MICROSTRUCTURAL EVOLUTION AND MECHANICAL-PROPERTIES OF A CR-CR2HF ALLOY

The cast, homogenized and forged microstructures of a two-phase Cr-Cr2Hf 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 Cr2Hf 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 Cr2Hf. 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 approximately 7 MPa square-root m at 293 K that increased almost linearly with temperature to approximately 15 MPa square-root 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.