Elsevier

Surface and Coatings Technology

Volume 244, 15 April 2014, Pages 69-77
Surface and Coatings Technology

The variations of microstructures and hardness of titanium matrix composite (TiB + TiC)/Ti–6Al–4V after shot peening

https://doi.org/10.1016/j.surfcoat.2014.01.053Get rights and content

Highlights

  • The reinforcements were yielded to the strain of matrix and inclined.

  • The inclination directions were random due to the random strain of matrix.

  • The variations of hardness were investigated via the nano-indentation method.

  • The indenter can avoid the reinforcements, just testing the hardness of matrix.

Abstract

After shot peening (SP), the variations of microstructures and hardness of (TiB + TiC)/Ti–6Al–4V were investigated. The microstructures of the surface and cross-section were studied, and the results revealed that the convex and concave features appeared on the surface after SP. Based on the microstructures of cross-section, the micrograph showed the random inclination of reinforcements in the surface layer, which illuminated that the reinforcements were yielded to the random deformation of matrix during SP. The variations of hardness before and after SP were investigated via the nano-indentation method. The results indicated that the variations of hardness were caused by the different extent of deformation under surface. All results were discussed in detail.

Introduction

Titanium matrix composites (TMCs) have wide application prospects in the field of aerospace, automobile and other industries because of their good combination of excellent properties such as high specific strength, good ductility, and excellent fatigue properties etc. [1], [2], [3], [4]. However, during the process of manufacturing and subsequent heat treatments, tensile stresses are always generated [5]. As the formed tensile stresses may deteriorate the fatigue properties, in the interest of changing the unfavorable stress states, surface treatments are utilized [6], [7]. As an important technology of surface treatments, shot peening (SP) is well known as a method to improve surface properties of metallic components, and in the process of SP, compressive residual stresses (CRS) and work hardening are introduced into the surface layer.

The residual stresses and hardness are very important properties of materials after SP treatment. The distribution of residual stresses of TMCs has been discussed in some previous works in detail [8], [9], [10], [11]. However, in the previous work, the obtained hardness was Vickers' hardness, and the indenter was Vickers' indenter. Because the size of Vickers' indentation was bigger than the size of reinforcement, it was difficult to avoid the reinforcements while hardness testing, and the values of Vickers' hardness were the average values of matrix and reinforcements. But in this work, the nano-indentation method [12] will be adopted, and the nano-indenter can avoid the reinforcements during testing. So the obtained hardness is just the matrix's hardness, which is very useful and different from our earlier studies.

Besides, even though a few previous studies were involved in the hardness of surface layer of TMCs after SP [8], [11], in order to obtain the distribution of hardness along the depth, the thin top surface layer was removed one by one via the method of chemical etch, which was difficult to operate and inconvenient. Some references reported that the measurements of hardness along cross-section were operated easily [13], [14], so in this work, the measurements of hardness using nano-indentation method will be operated on the cross-section after SP. In addition, investigation on the variations of microstructures on the surface and cross-section is significant. Therefore, before and after SP, the microstructures' variations of the surface and cross-section will be observed, and the effects of reinforcements and SP on the variations of microstructures and hardness will be related and discussed.

Section snippets

Preparation of materials and SP treatment

In this work, (TiB + TiC)/Ti–6Al–4V (TiB:TiC = 1:1 (vol.%) was chosen as the sample because of its high specific strength, good ductility and excellent fatigue properties. In addition, the reinforcements of TiB whiskers and TiC particles offered high modulus, similar density to titanium matrix and clean interfaces without any unfavorable reaction between reinforcements and matrix [15], [16], [17], [18], [19], [20], [21]. The samples were fabricated via in situ technology [22], [23], and two types

The variations of microstructures

Three dimensional contours of the surfaces before and after SP have been shown in Fig. 2. Before SP, the surface of matrix Ti–6Al–4V is smooth (Fig. 2(a)); however, there are different surfaces with the addition of reinforcements (Fig. 2(c) and (e)). The reinforcements make the surface rougher, and the higher concentration of reinforcements, the more roughness of the surface. Moreover, the distribution and concentration of reinforcements can be shown approximately in Fig. 2(c) and (e), and the

Conclusions

The variations of microstructures and hardness of (TiB + TiC)/Ti–6Al–4V were investigated. Based on the microstructures, the addition of reinforcements enhanced the surface roughness, and after SP, the surface roughness promoted more, which resulted from the impact of small shot balls on the surface with high kinetic energy. Meanwhile, the micrographs of the cross-section showed the random inclination of reinforcements after SP, which was ascribed to the discontinuous, random pressure events

Acknowledgment

This work is supported by the projects of 973 Program under Grant No. 2012CB619600 and Shanghai Academic Leaders Funding 12XD1402900. The financial support of the China Scholarship Council (No. 2011623074) and the Shanghai International Cooperation Project (08520705300) are gratefully appreciated.

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