Axial fatigue 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 heterog eneous microstructures typical of ferrous powder metallurgy materials. Bind er treatment can provide a variety of advantages in manufacturing, over dif fusion alloyed powders, including faster and more consistent flow into the die cavity, increased green strength, and reduction of fine particle dustin g. In addition to conventional porosity, smaller, "copper diffusion" pores were observed where copper particles had been prior to forming a liquid pha se during sintering and diffusing into the Fe particles. The microstructure in both alloys was typical of PM alloy steels, with a heterogeneous micros tructure consisting of areas of "divorced pearlite," martensite, and nickel -rich ferrite. The modulus and tensile strength of both types of alloys wer e equivalent. Yield strength in the binder-treated alloy was higher which c oincided with somewhat lower ductility. The fatigue behavior in terms of st ress versus cycles (S-N curves) was almost identical for the two systems. F ractographic observations showed fracture to have initiated primarily at po re clusters in the surface region. Fracture surfaces after fatigue tests sh owed ductile fracture in the interparticle bridge regions, cleavage facets in pearlitic regions, and striations. (C) 2001 Elsevier Science B.V. All ri ghts reserved.