The evolution of a defect structure and its relation to the deformation parameters in TiAlNbZr alloy at 4.2 K

Abstract

The phase composition of the Ti-(5–6.5) wt.%Al-(2.5–4)wt.%Nb-(1–2.5)wt.%Zr structural alloy and the evolution of the alloy defect structure during its plastic deformation at 4.2 K were studied using transmission electron microscopy. It is shown that according to its phase composition this alloy should be attributed to the category of pseudo α alloys which contain a small (by volume) amount of b.c.c. β phase in an h.c.p. α matrix, the β phase being revealed in the form of fine needles and plates. The high values of the mechanical properties of the alloy at liquid helium temperature are due in part to the presence of strong dislocation pile-ups, pinned at β phase precipitates and creating appreciable internal stresses. In contrast, the high concentration of internal stresses in slip bands containing a high density of dislocations and pinned at the precipitates is the cause of cracking which results in a decrease in the plasticity of the alloy at 4.2 K. The quantitative relationships between the deforming stress σ, the prismatic dislocation density π and the strain ϵ were obtained for an alloy deformed at 4.2 K: σ = σ 0 + αGbϱ 1 2 ϱ = ϱ 0 + Aϵ 3 2 where σ 0 is the stress approximately corresponding to the yield strength, G is the shear modulus at 4.2 K, b is the prismatic dislocation Burgers vector, α (=0.5) is a factor characterizing the contribution from the dislocation elastic interaction to the deforming stress value, ϱ 0 is the dislocation density in an unstrained material and A is the dislocation multiplication factor.