Microstructural evolution and mechanical properties of a CrCr 2Hf alloy

Abstract

The cast, homogenized and forged microstructures of a two-phase CrCr 2Hf alloy (Cr-6.5 at.% Hf) were characterized via optical, scanning and transmission electron microscopy. The as-cast microstructure was substantially broken down by the hot working operation and subsequent heat treatment led to further refinement in the second-phase morphology. In the forged condition, the eutectic Cr 2Hf had the C14 structure; in addition, precipitation on a fine scale was observed in the Cr phase due to solid state decomposition, and these fine precipitates were tentatively identified as C15 Cr 2Hf. Extended heat treatments at high temperatures (1273–1373 K) led to the metastable C14 eutectic phase transforming to the C36 phase. Compression specimens from the forging, tested in the temperature range 293–1473 K, exhibited a yield strength-temperature profile characteristic of BCC alloys. Four-point bend tests were conducted as a function of temperature and strain rate to obtain an estimate of the ductile-to-brittle transition temperature. Notched bend specimens tested as a function of temperature provided a toughness of ∼7 MPa√m at 293 K that increased almost linearly with temperature to ∼15 MPa√m at 873 K. The resulting fracture surfaces were examined in the scanning electron microscope. The measured properties were correlated with the observed microstructures; microstructural changes that accompany the thermal-mechanical processing of such an alloy were understood.