Thermal stability and corrosion resistance of nanocrystallized zirconium formed by surface mechanical attrition treatmen

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an Lu Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Kom, Kowloon, Hong Kong, China

Wengting Zhang State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China (Received 9 February 2009; accepted 17 June 2009)

The thermal stability and corrosion behavior of the nanostructured layer on commercially pure zirconium, produced by surface mechanical attrition treatment (SMAT), were investigated. It is indicated that the nanograined Zr is stable at annealing temperatures up to 650  C, above which significant grain growth occurs and the grain size shows parabolic relationship with annealing time. The activation energy for grain growth of the nanograined Zr is 59 kJ/mol at 750–850  C, and the grain growth is dominated by grainboundary diffusion. The as-SMATed nanograined Zr exhibits higher corrosion resistance than the 550–750  C annealed SMATed Zr and the unSMATed coarse-grained Zr. It is indicated that the corrosion resistance of Zr tends to increase with the reduction of grain size, which is related to the dilution of segregated impurities at grain boundaries due to grain refinement and the formation of passive protection film. I. INTRODUCTION

Ultrafine grained (UFG, 1–1000 nm) materials are of great interest due to their unique physical, mechanical, and biological properties relative to coarse-grained counterparts.1–5 The issues of thermal stability and corrosion in UFG materials are important for two primary reasons. First, because on the basis of a Gibb’s free energy argument, such fine grains may be metastable, and hence prone to coarsening with a low energy threshold. Second, engineering applications require the consolidation of UFG materials, which typically involves exposure to elevated temperature or corrosive environment. A number of investigations on the thermal stability of UFG materials have been carried out, and the findings reveal that in pure metals produced by severe plastic deformation (SPD), significant grain growth occurs at a relatively low temperature of 0.31 to 0.34 Tm (where Tm is the melting temperature in K).6–8 Although significant grain growth in UFG Ti formed by equal channel angular pressing was reported to occur at 400  C (0.34 Tm),8 it is worth noticing that no significant grain growth was observed in cryomilled nc-Ti at the annealing temperature between 450 and 720  C, a)

Address all correspondence to this author: e-mail: [email protected] DOI: 10.1557/JMR.2009.0368

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J. Mater. Res., Vol. 24, No. 10, Oct 2009 Downloaded: 23 Jun 2014

keeping grain size from 15.2 to 27.5 nm.9 In the case of multicomponent systems, metallic alloys such as Al–Mg,10 Ni-based alloys,11 and Ti–24Nb–4Zr–7.9Sn alloy12 had a relatively high critical temperature, with slow to significant grain growth between 0.5 to 0.65 Tm, due to the barrier effect of two-phase duplex microstructure on grain growth. It is indicated that the thermal stability of UFG metallic materials is strongly dependent