or cavity growth rates.

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

Influences of material parameters and microstructure on superplastic forming

Abstract: The strain rate sensitivity of flow stress has long been recognized as an important factor in determining superplastic ductility, and its relationship to high tensile elongations is well understood from the mechanics point of view. However, the measurements of this parameter and other properties of superplastic materials are challenging, and quite varied results are observed from different test procedures used. In this paper a discussion of the various characterization methods is presented, and the relationship between the superplastic characteristics and the microstructure is brought forth. The applicability of the uniaxial superplastic properties on the biaxial deformation behavior as in the superplastic forming process is addressed, including a relationship of such properties to the selection of forming parameters.

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).

Tensile ductility of superplastic ceramics and metallic alloys

Abstract: Superplastic ceramics and metallic alloys exhibit different trends in tensile ductility in the range where the strain-rate-sensitivity exponent, m, is high (m⩾0.5). The tensile ductility of superplastic metallic alloys (e.g. fine-grained zinc, aluminium, nickel and titanium alloys) is primarily a function of the strain-rate-sensitivity exponent. In contrast, the tensile ductility of superplastic ceramic materials (e.g. zirconia, alumina, zirconia-alumina composites and iron carbide) is not only a function of the strain-rate-sensitivity exponent, but also a function of the parameter ⋗e exp (Qc/RT) where ⋗e is the steady-state strain rate and Qc is the activation energy for superplastic flow. Superplastic ceramic materials exhibit a large decrease in tensile elongation with an increase in ⋗e exp (Qc/RT). This trend in tensile elongation is explained based on a “fracture-mechanics” model. The model predicts that tensile ductility increases with a decrease in flow stress, a decrease in grain size and an increase in the parameter (2γs−γgb), where γs is the surface energy and γgb is the grain boundary energy. The difference in the tensile ductility behavior of superplastic ceramics and metallic alloys can be related to their different failure mechanisms. Superplastic ceramics deform without necking and fail by intergranular cracks that propagate perpendicular to the applied tensile axis. In contrast, superplastic metallic alloys commonly fail by intergranular and transgranular (shearing) mechanisms with associated void formation in the neck region.