Thermal Stability of Microstructure and Mechanical Properties of Ultrafine-Grained Pure Titanium Shankun Chen, Qiaoyan Sun*, Lin Xiao, Jun Sun State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China * Corresponding author: qysun@mail.xjtu.edu.cn Abstract Thermal stability of microstructure was investigated of ultrafine-grained (UFG) pure titanium subjected to anneal at 300oC, 400oC, 500oC and 600oC. Hardness variety was used to evaluate the effect of annealing temperature on mechanical properties in UFG titanium. Microstructural observation showed that the average grain size slightly increased in UFG pure titanium annealed below 400oC, while the dislocation density significantly decreased. As the annealing temperature increased to 500oC and 600oC, the average grain size dramatically increased and dislocation density sharply reduced. The hardness almost remained constant, when the annealing temperature was 300oC, while it gradually decreased, as the annealing temperature increased to 400oC. The hardness dramatically decreased in the samples annealed at 500oC and 600oC. The relationship between the microstructural stability and mechanical property variety in UFG Ti during annealing is discussed. Keywords Ultrafine-grained pure titanium, Thermal stability, Grain size, Hardness 1. Introduction Research on the ultrafine processing of grains in metals and alloys has attracted a lot of attentions since Prof Valiev and co-workers firstly produced the refined microstructures of less than 1 micron, which has a superior high strength, in copper via equal channel angular pressing (ECAP) technique two decades ago [1-4]. Ultrafine-grained (UFG) metals and alloys have demonstrated the higher combination of strength and ductility than the coarse-grained counterparts. The UFG titanium shows higher corrosion resistance than coarse-grained one due to rapid passivation [5]. Several severe plastic deformation (SPD) techniques have been used to fabricate UFG or nano-grained metals. For example, high pressure torsion (HPT) used for Cu, Cu-Zn, and Cu-Sn alloys [6,7], cumulative roll bonding (ARB) used for some fcc and hcp metals [8-10]. Thermal stability is a big concern for ultrafine-grained materials. Majid Hoseini et.al found that UFG pure titanium had a good thermal stability of microstructure below 450oC [11]. The grain growth activation energy (Q) is close to the self-diffusion activation energy in Ti [12,13]. Although the ultrafine processing of pure titanium has been well studied, little information is available on the thermal stability of mechanical properties and microstructure in UFG pure Ti. The objective of this paper is to study microstructural stability and mechanical property variety in UFG titanium subjected to anneal at different temperatures. 2. Experimental procedure The UFG pure Ti sheets of 0.2mm in thickness were produced by repeat-rolling with a reduction of 97.5% in thickness at 450oC. Then these sheets were isothermally annealed at 300oC, 400oC, 500oC
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