EFFECT OF DENSITY ON THE MICROSTRUCTURE AND MECHANICAL BEHAVIOR OF POWDER METALLURGY FE-MO-NI STEELS N. Chawla and X. Deng Mechanical Behavior of Materials Facility Arizona State University P.O. Box 876006 Tempe, AZ 85287-6006 M. Marucci and K.S. Narasimhan Hoeganaes Corporation Cinnaminson, NJ 08077 ABSTRACT The microstructure and mechanical properties of Fe-0.85Mo-Ni powder metallurgy (P/M) steels were investigated as a function of sintered density. A quantitative analysis of microstructure was correlated with tensile and fatigue behavior to understand the influence of pore size, shape, and distribution on mechanical behavior. Tensile strength, Young’s modulus, strain-to-failure, and fatigue strength all increased with a decrease in porosity. The decrease in Young’s modulus with increasing porosity was predicted by analytical modeling. Two-dimensional microstructure- based finite element modeling showed that the enhanced tensile and fatigue behavior of the denser steels could be attributed to smaller, more homogeneous, and more spherical porosity which resulted in more homogeneous deformation and decreased strain localization in the material. The implications of pore size, morphology, and distribution on the mechanical behavior and fracture of P/M steels is discussed. INTRODUCTION Ferrous powder metallurgy (P/M) components are typically characterized by residual porosity after sintering, which is quite detrimental to the mechanical properties of these materials [1-7]. The nature of the porosity is controlled by several processing variables such as green density, sintering temperature and time, alloying additions, and particle size of the initial powders [1]. In particular, the fraction, size, distribution, and morphology of the porosity have a profound impact on mechanical behavior [2-7].With an increase in porosity fraction (> 5%), the porosity tends to be interconnected in nature, as opposed to the situation where pores are isolated (< 5%) [1]. Under monotonic tensile loading, porosity reduces the effective load bearing cross-sectional area and acts as a stress concentration site for strain localization and damage, decreasing both strength and ductility [2]. Interconnected porosity causes an increase in the localization of strain in the relatively small sintered regions between particles, while isolated porosity results in more homogeneous deformation. Thus, for a given amount of porosity, interconnected porosity