Mechanical behaviour of a bimodal microstructured powder metallurgy Ni3Al

The tensile and compressive properties at room temperature of powder metall urgy Ni3Al synthesised using rapidly solidified powder particles, which wer e milled for up to 20 h, have been investigated. Furthermore, the mechanica l behaviour at high temperatures has been studied by strain rate change tes ts in compression at strain rates ranging from 5 x 10(-6) to 2 x 10(-3) s(- 1) and temperatures from 1073 to 1373 K. For comparative purposes, tensile tests at room temperature were carried out in materials consolidated from p articles in both the as rapidly solidified condition (RS) and after 2 h of milling time. Both materials showed a very similar microstructure composed of equiaxed grains. However, an increase in the yield strength of close to 40% was exhibited by the 2 h milled particle material. The intermetallic ma de of 20 h milled particles presented a bimodal microstructure and showed a dramatic increase in yield strength at room temperature to 740 MPa, more t han twice the value reached by the material consolidated with RS particles. The analysis of the stress-strain rate data of tests performed at elevated temperatures revealed a transition from a stress exponent n approximate to 5 to an exponent n approximate to 3.5 at low stresses and/or high temperat ures of testing. An activation energy for plastic flow of 370 kJ mol(-1) wa s deduced in the low stress regime.