Effects of microstructure on the erosion of Al-Si alloys ny solid particles
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INTRODUCTION
IT is now well documented t-t~ that the mechanism of material removal by impacting solid particles on targets whose microstructure consists of a single phase is in general different for ductile materials compared to brittle materials. For ductile materials, volume loss at small angles of impingement is considered to occur by micro-machining* of highly *The term, machining, is here used loosely to include all shear-type cutting mechanisms.
deformed (ploughed) surface material and at high angles by the ductile fracture of micro forged or extruded material (platelets). 5's-16Volume loss in brittle materials generally occurs by indentation cracking and chipping at all angles, 1-7,1-/ although plastic deformation or sheafing mechanisms may also occur at shallow angles2 due to the high compressive and shear stresses which may be generated beneath the impacting particle or at the crack tip. In general, the erosion of ductile materials is characterized by a maximum erosion rate at shallow angles of impingement (ductile signature) and that of brittle materials at normal impact (brittle signature); however, this behavior is not universal. For example, nominally brittle materials can exhibit maximum erosion rate at shallow angles as the impacting particle size is decreased, zs'19 and nominally ductile materials may exhibit maximum erosion rate at normal incidence for spherical
particles .20 In contrast to the situation for single phase materials, only little is known concerning the erosion behavior of materials whose microstructure consists of both ductile and brittle phases in significant fractions, which structure is commonly found in commercial alloys. Previous work by the present authors2~'22 on sintered WC-Co alloys in the range of 8 to 21 vol pct Co eroded by 240 grit (40/xm) A1203 particles revealed that the erosion rate vs angle of particle incidence Y.W. SHIN, formerly with the Department of Materials Engineering, North Carolina State University, is Research Scientist, Korea Advanced Energy Research Institute, Taejun, Chungnam, Korea. G.A. SARGENT is Dean of Engineering, University of Dayton, Dayton, OH 45469. H. CONRAD is Professor, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907. Manuscript submitted December 23, 1985. METALLURGICALTRANSACTIONSA
exhibited a brittle signature. Further, the erosion rate approximated a linear rule of mixtures. Although some effect was indicated, no clear influence of Co mean spacing or WC grain size on erosion rate was established. In more recent work on WC-Co alloys, 23'24 it was found that the signature of erosion rate v s angle of particle incidence changed from brittle to ductile, and v i c e v e r s a , depending on changes in the size of the microstructural constituents and the impacting particle size and velocity. Similar behavior was noted for fully-aligned composites consisting of A1203 rods in a stainless steel matrix. 23 The changes in signature were linked to the relative size of the impact damag
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