Influence of Recrystallization on Phase Transformation and Texture of Ni47Ti44Nb9 Shape Memory Alloy Bar
Abstract:
The influences of recrystallization on phase transformation and texture of as-hot-rolled Ni47Ti44Nb9 alloy bar with a diameter of 19 mm were systematically investigated using optical microscope (OM) , differential scanning calorimetry (DSC) and X-ray diffraction (XRD) to provide theoretical basis for application of the alloy bar.The results showed that the recrystallization of the hot-rolled bar started obviously at 450 ℃, and recrystallized grain size was decreased and Ms dropped.Temperature ranges of martensitic transformation became narrower and thermal hystereses increased from the center to the edge of the bar vacuum annealed at 850 ℃/90 min F.C.More intensive γ fibre texture was formed in the as-hot-rolled bar.But intensity of γ fibre texture in the bar annealed at 600 ℃/90 min F.C was slightly decreased.When annealing temperature was 850 ℃, γ fibre texture was markedly weakened, the diffusion of the isopycnals occurred, and new texture components were formed, which was beneficial to improving restoration strain of the bar along radial direction.
(DSC sample are cut from 1 to 7 plate samples whose sizes are 10mm×7 mm×2 mm;10 and 17 mm are the length and height of cuboid sample, respectively)
图2 热轧棒在不同条件下距中心 (1/3~2/3) R处且垂直轴向截面的金相组织
Fig.2 OM microstructures of the sample sections normal to axis direction of the as-hot-rolled bar at (1/3~2/3) R sites and under different conditions
(a) Barhot-rolled at850℃, Ais dentritic B2 matrix, B the eutectic of B2 andβ-Nb distributing between B2 phases, C broken B2 dentrite; (b) 450℃/90 min W.C., D are local recrystallized zones; (c) 600℃/90 minW.C, more and more fine grain zones formed by recrystallization; (d) 850℃/90 min W.C.
Fig.3 Variations of HV hardness at the (1/3~2/3) R sites of hot-rolled bar with quenching temperature
图4 850 ℃/90 min真空退火后, 热轧棒从边缘到中心垂直轴向截面的金相组织
Fig.4 Microstructures of sections normal to axis direction of the hot-rolled bar at 850 ℃/90 min F.C. from the edge to center of bar
(a) and (g) Fine recrystallized grains at the edge of bar; (b) and (f) Finer grains close to edge; (c) and (e) Greater grains near the center of bar; (d) Grains with greater size difference at the center of bar
Fig.5 Variations of Ms, As, thermal hysteresis (As-Ms) and martensitic transformation temperature range (Ms-Mf) from edge to the center of bar annealed at 850 ℃/90 min F.C.
Fig.6 Constant φ section chart groups of grain ODF of the as-rolled rods at different conditions
(a) Bar hot-rolled at 850℃; (b) 600℃/90 min F.C.; (c) 850℃/90 min F.C., texture components of A and B points are (201) [122]and (113) [1 10], respectively