The effect of solidification rate on structure and high-temperature strength of the eutectic NiAl-Cr

Abstract

The eutectic NiAl-Cr, consisting of chromium rods in a NiAl matrix, was directionally solidified at rates varying from 1/4 to 30 in. per hr. The inter-rod spacing and elevated temperature tensile properties were measured and the structure determined for each solidification rate. The spacing, λ, obeyed the relation λ²V= Constant, whereV is the solidification rate. AtV greater than 1 in. per hr, a cell or colony structure formed and the cell size decreased with increasing solidification rate. At 600°C, the tensile strength increased substantially with decreasing inter-rod spacing, reaching 100,000 psi at the highestV. At 800° and 1000°C, the strength first increased then decreased at the highest solidification rates reaching maximum strengths of 63,000 psi and 47,000 psi, respectively. At 1200°C, the strength decreased slightly with increasing solidification rate. Fracture occurred by shear along grain boundaries and cell walls. The decreased strength at the highest solidification rates may be related to the increased cell boundary area.