Comparison of mechanical properties of Ni3Al thin films in disordered FCC and ordered L12 phases

We report the results of several experiments isolating the effect of long-range order on mechanical properties of intermetallic compounds. Kinetically disordered FCC Ni3Al (Ni 76%) thin films were produced by rapid solidification following pulsed laser melting. For comparison, compositionally and microstructurally identical films with ordered L1(2) structure were produced by subsequent annealing at 550 degreesC for 2 h. These FCC and L1(2) Ni3Al thin films were tested by nanoindentation for hardness and Young's modulus, and the critical strain to fracture was measured by straining the substrate under four-point bending. Ni3Al thin films in the disordered phase were found to have nearly twice the critical strain to fracture, more than three times the fracture toughness, and about 20% lower hardness than in the ordered counterpart. Blunter crack tips and crack bridging observed in the disordered phase also illustrate increased ductility. The increased plasticity of Ni3Al due to chemical disorder is manifested both within the grains and at the grain boundaries. Young's moduli of the ordered and disordered materials were found to be indistinguishable. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.