Gradient nano microstructure and its formation mechanism in pure titanium produced by surface rolling treatment
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rge depth of 800 lm gradient microstructure was produced in pure titanium (Ti) by means of surface rolling treatment (SRT). The microstructural characteristics with different depths from the top surface were analyzed by optical microscopy, transmission electron microscopy, and electron backscattered diffraction. The results showed that, on the outmost surface, nanograins with an average grain size of ;100 nm were achieved. In the subsurface, i.e., the deformation twinning region, a large volume fraction of {1012} deformation twins together with a low fraction of {1122} and {1124} twins were identified. Based on the microstructural analysis, the grain refinement mechanisms with increasing strain are summarized as: (i) prior division by deformation twinning, (ii) refinement effect of subgrain boundaries resulting from the accumulation of high density of dislocation, and (iii) transformation effect from low angle grain boundaries to high angle grain boundaries. The results of tension tests also show that the titanium sample after SRT shows higher strength than the as-received titanium sample.
I. INTRODUCTION
Commercial-purity titanium (Ti) has been widely used in medical industry owing to its high ratio of strength and weight, high corrosion resistance, and excellent biocompatibility.1 However, it has a low yield stress, ultimate strength, and fatigue limit due to its coarse grain (CG) size, which restricts its application. According to the Hall–Petch law,2,3 grain refinement is an effective way to enhance both strength and ductility of metals and alloys. Severe plastic deformation (SPD), including equal channel angular pressing (ECAP), high press torsion, accumulative roll-bonding (ARB),4 and surface nanocrystallization (SNC),5 has proved to be one of the effective techniques to produce ultrafine grains (UFG, 100 nm , d , 1000 nm) and nanograins (NG, d , 100 nm).6 The strength of UFG/NG materials produced by SPD can be significantly improved. Recent research7,8 shows that a gradient UFG/NG microstructure produced on the surface of Ti by SNC can possess excellent strength and fatigue life. The results in literature9 demonstrated that the gradient UFG/NG layer has a crack-stopping effect to prevent catastrophic failure. Wen et al.10 found that the pure Ti after a two-step treatment including surface mechanical attrition treatment (SMAT) combined with thermal oxidation process showed excellent wear-resistance and biocompatibility. The SNC technique includes shot peening, cryogenic burnishing, SMAT, surface mechanical grinding treatment a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.19 J. Mater. Res., Vol. 29, No. 4, Feb 28, 2014
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(SMGT),11 and low plasticity burnishing.12,13 The SPD behavior of Ti has been extensively studied during ECAP14 and ARB,15 which are often carried out at 450–500 °C to avoid the billet to be broken. In fact, the dislocation slip and shear bands are activated, and the deformation twinning is su
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