Thermal relaxation of residual stresses in shot peened surface layer of (TiB + TiC)/Ti–6Al–4V composite at elevated temperatures

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Abstract

As an effective and important surface treatment method, shot peening can introduce high residual compressive stress and microstructure variation at near surface deformation layers. In this work, residual stresses relaxation behaviors of the shot peened layer of (TiB + TiC)/Ti–6Al–4V composite were investigated during thermal exposure, and the microstrain was calculated according to the integral breadth after isothermal annealing. The microstrain decreased fast and reached the minimum at 500 °C, which resulted from the thermal recovery and dynamic recrystallization. At elevated temperatures, the residual compressive stresses were relaxed in the whole deformation layers, which were caused by the thermally activated gliding of dislocations. The processes of relaxation can be described using a Zener–Wert–Avrami function and the activation energy of the residual stresses relaxation was higher than that of titanium self diffusion, which was ascribed to the hindrance effects of reinforcements as sink sources of dislocations during annealing.

Highlights

► The residual stresses of shot peened samples are relaxed obviously after isothermal annealing. ► The processes of relaxation can be described using a Zener–Wert–Avrami function. ► The thermal relaxation process is caused by the thermally activated gliding of dislocations. ► The activation energy of relaxation is higher than that of both α-Ti and β-Ti self diffusion. ► The increment of activation energy is ascribed to the hindrance effects of reinforcements.

Introduction

As one kind of metal matrix composites, titanium matrix composites have been widely concerned because of their excellent physical and mechanical properties [1], [2]. However, in the process of manufacturing and subsequent heat treatments, the tensile stresses always generated and deteriorate the fatigue properties [3]. In order to improve their fatigue strength and fatigue life, crack initiation and growth at surface layers must be suppressed using surface mechanical treatments [4]. As an effective and important surface treatment method, shot peening (SP) can introduce high residual compressive stress (RCS) and microstructure variation at near surface layers, which can enhance their fatigue properties compared to un-peened materials. The process of SP involves the bombardment of spherical balls of a hard material against the surface of components, which induces the strong elastic–plastic deformation at the surface and sub-surface regions. In the deformation layers, high RCS and microstructure refinements are introduced after SP.

However, RCS could relax significantly under thermo-mechanical loadings, thermal exposure, static loading and cyclic loading [5], [6]. The stability of the RCS on near-surface regions against thermal and mechanical loading is crucial to the fatigue properties improvement [7]. The relaxation behaviors in traditional alloys and metals have been extensively investigated [8], [9], [10], [11], [12]. But few investigations have been made on the relaxation behaviors in shot peened titanium matrix composites at elevated temperatures. Therefore it is significant to understand the residual stresses relaxation behaviors of titanium matrix composites reinforced with TiB and TiC.

The residual stresses relaxation involves mechanical and thermal relaxation mechanisms [8]. In this work, the thermal residual stresses relaxations of the shot peened (TiB + TiC)/Ti–6Al–4V have been studied and thermal relaxation mechanisms have been discussed in detail. Besides, the residual stress distribution and microstrain variations have also been investigated after isothermal annealing.

Section snippets

Preparation of materials

The samples of (TiB + TiC)/Ti–6Al–4V (TiB:TiC = 1:1 (vol%)) were fabricated via in situ technology [13], [14], [15], and the volume fraction of reinforcements (TiB + TiC) was 8%. Stoichiometric raw materials of sponge titanium, B4C and graphite powder were melted homogeneously in a consumable vacuum arc remelting (VAR) furnace to produce titanium matrix composites via self-propagation high-temperature synthesis. In order to obtain the even composition, the process of melt was three times. The samples

The surface roughness and microhardness after the first and second SP

The surface roughness of titanium matrix composites after first and second SP have been measured and shown in Fig. 2. From the figure, it reveals that after the second SP, the surface roughness decline obviously and the second SP makes the surface smooth after first SP. In addition, the microhardness at the surface after first and second SP has also been tested, and the values are 600.8 HV, 654.4 HV respectively. Compared the microhardness, it can be found that although the microhardness is

Conclusions

The residual stress relaxation behaviors and the variations of microstrain in the shot peened layer of (TiB + TiC)/Ti–6Al–4V composite were investigated during isothermal annealing. The results revealed that the residual stresses were relaxed in the whole deformation layers. At different annealing temperature, the RCS relaxed and the higher temperature, the more obvious of relaxations were. The maximum stress relaxation rate occurred mainly in the initial stage of annealing. Based on the

Acknowledgement

The authors are indebted to Professor Weijie Lu for the kindly guidance.

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