Mechanical behaviour of a bimodal microstructured powder metallurgy Ni3Al

The tensile and compressive properties at room temperature of powder metallurgy Ni3Al synthesised using rapidly solidified powder particles, which were milled for up to 20 h, have been investigated. Furthermore, the mechanical behaviour at high temperatures has been studied by strain rate change tests 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 particles 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 made of 20 h milled particles presented a bimodal microstructure and showed a dramatic increase in yield strength at room temperature to 740 MPa, more than 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 temperatures of testing. An activation energy for plastic flow of 370 kJ mol(-1) was deduced in the low stress regime.