Phase transformation kinetics in a homogenised cast alloy 625 and its impact on deformation micromechanisms

Abstract

ABSTRACT This study focuses on understanding the impact of sequential ageing on the microstructural evolution of a large-grained structure homogenised cast alloy 625 and its effect on mechanical properties. SEM and TEM analyses of aged samples were conducted to comprehend various precipitation micromechanisms such as (i) nucleation and growth kinetics of γ′′ phase and (ii) transformation of primary carbides and grain boundary phase(s) evolved. The MC-based primary carbides are ascertained to transform into M23C6 secondary carbides following two completely different pathways: (I) MC + γ→ M23C6 + δ (mainly at grain boundary) and (II) MC + γ→M23C6 + γ″ (mainly within matrix). Furthermore, all three variants of γ″ phase are detected to evolve in the matrix displaying a crystallographic orientation of [001]γ″ // < 001>γ, and {100}γ″ // {100}γ. The formation of γ″ phase gradually improves the hardness of the material up to 650 h (224 HV) of ageing with a corresponding average precipitate size of 61 nm. The growth of γ″ particles complies with the Lifshitz–Slyozov–Wagner (LSW) theory of diffusion-controlled precipitate coarsening during the entire ageing period. Surprisingly, the hardness declined significantly beyond 650 h of ageing, even though the γ″ phase remained coherent with the matrix despite its coarsening (106 nm after 1000 h of ageing) and evidence of γ″→δ (mainly incoherent) transformation is not detectible throughout the ageing treatment. This lowering of hardness is attributed to a non-conventional slip-to-twin transition in the deformation mode of coherent γ″ particles after prolonged ageing, which is confirmed through tensile deformation micromechanisms analysis of the 650 and 1000 h aged samples.