MODELING THE EFFECT OF POROSITY ON DUCTILE FRACTURE OF POWDER PROCESSED TITANIUM ALLOY H.J. Niu 1 * and I.T.H. Chang 2 1 Department of Metals and Materials, Jilin University of Technology, Changchun 130025, Peoples Republic of China 2 School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (Received February 19, 1999) (Accepted in revised form May 5, 1999) Keywords: Titanium; Porosity; Fracture toughness; Modeling Introduction Ductile fracture of two-phase alloys occurs by initiation, growth and coalescence of voids which nucleate at second-phase particles. There are two types of particles: primary particles at which voids nucleate first; and secondary particles at which voids nucleate later in the fracture process (1,2). In powder metallurgically processed titanium alloys, there exist inherent microstructural defects of pores and inclusions, which cause the deterioration of the mechanical properties (2– 4). The microstructures of powder metallurgy of titanium alloys consist of Widmansta ¨tten  platelets in a  matrix, and the  platelets orientate along particular crystallographic variants in the same colony (2). The fracture surfaces of the tensile samples and the fracture toughness samples contain many small secondary and large primary voids. These large primary voids are either the residual pores, or these initiated at the inclusions. The / interfaces in the colony boundary are the preferred sites for secondary void nucleation, which is attributed to the strong barriers of the / interfaces for dislocation slip (5) and the anisotropic deformability of the lamellar colonies (6). The growth and coalescence of these residual pores are extremely important in controlling fracture toughness and this process is carried out by the formation and growth of the secondary voids. In this work, a model of void growth based on Wilkinson and Vitek crack theory (7) was used to describe the crack propagation in powder metallurgically processed titanium alloy. In this model, the effect of the stress field of the residual pores on the growth and coalescence of the secondary voids was considered. The purpose is to provide useful insight to the influence of the residual pores after powder metallurgical processing and the subsequently stress-induced secondary voids on fracture of titanium alloys. The Model Fig. 1 shows the scanning electron microscope (SEM) micrograph of the fracture surface of the tensile sample of Ti-6Al-4V alloy produced by the blended elemental powder metallurgy method (8). The *Present address: School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Pergamon Scripta Materialia, Vol. 41, No. 5, pp. 481– 486, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter PII S1359-6462(99)00178-5 481