Improvement of the ductility of a bimodal microstructured PM Ni3Al by thermal treatment

The effect of a thermal treatment at 1523 K on the microstructure and mecha nical properties of powder metallurgy (PM) Ni,AI with bimodal microstructur e synthesised by a route involving rapid solidification, milling of powders and consolidation by hot isostatic pressing has been studied. The microstr uctural evolution of the material after 4, 24 and 120 h at 1523 K compared with the material after consolidation has been characterised by grain size measurements and microhardness tests. The intermetallic annealed for 4 h sh owed a bimodal microstructure that was very similar to that exhibited by th e material after consolidation, although a slight grain growth took place. A more homogeneous microstructure was observed after annealing for 24 and 1 20 h. The tensile behaviour of the material annealed for 24 h was better th an that csf the hipped material. The material tensile tested at room temper ature broke above 1100 MPa with 18% of elongation to failure. An increase o f these values was observed at a testing temperature of 473 K. Above this t emperature, the material became brittle, breaking in the elastic regime at 673 K. It has been demonstrated that this microstructure can improve the du ctility and therefore the formability of PM Ni3Al, while keeping the streng th values high. (C) 2001 Elsevier Science B.V. All rights reserved.