The role of twinning in the cavitation erosion of cobalt single crystals
- PDF / 5,497,932 Bytes
- 12 Pages / 594 x 774 pts Page_size
- 4 Downloads / 192 Views
IN recent years a concentrated effort has been made to understand the deformation characteristics of materials exposed to erosive environments, l in order to optimize those factors contributing to their erosion resistance. In the case of cavitation erosion, it is observed that the dynamic and localized nature of the stresses generated by the imploding cavities produces a material response which is quite different from that obtained under bulk, quasi-static loading. A striking example of this is that cobalt and its alloys exhibit far greater erosion resistance than other metals and alloys of comparable strength. 2 In fact, pure cobalt is the most erosion resistant metal known. The superior erosion behavior of cobalt alloys, particularly "Stellite 6B", was originally attributed to a strain-induced martensitic transformation absorbing a significant fraction of the incident cavitation energy? However, subsequent experiments on these as well as several brasses and steels have shown that the contribution of the transformation to erosion resistance, if any, is minor. 2,4,5Instead, an improvement in erosion properties parallels a decrease in stacking fault energy (SFE). It has, therefore, been suggested 2 that the planar slip mode in low SFE materials delays the development of localized stresses required to initiate fracture. Alternatively, it has been proposed that the superior erosion resistance of martensitic alloys is derived from the reduction in effective mean free path for dislocation motion produced by the fine dispersion of martensite? Since cobalt has been observed to twin extensively under cavitation exposure, it has been conjectured that a similar reduction in effective grain size by deformation twinning may be responsible for its superior properties. 5 In contrast, twinning in zinc has been found to be highly detrimental to its erosion resistance: cleavage cracks nucleate at grain boundaries and at grain boundary-twin and twin-twin intersections and propagate large distances creating macroscopic pits. 5,6 S. VAIDYA, S. MAHAJAN, and C. M. PREECE are Members of the Technical Staff, Bell Laboratories, Murray Hill, NJ 07974. Manuscript submitted September 14, 1979.
In bcc iron, on the other hand, although mass loss is found to initiate at grain boundaries and twins, detectable erosion damage occurs by the formation of macroscopic pits, which nucleate at grain boundaries and grow by brittle fracture, apparently, unaffected by the presence of twins. 5 It is, thus, clear that the contribution of deformation twinning to erosion characteristics is not well understood. Since twins constitute homogeneously sheared regions, significant stress concentrations can build up wherever their growth is restricted, such as when they terminate within the lattice or impinge on other twins or grain boundaries. In the absence of any stress-relief mechanisms at these junctions, the nucleation of fracture at twins is quite feasible. The present work is aimed at rationalizing the superior erosion properties of cobalt, specifically in te
Data Loading...
