POROSITY AND MICROSTRUCTURE IN P/M ALLOYS: CRITICAL REVIEW OF THEIR EFFECTS ON FATIGUE AND FATIGUE CRACK GROWTH

摘要

Porosity and matrix microstructure are two critical factors which control the fatigue and fatigue crack growth behavior of P/M alloys. Various contributions of these factors are conceptually discussed in this paper with examples from the literature when available. Very few previous studies addressed these issues in a systematic manner and generally individual effects can not be easily decoupled, evaluated and understood. Since the information in the literature was generated on different alloys, the following discussions are not based on any particular one and the examples are mainly used to emphasize concepts rather than explain fatigue characteristics of given materials/structures. The effects of porosity are discussed with respect to amount (vol.% porosity), type (open vs. closed) and morphology (size and shape). The effects of microstructure are discussed in terms of microstructural uniformity (homogeneous vs. heterogeneous), type and amount of microstructural phases (martensite, bainite, pearlite, divorced eutectoid, ferrite, Ni-rich areas, etc.) and porosity-microstructure interactions (porosity levels beyond which microstructure effects become dominant). Additional discussions are directed towards the relevance of fatigue crack growth data obtained from long crack growth tests with emphasis on the role of crack closure. The importance of small crack growth is illustrated and methods to evaluate the small crack growth behavior from long crack growth data are reviewed. The way heat treatment (specifically the quenching media) can change closure level and affect fatigue crack growth is also addressed. The porosity and microstructure contributions discussed in this paper are being systematically investigated in a fundamental study at the Metal Processing Institute of Worcester Polytechnic Institute. The study is focused on the fatigue and fatigue crack growth mechanisms of pre-alloyed and admixed Fe-1.8Ni-0.5Mo-0.6C P/M alloys.