Influence of Temperature and Plastic Strain on Deformation Mechanisms and Kink Band Formation in Homogenized HfNbTaTiZr

Abstract

Due to its outstanding ductility over a large temperature range, equiatomic HfNbTaTiZr is well-suited for investigating the influence of temperature and plastic strain on deformation mechanisms in concentrated, body centered cubic solid solutions. For this purpose, compression tests in a temperature range from 77 up to 1073 K were performed and terminated at varying plastic strains for comparison of plastic deformation behavior. The microstructure and chemical homogeneity of a homogenized HfNbTaTiZr ingot were evaluated on different length scales. The compression tests reveal that test temperature significantly influences yield strength as well as work hardening behavior. Electron backscatter diffraction aids in shedding light on the acting deformation mechanisms at various temperatures and strains. It is revealed that kink band formation contributes to plastic deformation only in a certain temperature range. Additionally, the kink band misorientation angle distribution significantly differs at varying plastic strains.

Kink bands promote exceptional fracture resistance in a NbTaTiHf refractory medium-entropy alloy

“…Consistent with prior reports on Group IV–rich RHEAs in tension and compression, EBSD maps (Fig. 3) showed profuse deformation kink bands ( 20 , 21 , 39 – 42 ). However, the role played by kink bands on the overall behavior in the presence of a sharp crack is not established.…”

“…Tensile tests of this material only add further mystery because the alloy exhibits low uniform tensile ductility (~4%) and a marked absence of strain hardening, which, unlike the face-centered cubic (fcc) high-entropy alloys ( 19 ), indicates that the toughness should be very low ( 20 ). Analysis of the room-temperature deformation structures reveals that, although bulk plastic flow is controlled by the lethargic motion of screw-character dislocations, local ductility can nonetheless be accommodated by collective arrays of edge dislocations, which form pairs of tilt boundaries known as kink bands ( 20 , 21 ). As first described in zinc ( 22 ), the formation of these bands reorients crystalline slip planes along directions of high resolved shear stress, softening the material ( 23 – 25 ).…”

“…In polycrystalline materials, kink bands nucleate in regions of high dislocation density and propagate across grains to relax the associated stress concentration ( 26 ). Their formation in RHEAs with a high concentration of group IV elements ( 20 , 21 ), as well as in pure group V bcc elements ( 27 ), should be detrimental to the toughness, as this mechanism in tension promotes plastic instability by suppressing uniform ductility and is thus associated with the onset of fracture. We demonstrate that in a specifically engineered alloy, kink bands can induce a remarkable response as an exceptionally potent intrinsic toughening mechanism that does not necessarily inhibit high-temperature strength.…”