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Grain refinement in β-CuAlNi strain memory alloys Sure, Ganesh Narayan


A study has been made of the effect of grain refinement on the mechanical and the strain memory properties of 0-CuAlNi alloys. Addition of 0.5 % Ti to β-CuAlNi resulted in considerable grain refinement. This was due to the formation of titanium-rich particles which effectively retarded grain growth during annealing. By controlled annealing, a grain size as small as 15μm could be obtained, though some second phase γ2 was present due to incomplete precipitate dissolution. Stress-strain curves for most specimens in both the strain memory and pseudoelastic states showed a three-stage characteristic with a region of lower slope between two regions of higher modulus. It was found the σ, (the transition stress between stages 1 and 2) and dσ/dε (the slope of stage 2) increased with grain size according to a (g.s.)[sub – ½] relationship. The ultimate tensile strength and strain to fracture also followed a similar Hall-Petch relationship. The alloys showed higher strength in the martensitic state than in the pseudoelastic one. The presence of second phase particles had no effect on the mechanical properties and martensite deformation behaviour. Fracture strains as high as 7% were obtained at the finest grain sizes. It was found that the strain memory and pseudoelastic recovery properties were not affected significantly by decreasing grain size and the presence of second phase particles. Maximum recovery strains of 6.5% were obtained infine grain samples. The fatigue life of most specimens was relatively poor and did not vary significantly with grain size and the presence of titanium-rich particles. However, the presence of γ2 particles in the finest grain sizes of 15-40μm caused the fatigue life to increase by a factor of ten, so that specimens cycled to 280 MPa and 0.6% strain had a life of 40000-50000 cycles. This improvement in fatigue life appeared to be due to a change in the fatigue crack propagation mode.

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