Materials Science and Technology

A study has been made of the metallurgical reactions that control the diffusion bonding of WC–Co hardmetal butt-joints interlayered with Ni, Co, and steel at 1300°C. Vapour transport appears to make a significant contribution to the growth of the bonded regions during the early stages of the process, and the rates of growth are in reasonable agreement with predictions from sintering theories for single component systems. The chemical reactions that occur at the bond interface with Co and steel interlayers are sensitive to differences in the C contents of the interlayers and of the hardmetal binder phase. With low C interlayers the hardmetal near the interface is transformed to an η phase structure.

A study has been made of the microstructural stability of various Co–Cr–Ti alloys. The alloy Co–16Cr–5Ti, when solution treated and cooled, forms a microstructure consisting of a homogeneous precipitation of Co₃Ti (γ′) particles in an fcc (γ) matrix. This structure remains relatively stable during heat treatment between 600 and 800°C. The addition of 3%Mo to the alloy causes transformation of the structure in this temperature range both by an fcc → hcp transformation and by discontinuous reprecipitation. The kinetics of these transformations are reported. The addition of 11%Ni to the Mo containing alloy suppresses the fcc → hcp transformation but not the discontinuous transformation. The fcc → hcp transformation, though martensitic in appearance, exhibits thermally activated kinetics. Possible explanations for this are discussed and possible reasons for the enhancement of the discontinuous transformation by Mo are also examined.

The reverse transformation α′ → γ within two austenitic stainless steels of nominal composition Fe–18Cr–8Ni and Fe–18Cr–12Ni has been studied. A magnetic balance apparatus was used to determine the phase contents after rapid, salt bath heating. Information concerning the structures produced and the mechanisms of the transformation was obtained using transmission electron microscopical examination, including in situ hot stage experiments and X-ray microanalysis. The results obtained show that the α′ → γ transformation proceeds by both diffusional and shear processes depending upon the heating time and temperature, initially producing austenite having a twinned substructure which recovers into fine subgrains. Diffusion occurs at temperatures between the A _s and A _f to produce the equilibrium austenite and ferrite duplex structure. Further observations on the recrystallization behaviour, the effect of transformation cycling, and the phenomenon of a magnetic phase ‘peak’ at ∼ 400°C are also reported, and possible mechanisms operating during these processes are discussed.

The nature of the microscopic processes occurring at the crack tip during crack initiation and the early stages of propagation were investigated by detailed characterization of the stretched zone in J integral specimens of two thicknesses tested at two different temperatures. The J integral tests were conducted on HT–9, a martensitic stainless steel, using the single specimen compliance method. Tests were conducted on modified 0·5 in thick compact tension (CT) specimens at two thicknesses, 12·7 and 2·5 mm, fatigue precracked before the test and at temperatures of 25 and 200°C. The topography of the crack tip region was mapped by detailed stereographic measurements on scanning electron micrographs of the region containing the stretched zone. The measured J _1C values and the stretched zone dimensions for each test condition were then compared using the relationship of Knott: J _1C = mσ_fδ_c, where m is a constraint factor, σ_f is the flow stress, and δ_c is the crack opening displacement taken as twice the stretched zone height. Theory predicts values of m of about 2 for specimens under plane strain conditions and lower values of m for thin specimens under plane stress conditions. The current experiments showed good agreement with theory for the thicker specimens but larger values of m, near 6·0, for the thin specimens. A rationale for this lack of agreement and an analysis of the crack tip deformation processes are presented.

The growth rates of grain boundary cavities during slow–fast fatigue are calculated by assessing the rates of four growth mechanisms. It is shown that cavity growth can take place providing the strain rate during tension is below a critical value. Also at sufficiently low strain rates cavities can grow to a large size before the life of a fast–fast test is achieved. The paper suggests that a better understanding of cavity nucleation is desirable.

Phase equilibria have been investigated in the Ni rich regions of the Ni–Al–Mo system at 1473 K and of the Ni–Al–W system at 1523 and 1273 K using electron microprobe analysis, X-ray diffraction, and optical metallography. The phase relationships at 1473 and 1523 K are similar in the two systems showing three-phase equilibria of γ + γ′ + α (Mo or W) and γ′ + β + α (Mo or W); no ternary compounds were found in the regions studied. The maximum solubilities of Mo and W in γ′ were found to be ∼6 at.-%. Differences between the present and previous work are reviewed.

An algorithm for finding common tangents to Gibbs energy functions is presented. The approach differs significantly from those which have been used previously and has the advantage of being very robust in the sense that one can start with a quite rough approximation to the desired solution and still obtain iterative convergence to any required precision to the solution.

Commercially pure nickel was deformed to very large strains by cold rolling. Tensile tests on prestrained sheet were used to establish strain hardening behaviour and residual ductility. The hardening of nickel was found to be peculiar: initial rapid hardening was followed by an apparent plateau, which, in turn, was followed by another stage of rapid hardening at strains larger than 4. The residual elongation dropped precipitously with initial prestrain, levelled off, and then increased again during the final stage of hardening. Nickel rolled to a strain of 6·1 exhibited a remarkable tensile strength of 1400 MN m⁻² with 4% total elongation. Extensive in-plane and limited edge-on transmission electron microscopy was used to examine the evolution of substructure. Dislocation cells formed at strains <0·10 and continued to refine in size with increasing strain. Definite evidence of dynamic recovery was found. The formation of elongated, well defined subgrains appears to coincide with a saturation of hardening in the plateau region. During the final stage of hardening the substructure became cut up into small, more equiaxed domains with subgrains becoming more numerous. The hardening transition may be caused by the re-emergence of original grain boundaries as important structural features at very large strains.