Axial fatique behavior of binder-treated versus diffusion alloyed powder metallurgy steels

A comparative study has been conducted on the microstructure, tensile, and axial fatigue behavior of two Fe-0.5Mo-1.5Cu-1.75Ni alloys, made by binder-treated and diffusion alloying processes. The mechanical properties will be explained in terms of the pore size and morphology, as well as the heterogeneous microstructures typical of ferrous powder metallurgy materials. Binder treatment can provide a variety of advantages in manufacturing, over diffusion alloyed powders, including faster and more consistent flow into the die cavity, increased green strength, and reduction of fine particle dusting. In addition to conventional porosity, smaller, "copper diffusion" pores were observed where copper particles had been prior to forming a liquid phase during sintering and diffusing into the Fe particles. The microstructure in both alloys was typical of P/M alloy steels, with a heterogeneous microstructure consisting of areas of "divorced pearlite," martensite, and nickel-rich ferrite. The modulus and tensile strength of both types of alloys were equivalent. Yield strength in the binder-treated alloy was higher which coincided with somewhat lower ductility. The fatigue behavior in terms of stress versus cycles (S-N curves) was almost identical for the two systems. Fractographic observations showed fracture to have initiated primarily at pore clusters in the surface region. Fracture surfaces after fatigue tests showed ductile fracture in the interparticle bridge regions, cleavage facets in pearlitic regions, and striations.