N. Ridley

Cavitation in alloy steels during superplastic deformation

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

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.

Effect of phase proportions on deformation and cavitation of superplastic α/β brass

Studies have been made, using metallographic and precision density techniques, of the deformation and cavitation behaviour during superplastic tensile straining at 873 K of three microduplexα/β brasses which, as a consequence of varying composition, contained varying proportions ofα andβ phases. It was observed that both strain-rate sensitivity and elongation-to-failure passed through a maximum when approximately equivolume proportions of the two phases were present. Cavitation, on the other hand, decreased rapidly as the volume fraction ofβ phase was increased. The cavitation behaviour was attributed to the relative abilities of the phases to accommodate grain boundary sliding. When a high proportion ofα phase is present accommodation is minimal and cavity nucleation. occurs readily. Evidence is presented to show that grain-boundary sliding plays a predominant role in cavity growth. When a high proportion ofβ phase is present accommodation is almost complete and cavity nucleation is minimal.

Superplastic deformation characteristics of two microduplex titanium alloys

A comparison of the superplastic deformation behaviour of Ti-6Al-4V (wt%) between 760 and 940‡ C and Ti-6Al-2Sn-4Zr-2Mo between 820 and 970‡ C has been carried out on sheet materials possessing similar as-received microstructures. High tensile elongations were obtained with maximum values being recorded at 880‡ C for Ti-6Al-4V (Ti-6/4) and at 940‡ C for Ti-6Al-2Sn-4Zr-2Mo (Ti-6/2/4/2), under which conditions both alloys possessed aΒ phase proportion of approximately 0.40. For a given deformation temperature the Ti-6/4 alloy had a slightly lower flow stress than the Ti-6/2/4/2, and this was attributed to the lowerΒ phase proportion in the latter alloy. However, at the respective optimum deformation temperatures the Ti-6/2/4/2 alloy had the lower flow stress. The results show that suitably processed Ti-6/2/4/2 alloy is capable of withstanding substantial superplastic strains at relatively low flow stresses, although the optimum deformation temperature is higher for this alloy than for Ti-6/4 material possessing a similar microstructure.

Effect of grain size on cavitation in superplastic Zn-Al eutectoid

The effect of initial grain size on cavitation during superplastic deformation in two commercially available Zn-Al eutectoid alloys has been studied using metallography and precision density measurements. Cavitation was found to be minimal for initial grain sizes below about 5 μm. Superplastic deformation caused grain growth in both alloys under all testing conditions, and when the grain size exceeded about 8 μm a significant level of cavitation was produced. The grain size and extent of cavitation increased with increasing strain along the specimen gauge length, with cavities concentrated in regions adjacent to the fracture tip. Although never very large, the cross-sectional area at fracture increased with increasing levels of cavitation. It was concluded that cavitation in Zn-Al eutectoid results from incomplete accommodation of grain-boundary sliding when excessive grain growth leads to restricted grain-boundary diffusion and/or to restricted grain-boundary migration.

Cavitation during superplastic flow of ternary alloys based on microduplex Pb-Sn eutectic

The effect of phases having a range of hardnesses on the superplastic tensile behaviour of microduplex Pb-Sn eutectic has been studied. The presence of relatively hard particles induced cavitation at particle/matrix interfaces during deformation in an otherwise noncavitating system, and the growth and interlinkage of cavities led to brittle superplastic fractures. Density measurements showed that cavitation increased as the volume fraction, hardness and size of intermetallic particles was increased. Increasing strain rate and decreasing deformation temperature also led to an increased level of cavitation. Cavity nucleation was attributed to the limited ability of the relatively hard phases to contribute to the accommodation processes occurring during superplastic flow.

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

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.

The kinetics of isothermal and isovelocity pearlite growth in Cu-Al eutectoid alloy

Measurements of pearlite growth rate and inter lamellar spacing have been made on isothermally transformed specimens of Cu-11.8 wt pct Al eutectoid alloy. Growth rates which have been calculated on the assumption that volume diffusion in the parent β phase is the rate-controlling process show excellent agreement with the measured values. Directional control of eutectoid growth by translating specimens through steep temperature gradients resulted in the formation of aligned lamellar structures, and the effect of variations in imposed velocity and temperature gradient on alignment has been examined. A comparison of the kinetics of the two modes of transformation has shown that isothermal and isovelocity pearlite growth in this alloy may be readily interrelated. Analysis of the isovelocity growth data supports the conclusion that growth is controlled by volume diffusion.

Cavitation behaviour in fine grain 3Y-TZP during tensile and compressive superplastic flow

Studies of cavitation in Y-TZP during superplastic flow have been made for both tensile and compressive deformation conditions. It was observed that the morphologies of cavities near the fracture faces of tensile specimens varied markedly with testing conditions and in most cases differed from those near the gauge heads. Two quite different forms of cavitation behaviour were observed leading to high and low strains to failure, respectively. For optimum conditions of superplastic flow, of high temperature/low strain rate (low stress), when large elongations were observed, cavities were either spherical or elongated parallel to the tensile axis. Those near the fracture face interlinked in a plastic (necking) mode to give transverse cavities and subsequent failure. At high strain rate/low temperature (high stress), transverse intergranular cracking played a dominant role in failure at low elongations. For intermediate conditions of temperature/strain rate, elongated cavities developed parallel to the tensile axis, but near the fracture face these usually interlinked by transverse cracking. These conditions were associated with intermediate elongations to failure. For the assessment of cavity growth mechanisms, artificial pores were introduced into fine grain Y-TZP specimens and changes in their shape and size during tensile or compressive deformation were investigated. Results show that the change of pore volume, in the superplastic regime, is controlled by plastic deformation of the matrix and can be described by the relationship of dR/dɛ = ;ηR, where ɛ is the true strain, η the cavity growth rate parameter and R is the radius of the pore.

Evaluation of diffusion bonds formed between superplastic sheet materials

Diffusion bonds produced in microduplex titanium and stainless steel sheet materials for various bonding conditions have been evaluated using a range of techniques. These include light and scanning electron microscopy (SEM), scanning acoustic microscopy (SAM) and compressive lap shear testing. The potential of other procedures such as ultrasonic inspection and resistivity measurement are also discussed. For imperfect bonds, the bond line in titanium alloys consists of clearly defined interfacial voids separated by metallurgically sound bonded regions, while the unbonded regions in stainless steel often consist of long flat voids in which the opposing surfaces have contacted but not bonded. It was observed that light microscopy and SEM observations provide a convenient and reliable method for the assessment of the bond quality, and in the case of titanium alloys it is possible to obtain quantitative data on the extent of bonding. High frequency SAM also proved to be an effective procedure for qualitative assessment. A linear relationship between the fraction of parent metal strength achieved and bonded area fraction as determined by metallography was observed for titanium alloys.