Effect of Cold Deformation and Annealing on the Microstructure and Tensile Properties of a HfNbTaTiZr Refractory High En
- PDF / 4,028,992 Bytes
- 17 Pages / 593.972 x 792 pts Page_size
- 82 Downloads / 277 Views
I.
INTRODUCTION
REFRACTORY high entropy alloys (RHEAs) have recently received much attention because of their attractive elevated-temperature mechanical properties.[1,2] This innovative class of alloys includes high-melting-point refractory elements (to achieve high-temperature strength and microstructure stability) and usually a sufficient level of light elements, such as Al, Ti and Zr, to reduce overall material density. Due to their superior mechanical performance, RHEAs are now considered as potential candidates for replacement of advanced Ni-based superalloys, especially in view of the fact that the temperature capability of the latter alloys is rapidly approaching the physical limit defined by their solvus and solidus temperatures. To the present, most RHEAs have exhibited very limited ductility or totally-brittle behavior at room temperature. Notable exceptions
O.N. SENKOV, A.L. PILCHAK, and S.L. SEMIATIN are with the Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433. Contact e-mail: [email protected] Manuscript submitted January 26, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
include RHEAs such as HfNbTaTiZr,[3–6] HfNbTiZr,[7] and HfMoxNbTaTiZr.[8] It should be emphasized, however, that the mechanical properties of most RHEAs reported in the literature have been for material in the as-cast or cast-and-annealed condition. Generally, both the strength and ductility of cast alloys are inferior to those of the corresponding wrought alloys because of the presence of casting defects, large grain size, etc. Thermomechanical processing can often improve mechanical properties by healing casting defects and refining the microstructure. Indeed, noticeable improvements in the tensile properties of a HfNbTaTiZr RHEA after cold working and annealing have been reported recently.[5] For example, this alloy had a yield stress of 820 MPa and tensile fracture strain between 0 and 9.5 pct (depending on the sample position in the casting)[6] in the as-cast condition with a dendritic microstructure. Following cold rolling to 86 pct thickness reduction and annealing at 1000 C for 2 hours, the same alloy exhibited a homogeneous recrystallized microstructure with a yield stress of 1145 MPa and a fracture strain of 10 pct.[5] The HfNbTaTiZr alloy has a single-phase BCC crystal structure in the as-cast condition as well as after cold rolling and annealing at temperatures above 1000 C.[4–6,9] By contrast, second-phase precipitates with a BCC crystal structure,
enriched in Nb and Ta, were identified in the material that was rolled to a reduction of 86 pct and annealed at 800 C for 2 hours[5] or processed by high-pressure torsion (HPT) and annealed at 800 C or 900 C for 1 hours.[10] It is interesting to note that without preliminary cold deformation, the alloy retained a single phase BCC structure during annealing at 800 C for up to 10 hours while a hexagonal phase enriched in Hf and Ta precipitated at sub-grain boundaries during longer annealing times.[11] A hexago
Data Loading...
