The cast, homogenized and forged microstructures of a two-phase Cr-Cr2
Hf alloy (Cr-6.5 at.% Hf) were characterized via optical, scanning and
transmission electron microscopy. The as-cast microstructure was subs
tantially broken down by the hot working operation and subsequent heat
treatment led to further refinement in the second-phase morphology. I
n the forged condition, the eutectic Cr2Hf had the C14 structure; in a
ddition, precipitation on a fine scale was observed in the Cr phase du
e to solid state decomposition, and these fine precipitates were tenta
tively identified as C15 Cr2Hf. Extended heat treatments at high tempe
ratures (1273-1373 K) led to the metastable C14 eutectic phase transfo
rming to the C36 phase. Compression specimens from the forging, tested
in the temperature range 293-1473 K, exhibited a yield strength-tempe
rature profile characteristic of BCC alloys. Four-point bend tests wer
e conducted as a function of temperature and strain rate to obtain an
estimate of the ductile-to-brittle transition temperature. Notched ben
d specimens tested as a function of temperature provided a toughness o
f approximately 7 MPa square-root m at 293 K that increased almost lin
early with temperature to approximately 15 MPa square-root m at 873 K.
The resulting fracture surfaces were examined in the scanning electro
n microscope. The measured properties were correlated with the observe
d microstructures; microstructural changes that accompany the thermal-
mechanical processing of such an alloy were understood.