Influence of HPT and Accumulative High-Pressure Torsion on the Structure and Hv of a Zirconium Alloy

Abstract

The authors previously used the accumulative high-pressure torsion (ACC HPT) method for the first time on steel 316, β-Ti alloy, and bulk metallic glass vit105. On low-alloyed alloys, in particular, the zirconium alloy Zr-1%Nb, the new method was not used. This alloy has a tendency to α → ω phase transformations at using simple HPT. When using ACC HPT, the α → ω transformation can be influenced to a greater extent. This article studies the sliding effect and accumulation of shear strain in Zr-1%Nb alloy at various stages of high-pressure torsion (HPT). The degree of shear deformation at different stages of HPT was estimated. The influence of various high-pressure torsion conditions on the micro-hardness and phase composition by X-ray diffraction (XRD) of Zr-1%Nb was analyzed. It is shown that at high-pressure torsion revolutions of n = 2, anvils and the specimen significantly slip, which is a result of material strengthening. It was found that despite sliding, regular high-pressure torsion resulted in the high strengthening of Zr-1%Nb alloy (micro-hardness more than doubled), and after high-pressure torsion n = 10, up to 97% of the high-pressure ω-phase was formed in it (as in papers of other researchers). Accumulative high-pressure torsion deformation leads to the strongest transformation of the Zr-1%Nb structure and Hv and, therefore, to a higher real strain of the material due to composition by upsetting and torsion in strain cycles.

Slippage during high-pressure torsion of commercially pure titanium and application of accumulative HPT to it

“…The specimen became of complex shape, but no significant shear of the upper and lower surfaces of the specimen was found. Let us note that in the same experiment with Cu, the appearance of Cu segments after combined HPT at θ=90º displaced relative to each other by θ=90º [16]. Hence, the real shear strain Ti after HPT n=1/4 was much lower than  1 =8, which is predicted by formula (1).…”

“…To determine the real achieved strain by torsion, two Ti segments were subjected to joint HPT by a quarter of a turn according to the scheme (Figure 2, a) [16]. The specimen became of complex shape, but no significant shear of the upper and lower surfaces of the specimen was found.…”

“…a) b) c) Figure 2. Scheme of combined HPT of two segments to find the degree of shear strain [16] (a), two segments of the Ti disc, prepared for joint HPT (b), segments after joint HPT n= ¼ (c). After HPT, micro-hardness of Ti more than doubles (Table 2), which proves a strong refinement of the structure.…”

“…The paper [15] proposed a new method of joint deformation of the halves to evaluate the slippage degree at HPT with which it was shown that shear strain  in hard bulk metallic glass (BMG) based on Zr (YTS>1500 MPa) at HPT with n=0.25-5 was much lower than expected, and while there is no shear strain at HPT, the structure of BMG significantly changed. In [16], slippage at HPT Zr-1%Nb alloy was studied, and it was shown that the specimen of Zr-1%Nb at the initial HPT stages (n=1, n=2) received a significant degree of torsional deformation, However, after HPT with more than 5 revolutions, torsional deformation was not implemented due to slippage. After HPT n=10 the structure of the same alloy is slightly finer, but the defect density is much higher compared to the same alloys after HPT n=5.…”

“…As said above, for usual HPT conditions, when n rises in such materials, anvils start to slip. Previously ACC HPT was applied to steel 316, β-Ti, Zr-1%Nb [16,18]. It was shown that ACC HPT led to a stronger refinement of the structure of these materials than conventional HPT.…”