Elsevier

International Journal of Fatigue

Volume 66, September 2014, Pages 127-137
International Journal of Fatigue

Effect of reinforcements on rolling contact fatigue behaviors of titanium matrix composite (TiB + TiC)/Ti–6Al–4V

https://doi.org/10.1016/j.ijfatigue.2014.03.019Get rights and content

Highlights

  • The microstructure variations analyzed with SEM reveal the plastic flows.

  • The reinforcements are prone to flake off and fracture in the damaged layers.

  • The rolling pressure and cyclic stress are adverse to the RCF lives.

  • The reinforcements cause the reduction of RCF lives due to stress concentration.

  • The relative RCF lives are predicted via a new numerical method.

Abstract

Rolling contact fatigue (RCF) behaviors of titanium matrix composites (TiB + TiC)/Ti–6Al–4V are investigated, including the microstructural variables, the stress distribution and the RCF life. The effect of reinforcements on RCF is observed in detail. The microstructural information, especially pertaining to the reinforcements, is obtained through scanning electron microscope. The influence of reinforcements on stress distribution is studied via analyses of subsurface stress distributions based on an approximate numerical method, in which the reinforcement distributions are taken into account. The results reveal that the existence of reinforcements causes RCF life reduction due to the stress concentration in and around the reinforcements.

Introduction

Titanium matrix composites (TMCs) reinforced with ceramic particulates are promising structural materials owing to the combination of low density, high specific strength, high stiffness, and high resistance to thermally induced degradation [1], [2], [3]. Successful implementation of these materials requires a full understanding of their properties. Many connective joints are formed in a structure, in which the surfaces are likely the source of failure initiation because they are usually subjected to variable loading. Therefore, an in-depth rolling contact fatigue (RCF) study of these materials should be conducted.

TMCs can be reinforced with continuous fibers, whiskers or particles [3], [4]. Compared with continuous fibers, TMCs reinforced with whiskers or particles exhibit more isotropic behaviors. The fabrication of these materials is more convenient and cost effective; therefore, they have drawn extensive attentions recently [5]. Titanium monoboride (TiB) whiskers and titanium carbide (TiC) particles offer high modulus, relative chemical stability, and high thermal stability while maintaining similar density and thermal expansion coefficient to those of the titanium matrix, as well as clean interfaces without any unfavourable reaction between the precipitates and the titanium matrix [6], [7], [8], [9]. TMCs co-reinforced with TiB whiskers and TiC particles have been fabricated and investigated for their mechanical properties [10], [11], [12]. This study will be focused on the effect of reinforcements on RCF properties of TMCs (TiB + TiC)/Ti–6Al–4V.

Many studies on RCF lives treated materials as homogeneous materials and developed corresponding prediction models [13], [14], [15], [16]. However, inhomogeneous inclusions usually exist, therefore, material inhomogeneities and their effect on RCF life should be considered [17], [18], [19], [20], [21]. Jalalahmadi and Sadeghi [22], [23], [24] proposed a RCF damage model to incorporate the effect of inclusion microstructures, which suggests that the influence of inclusion distributions should be further explored. Understanding of the RCF of inhomogeneous materials requires the information of the stress distribution related to microstructural information [20], [25], [26]. However, little has been done so far for the RCF of titanium matrix composites, and fewer can be found for the influence of the TiB + TiC reinforcements. Therefore, an in-depth RCF study is needed to investigate the microstructural variations of (TiB + TiC)/Ti–6Al–4V composites in RCF, the stress distributions affected by reinforcement distributions, and their influence on the RCF life of these materials.

Section snippets

Preparation of materials

The materials of (TiB + TiC)/Ti–6Al–4V (TiB: TiC = 1:1 (vol%)) were fabricated via an in situ technology, by which the raw materials of grade II sponge titanium (99%), B4C powder (98%), and graphite powder were melted homogeneously in a consumable vacuum arc re-melting (VAR) furnace to produce titanium–matrix composites by means of high-temperature synthesis reactions [10], [11]. Two total volume fractions of reinforcements, 2% and 3% (TiB + TiC), were obtained, respectively. In order to obtain a

Theory of RCF life prediction

The distributions of subsurface von-Mises stresses were calculated with a simplified numerical method [34] based on the regenerated distribution models of TiB and TiC reinforcements. The detailed distribution models of TiB and TiC will be discussed in Section 4. Two steps are involved in the simplified numerical method: (i) determination of the equivalent eigenstrains for multiple inhomogeneities and (ii) calculation of the disturbed elastic field caused by the inhomogeneities. The advantage of

RCF lives

The RCF lives of the samples obtained from the experiment are shown in Fig. 3. The RCF lives are shown as 7.00-2.36+5.85×105, 1.69-0.59+1.12×105, 0.75-0.28+0.40×105 for Ti–6Al–4V, 2% (TiB + TiC)/Ti–6Al–4V and 3% (TiB + TiC)/Ti–6Al–4V, respectively. It can be found that the result variation of Ti–6Al–4V is higher than that of other materials. Compared to the composite, the average RCF life of Ti–6Al–4V is nearly ten times that of 3% (TiB + TiC)/Ti–6Al–4V. Expansion of the plasticity zone and wear that

Conclusions

This paper reports a study on the rolling contact fatigue (RCF) of (TiB + TiC)/Ti–6Al–4V materials with a focus on the effect of reinforcements on the RCF behaviors via microstructural observations, stress distribution analyses and RCF life comparisons. The damaged layers observed after RCF are about 10–20 μm. Reinforcements flaking and fracture are found in the damaged layers. The experimental results revealed that the average life reduces from 7.00 × 105 cycles for the Ti–6Al–4V matrix to 0.75 × 105

Acknowledgments

The authors would like to acknowledge the support by the Center for Surface Engineering and Tribology at Northwestern University, USA, the State Key Laboratory of Mechanical Transmission at Chongqing University (SKLMT-ZZKT-2012 ZD 03), China, and the scholarships from China Scholarship Council for L. Xie and Q. Zhou (Nos. 2011623074 and 2011605076). Prof. X. Jin acknowledges the startup funding from Chongqing University, China. Zhanjiang Wang would also like to acknowledge the support from

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