Cryogenic tensile properties and deformation behavior of a fine-grained near alpha titanium alloy with an equiaxed microstructure

Abstract

Different from previous research works on cryogenic titanium alloys with a lamellar microstructure, this study focused on cryogenic tensile properties and the related deformation mechanism of a fine-grained near alpha titanium alloy of equiaxed microstructure. The fine-grained equiaxed Ti–3Al–3Mo–3Zr alloy exhibited a remarkably enhanced cryogenic tensile strength until 20 K, but cryogenic ductility decreased sharply as temperature drops. Ultimate tensile strength jumped from 740 MPa at 298 K to 1290 MPa at 77 K and to 1535 MPa at 20 K, while elongation-to-fracture dropped from 20.0% to 12.5% and to 4.0%, respectively. Sharp decline of cryogenic ductility was considered to be directly influenced by the high-angle misoriented equiaxed α-Ti grains. Because dislocation motion got much difficult and twinning could not operate extensively in the fine equiaxed microstructure at cryogenic temperatures. Besides, slip and twinning could not transfer across α-Ti grain boundaries or α/β phase interfaces, resulting in premature cracking. Different from deformation of traditional counterpart with a lamellar microstructure, prismatic and basal slips invariably dominated at 298, 77 and 20 K. Only in certain α-Ti grains whose <0001> parallel to loading direction, {10 1‾ 2} type tension twins operated at 77 and 20 K. The present fine-grained near α-Ti alloy with an equiaxed microstructure is suggested to be applied at temperature not lower than 77 K.