Volume dependent fracture energy and brittle to quasi-brittle transition in intermetallic alloys

Abstract

Intermetallic alloys such as titanium aluminide (TiAl) are lightweight with superior specific strength and high temperature oxidation resistance. They have a potential to replace heavier nickel based super alloys in low pressure high temperature sections of an aero engine. To facilitate an industry wide implementation, it is however indispensable to develop physically accurate computational methods which account for key deformation and fracture mechanisms. This assists the virtual prototyping required for the new product development using TiAl alloys. In this work, the quasi-brittle and volume dependent behavior of TiAl alloys is experimentally and numerically investigated. A total number of 29 geometrically identical TiAl specimens of three different sizes are tested in a three-point bending setup to investigate the volume dependence of fracture properties. In addition, three compact tension tests with identical specimen geometries are performed to analyze brittle to quasi-brittle transition and a switch in failure mechanism. Since the material fracture is observed to be preceded by plasticity, a theoretical and numerical framework of phase field ductile fracture which accounts for both elastic and plastic work densities is applied. The averaged fracture energy density for each test is evaluated numerically which is found to be lower for larger volumes. The work concludes with the proposal of an empirical law relating the fracture energy density with the specimen volume. Data from literature for different classes of materials is used for validation of the proposed empirical law. The mechanisms of transition from brittle to quasi-brittle fracture are investigated on the basis of local stress triaxiality in three-point and compact tension simulations.