Fracture and Fractography of Ordered Ni 4 Mo
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FRACTURE AND FRACTOGRAPHY OF ORDERED Ni 4 Mo H. P. Kao*, C. R. Brooks** and M. Sanganeria** *Chung-Shan Institute of Science and Technology, Lung-Tan, Taiwan, Republic of China "**Materials Science and Engineering Department, The University of Tennessee, Knoxville, TN 37996-2200 Abstract The domain structure of the alloy Ni-20 at. % Mo depends upon the heat treatment below the critical temperature (868 0 C). Upon aging around 750-850 0 C, initially a very fine domain structure forms inside the a grains, which then coarsens, and then migration of these boundaries occurs with a coarser domains structure behind them. In the ordered condition, the alloy is very brittle. Fractographs are presented which correlate the fracture morphology to the microstructures of the ordered alloy, showing that fracture occurs along the migrated, former a grain boundaries. Introduction There is considerable interest in ordered alloys, as many are strong and show potential for use as high temperature structural materials [1]. One such alloy is Ni-20 at. %Mo, for which the ordering reaction and strengthening have been extensively studied. The physical metallurgy and mechanical properties of this alloy (and other NiMo alloys) have been reviewed [2]. Although this alloy strengthens considerably upon ordering, a serious limitation of the use of it in the ordered condition is its extreme brittleness. The alloy Ni-20% Mo undergoes an ordering reaction below 868 0 C from a shortrange ordered (SRO), FCC structure (c) to a tetragonal structure (B). This reaction occurs by the ordering of the Ni and Mo atoms on the FCC lattice, so the B retains a close-packed structure and a crystallographic relationship to the former a. This leads to six crystallographic variants of B and three types of domain boundaries. The a phase can be retained by rapid cooling from above 8680 C, and then the rate of ordering controlled by choice of the aging temperature and time. During aging in the range 600850 0 C the strength increases, then passes through a maximum. Accompanying this strengthening is a continuously increasing brittleness, with fracture occurring along the high angle, former a boundaries [2]. The microstructural development during aging involves first the formation of very fine B domains in the a grains. These domains then grow, but in the range 700-800 0 C an independent grain boundary reaction commences, which involves the movement of the high angle boundaries [2]. Examination of the microstructure of the samples using transmission electron microscopy has established that the grain boundary reaction involves the migration of the high angle boundary, with the concomittant formation of a considerable coarser domain structure behind it [3]. This boundary migration, initiating along all boundaries, eventually leads to contact between advancing interfaces. Since these are the migrated, high angle, former a boundaries, at this stage the structure consists of a grain size less than that present prior to the grain boundary migration. Note that the use of the te
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