Fracture behavior of Alloy 625 with different precipitate microstructures

To study the effect of various ordered phases and their growth on room temperature tensile properties and fracture behavior of Alloy 625, specimens of this alloy were held isothermally at 813 K, 973 K and 1123 K for 10 h, 100 h and 1200 h, respectively. Specimens held at the aforementioned temperatures were subjected to transmission electron microscopic investigation to characterize Ni2(Cr,Mo), γ″ and δ ordered phases. Tensile testing of the heat-treated samples revealed the influence of these ordered phases on the tensile properties and fractures. Primary carbides, which were identified semi-qualitatively under electron-probe micro-analyzer (EPMA) have been found responsible for void nucleation resulting in large dimples in Alloy 625 with solid-solution matrix. Early stages of Ni2(Cr,Mo) and γ″ and their subsequent growth at 813 K, was responsible for increase in strength without reducing ductility considerably. This behavior has been correlated with the reduction in size and depth of the dimples in the fractographs. Transmission electron micrographs showing three variants of γ″ in three orthogonal directions in the alloy matrix have revealed that the morphology of the ordered phase is lens shaped and the precipitate can grow up to ~150 nm. With the growth of γ″ the alloy is seen to fracture in transgranular cleavage manner following ~23% of uniform strain. Influence of plate shaped, long and thick δ phase on the strength and ductility appears similar to that of γ″. δ—precipitation after 1200 h of isothermal holding at 1123 K is also seen to cause faceted appearance of the fracture surface, which is similar to that of γ″ after 1200 h of isothermal holding at 973 K. However, the facets are seen to be consisting of dimples, which are small and uniformly distributed over the surface.