The recrystallization behavior of surface deformation layer of (TiB + TiC)/Ti–6Al–4V composite during isothermal annealing

Abstract

The recrystallization behavior of surface deformation layer of (TiB + TiC)/Ti–6Al–4V and Ti–6Al–4V were both investigated during isothermal annealing using X-ray diffraction line profile analysis. The surface deformation layer was introduced by shot peening treatment. The results revealed that with increasing the time of isothermal annealing, the microstructure variations at shot peened layer were obvious. Based on the results of line profile analysis, the recrystallization activation energies were calculated by computer regression analysis, and it showed that the recrystallization activation energy of (TiB + TiC)/Ti–6Al–4V was larger than that of Ti–6Al–4V, which was ascribed to the effect of reinforcements hindering the movements of dislocations, grain and subgrain boundaries in the process of recrystallization. The hindrance effect of reinforcements as sink sources of dislocations gliding resulted that the decrease rate of dislocation density of the composite was slower than that of the matrix. In addition, the relaxation activation energies were obtained according to the analysis of microstrain relaxation, and after isothermal annealing, the depth distribution of domain sizes from the top surface was investigated and discussed in detail. According to above analysis, the results showed that the thermostability of the composite was higher than that of the matrix because of the effect of reinforcements.

Introduction

At present, the research on titanium matrix composites becomes more and more significant and promising because of their excellent physical and mechanical properties [1], [2], [3], [4]. However, in order to improve their fatigue strength and life, crack initiation and growth at surface layers must be suppressed using surface treatments [5]. Shot peening (SP) is an effective surface mechanical treatment method widely used in industry which can improve surface properties, such as fatigue strength, stress corrosion resistance, surface hardness, etc. [6], [7]. However, the physical metallurgical processes such as recrystallization and grain growth, which happens at high temperatures can deteriorate the properties of shot peened components. Thus, the investigation on the thermostability of microstructure at elevated temperatures is important for retaining high properties of titanium matrix composites.

Till now, the researches on titanium matrix composites are mostly on the mechanical properties [3], [4], [8], [9], little investigation has been made on recrystallization behavior and thermostability of composite materials after SP together with heat treatment. And X-ray diffraction (XRD) line profile analysis is an important method for describing the microstructure variations of crystalline materials because XRD is reliability and non-destruction to materials [10], [11], [12], [13]. Therefore, the present investigation deals with the recrystallization behavior of deformation layer of in situ (TiB + TiC)/Ti–6Al–4V and Ti–6Al–4V during isothermal annealing using XRD line profile analysis.