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Bernard E. Hopkins passed away recently at the age of 85. He was a very important figure in the National Physical Laboratories for 30 years, working in the Metallurgy and Materials Applications Division. His work on alloys of iron in the early stages of his career, and later on non-ferrous materials alloys, formed the bases of future advancements in those fields. He loved to write, and his published papers are a lasting legacy.

Zones free of γ′ precipitate formed adjacent to grain boundaries in the gauge-lengths of specimens of Ni–20 % Cr-base alloys subjected to creep tests or slow-strain-rate tensile tests in the temperature range 750–850°C (1023–1123 K) have been studied. Electron-probe microanalysis did not reveal any significant change in composition between the precipitate-free zones and the grain interior. Consideration of the variation in the stress-dependence of minimum creep rate with temperature and grain size indicates that a diffusion creep mechanism may explain the formation of the precipitate-free zones.

In the high–strength superalloys used for ‘hot-end’ components in gas turbine engines, small amounts of elements with low melting points can have a seriously detrimental effect on high–temperature performance. Some of the more recent work carried out to define harmful levels of trace element content and to develop an understanding of the mechanisms involved is briefly reviewed in this paper. The emphasis is on results available for cast alloys; the problem of gaseous impurities in recycled material is also considered. The implications for the specification and control of trace element content are briefly discussed.MST/161

The ductility of a single-phase Ni–20% Cr alloy and two precipitation-hardened Ni–20% Cr-base alloys, hardened by Ni3(Al, Ti) and Ni3(Al,Nb), respectively, has been studied over a range of temperature at various strain rates. The results show that the alloy hardened by Ni3(Al,Nb), while less ductile than the single-phase alloy, is superior to that hardened by Ni3(Al,Ti) with respect to both tensile strength and elongation at fracture. In the single-phase alloy, there are two ductility troughs for a given strain rate when the strain rate is slow but in the two-phase alloys and at high strain rate in the single-phase alloy a single trough is obtained.Metallographic examination has indicated that the ductility behaviour of the single-phase alloy can be correlated with structural changes, void formation leading to a low elongation at fracture but recovery and recrystallization increasing ductility, presumably by relieving internal stresses at the grain boundaries. It is suggested that an important contribution to the ductility of the two-phase alloys at slow strain rates is the formation of precipitate-free zones at the grain boundaries as a result of diffusion creep. The improvement in ductility as a consequence of replacing Ti by Nb as a hardening addition in these alloys can be attributed at least in part to the more rapid formation of these zones, possibly because of faster diffusion rates in the Nb-hardened alloy.

The creep behaviour of nickel under cyclic loading has been studied, the aim being to relate the creep strain to the metallurgical recovery that occurs during each load reduction. Square-wave loading between stresses σ1(40 or 80 N/mm2) and σ2(0–σ1) for times t1(0.01–0.1 h) and t2(0.1–20 h) was applied at 650 or 750° C, and a point of principle was to apply the full stress σ1initially until the metal was fully work-hardened so that all subsequent changes could be ascribed to recovery. The strain per cycle consists of three components, which could be expressed in terms of experimental conditions: (1) ∆ε1, the normal strain in time t1at a stress σ1; (2) ∆ε2, the strain in time t2at stress σ2; ε2/ε1= (σ2/σ1)m, where m= 10 at 650° C and 5.2 at 750° C; (3) ∆εr, the extra strain on reloading from σ2to σ1; ∆εr= bt2⅔+ c, where band chave been determined. The factor ccan be negative, probably because strain-ageing occurs. A consequence is that three functions are needed to describe accurately the state of nickel during creep.At high temperature and low stress, recovery did not accumulate from cycle to cycle. By fitting the acceleration factor (∆ε1+ ∆ε2+ ∆εr)/∆ε1into a standard creep-strain/time equation, the envelope strain/time relation for cycling is described with satisfactory accuracy. At low temperature and high stress the creep rate steadily accelerated, presumably because recovery accumulated from cycle to cycle. Quite good predictions were made from the first few cycles. In both cases the predictive accuracy is more reassuring than it is with the conventional strain-hardening or time-hardening methods. The physical metallurgical approach enables this final result to be obtained with relatively little experimentation, though on this first occasion many extra experiments were performed to check various points.

The creep properties and the microstructure of two groups of austenitic steels containing manganese as a replacement for nickel have been studied at 700°C and for a range of stresses. In one group, nitrogen was added to stabilize the austenite; in the other, the content of manganese was higher and the steels were expected to be stably austenitic without nitrogen. The objective of the programme was to demonstrate that austenitic steels containing manganese could be produced having creep properties comparable with those of the more conventional austenitic steels. The work has shown that in terms of life to rupture the present steels compare favourably with equivalent steels containing nickel. The addition of carbide- and nitride-forming elements such as tungsten, vanadium, and molybdenum gave a significant improvement in creep strength relative to the base compositions, and X-ray analysis of extracted residues has been used in order to identify the precipitates formed.

The creep behaviour of a series of alloys based on IN 597 (Nimonic 101) and with varying volume fractions of γ′ precipitate has been studied at 800°C in the standard condition of heat treatment and for two grain sizes. In order to assess the long-term structural stability of alloys of this type, which have high contents of Cr, creep tests were also carried out on material overaged for 5000 and 10000 h at 850°C. Optical and electron metallography have been used to examine the microstructural characteristics of the materials. The results show a consistent pattern of increasing creep resistance with increasing volume fraction of γ′ precipitate, although the effect was more significant in the coarse grained material. After overaging, the creep resistance and the life to rupture of the alloys were reduced, and acicular particles, believed to be γ′ phase, were observed in the microstructure. Some limitations of current mechanisms describing dislocation–particle interactions are discussed with respect to creep deformation.

The effects of microstructural variations arising from the casting conditions on the creep performance of alloy IN939 have been assessed in order to determine the structural features having the greatest influence on creep properties. The results of creep tests at 850°C show that while grain size has an effect, the dendrite spacing is a more significant parameter in influencing performance. In particular, the life-to-rupture was greatest in specimens with fine dendrites as a consequence of the improved ductility with this type of structure. Thus the optimum combination of creep properties was obtained for material with fairly coarse grain size (~1mm) and fine dendrites (~20 µm). The structural changes associated with the various stages of the commercial heat treatment have been determined by optical metallography and electron microscopy of thin foils, and evidence has been found of dislocation bowing and shearing of ?' particles in fractured creep specimens. The creep results are discussed in the context of the microstructural changes observed and the relevance of mechanisms of deformation involving dislocation–particle interactions.MST/537

Damping capacity measurements have been carried out on copper-silicon alloy specimens before and after internal oxidation and the results compared with those for oxygen free, high conductivity copper specimens given the same treatment. High damping values have been noted for the fully internally oxidized copper-silicon material and it is suggested that this is caused, to some extent, by the generation of relatively free dislocations around the silica particles during the growth and subsequent cooling of these inclusions.