Fatigue and fracture of porous steels and Cu-infiltrated porous steels
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INTRODUCTION
POWDER metallurgy (P/M) processes are being explored in a variety of applications, because these techniques can produce parts to near-net shape and with microstructures not easily achievable otherwise. In cases where P/M is utilized, it is of primary importance to ensure that the structure is fully dense, while also reducing the instance of undesired inclusions and foreign particles. Materials containing pores exhibit reduced mechanical properties, especially strength, modulus, and toughness, possibly making them unsuitable for structural applications. The controlling effects of porosity are to reduce the load-bearing area of the structure and to provide local stress concentrations. Secondary processing of sintered parts, such as hot isostatic pressing and forging, can reduce porosity levels, but the additional costs involved may eliminate the economic advantages of P/M over cast processes. An alternate solution to these thermomechanical processes is metal infiltration. Infiltration methods to enhance the mechanical properties of P/M steel parts have been used for over 40 years.[1] Improved machinability and a number of unique benefits related to processing, surface condition, and multicomponent assembly have been realized. In this secondary process, molten metal infiltrates are drawn into pores by capillary force. For P/M irons and steels, copper has been the infiltrant of choice, since molten Cu has a small contact angle PERAVUDH LOWHAPHANDU, Graduate Student, and JOHN J. LEWANDOWSKI, Professor, are with the Department of Materials Science and Engineering, The Case School of Engineering, Case Western Reserve University, Cleveland, OH 44106. This article is based on a presentation made in the symposium ‘‘Fatigue and Creep of Composite Materials’’ presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
with iron which enhances the thermodynamic driving force for infiltration. In addition, the lack of reaction product formation between the Cu- and Fe-based materials facilitates infiltration of the interconnecting pores. The resulting composite structure consists of a ductile metal (e.g., Cu) in a steel matrix which can be heat treated to a variety of strength levels. Much of the work that has been conducted on copperinfiltrated steels has focused on improving the porous compact production and infiltration process. Compaction pressure, sintering temperature, and sintering time were the process control variables optimized.[2–5] The conventional infiltration process has been modified by SCM Metal Products, Inc. (Research Triangle Park, NC), with resulting increases in unnotched impact strength of the infiltrated steels of 500 pct over the conventional copper infiltration method.[6] This has been accomplished by careful control of the processing to minimize porosity, improve the infiltrant properties, and to reduce the diffusion depth of the Cu infiltrant in the matrix
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