The effect of thermal cycling on the structure and properties of a Co, Cr, Ni-TaC directionally solidified eutectic alloy

Abstract

A Co, Cr, Ni matrix, TaC reinforced, directionally solidified eutectic alloy was tested under conditions of thermal fatigue. The material was cycled between 427 and 1093°C for up to 5000 cycles. The stress rupture properties are decreased by thermal cycling. This loss in stress rupture properties varies with the number of cycles; the initial loss in properties at about 200 cycles is relatively high. The stress rupture strength improves with further cycling up to 2000 cycles and decreases for a larger number of cycles. The TaC fibers in the cycled material developed surface serrations which increased with maximum cycling temperature and number of cycles. The formation of these serrations is attributed to the increasing solubility of the TaC fibers in the matrix with temperature. Since the easy growth planes of the TaC fibers were not coincident with the fiber faces exposed to the matrix, the TaC which was dissolved and reprecipitated during thermal cycling formed these serrations on the easy growth planes of the fibers. These serrations act as notches which reduce the tensile strength of the fibers.