MICROSTRUCTURE AND MECHANICAL .BEHAVIOUR OF P/M SUPERALLOYS BASED ON ZhS6-K

Abstract

The present work is aimed at studying the metallurgical behaviour of P/M superalloys which are low in cobalt and are comparatively richer in refractory elements such as tungsten, molybdenum, niobium etc. Chemistry of a high performance Soviet cast superalloy ZhS6-K (Ni-1 0 Cr- 5W-5A1-4.SCo-4Mo-2.STi-0.15C-0.02B) has been modified by slight reduc-tions in carbon, titanium and aluminium and by minor additions of boron, niobium and hafnium. These chemistry variants of the P/M alloys designated as DM-P-1 (with 0.7% Hf and 0.02 % B), D M P-2 (with 0.7% Hf and 0.085% B) and D M P-3 (sigma-prone composition but without Hf) have been prepared by argon atomization. Followed by characteriza- tion, the powders have been hot isostatically pressed at different tempera-tures below and above the r •solvus. It has been observed that unlike carbon, increase in boron does not promote the continuous precipitation at the prior particle boundaries. However, boron beyond 0.02% does narrow down the consolidation temperature range and changes the morpho-logy of particles from cuboidal to dendritic. The precipitation of eutectic Y + structure and boride films at the grain boundaries have been shown to adversely affect the mechanical properties of the high-boron alloy pressed at temperature a little above the 'y solvus. These borides have been analysed to contain tungsten, molybdenum, niobium and chromium but no titanium . Addition of hafnium has also been found to decrease i the alloy solidus reflected by the precipitation of eutectic +'Y structure and formation of hafnium-rich phase at the triple point boundaries. In contrast, the addition of hafnium has been observed to reduce the activity of sulphur. However, the hafnium-free alloy has been found to be relatively less prone to the development of eutectic +~' structure The consolidation of the three alloys at temperatures very close to their 'Y' solvus has produced significant property improvement. Out of these alloys ,sigma-prone E3 f-free alloy has been found to offer the best combination of tensile and stress-rupture properties after a suitable heat treatment. Additions of boron and hafnium have been found to restrict the use of solutioning temperatures above the Y solvus of the alloys thereby de in anding not only a control on the consolidation temperature but also on the solutioning temperatures.