Elsevier

Materials Science and Engineering: A

Volume 648, 11 November 2015, Pages 443-451
Materials Science and Engineering: A

Compressive behaviors and mechanisms of TiB whiskers reinforced high temperature Ti60 alloy matrix composites

https://doi.org/10.1016/j.msea.2015.09.026Get rights and content

Abstract

The hot deformation behaviors of TiB whiskers reinforced Ti60 alloy matrix composites with network microstructure were investigated by hot compression tests in the temperature range of 900–1050 °C and strain rate range of 0.001–1 s−1. The flow stress curves exhibited obvious flow softening phenomenon at low strain rates in the α+β phase region, which was mainly attributed to the globularization of α phase. The apparent activation energy of the composite was calculated to be 681 kJ/mol in α+β phase region, which is much higher than that of Ti60 alloy due to the network distribution of TiBw reinforcement. The constitutive equation for the composites in α+β phase region was also established. The processing maps of the composites exhibit one peak efficiency of power dissipation at 950 °C/0.03 s−1 about 57%, which corresponded to globularization or dynamic recrystallization of lamella α phase. The equiaxed α grain preferably formed at TiBw-rich region (network boundary), which indicated that the TiBw can promote globularization of lamella α phase. In the instability region, inhomogeneous deformation and macroscopic crack can be observed. Additionally, the EBSD results reveal that the large equiaxed α grains contain lots of low angle grain boundaries (LAGBs), which indicates that globularization of lamella α phase is attributed to the continuous dynamic recrystallization (CDRX) mechanism. In single β phase region, the fraction of LAGBs is decreased with decreasing the strain rates, which indicates that DRX of β phase occurs at low strain rate.

Introduction

During the past decades, a series of near α high-temperature titanium alloy have been developed by many countries to meet the requirements of aerospace lightweight structural materials, such as IMI834 (UK), Ti-1100 (USA), Ti-60 (China) and BT36 (Russia) alloys [1], [2]. However, the service temperature of these titanium alloys is not more than 600 °C. One of the major problems hindering their application is insufficient strength at more than 600 °C. Comparing with conventional titanium alloys, titanium matrix composites (TMCs) exhibit higher specific strength, higher specific modulus and excellent high temperature durability [3], [4], [5]. In particular, by selecting near-α titanium alloy (Ti60) as matrix, the TiBw/Ti60 composites with network microstructure showed a surprised enhancement of high temperature strength, which can be expected to meet the requirements of aerospace lightweight structural materials between 700 °C and 800 °C [6]. Near α Ti alloy always exhibits more sensitivity to processing parameters than other Ti alloy due to narrow processing window [7]. Furthermore, ceramic reinforcements such as TiBw play important role on the evolution of microstructure and properties of TMCs, especially for the composites with the network microstructure. Therefore, it is necessary and meaningful to understand compressive behaviors and mechanisms of TiBw/Ti60 composites with a network microstructure during the hot deformation under different parameters.

Extensive researches aimed at guiding hot-working parameters have been carried out on the deformation behaviors of various titanium alloys and titanium matrix composites. Zeng et al. [8] analyzed hot deformation behavior of Ti60 alloy in the temperature range of 970–1120 °C and strain rate range of 0.01–10 s−1. They suggested that the flow softening was caused by break-up and globularization of lamellar α in α+β field. While in the β field, flow softening was caused by dynamic recovery (DRV) and recrystallization (DRX). In Shibayan's work [9], the flow stress curves of Ti6Al4V alloy and Ti6Al4V–0.1B alloy were compared. It was pointed out that the softening temperature decreased about 50 °C due to the boron addition. The processing map based on the dynamic materials model (DMM) have been proved to be useful to optimize the hot workability and control the microstructure. The instability regions of TiCp/Ti662 composites are quite different with those of Ti662 alloys according to Poletti work [10]. Ma et al. [11] pointed out that the variation in activation energy was caused by different deformation mechanisms of the composites, which was affected by TiC particles and the variation in the volume ratio of α/β phase. Although there were so many researches for Ti alloy and TMCs with homogeneous microstructure, no research on compressive behaviors and mechanisms of TiBw/Ti60 composites with network structure has been reported.

The objective of the present investigation is to study the hot deformation behaviors of TiBw/Ti60 composites with network microstructure using the compression tests. The processing maps of the composites are also constructed to determine the safe processing window using the compressive data. Additionally, the microstructures of the composites corresponding to different regions of processing map were observed, as well the deformation mechanism was analyzed.

Section snippets

Material and experimental procedures

In the present study, 3 vol% TiBw/Ti60 (Ti–5.8Al–4.0Sn–3.5Zr–1Mo–0.85Nd–0.4Si) composites with a novel network microstructure were fabricated by reaction hot pressing (RHP). Large spherical Ti60 powders (D=110 μm) and fine prismatic TiB2 powders (3 μm) were low energy milled and then hot pressed in vacuum. During low-energy milling process, TiB2 powders were adhered onto the surface of the large Ti60 particles, however, Ti60 particles were not smashed. This was beneficial to the formation of the

Flow stress–strain curves

The true stress–strain curves of TiBw/Ti60 composites at various conditions are presented in Fig. 2. All the flow curves exhibit peak flow stresses and then flow softening. The peak flow stresses of the composite are listed in Table 1. According to experience and testing results, the errors are always less than 3%, which indicates that the experimented data are stable and reliable. As expected, the peak flow stresses decreases with increasing the deformation temperatures and the decreasing the

Conclusions

(1) The flow stress curves exhibit flow softening at low strain rates in the α+β phase region, which is attributed to the dynamic recrystallization or globularization of lamellar α phase with the CDRX mechanism. The degree of flow softening decreases with the increase of the deformation temperatures and the reduction of the strain rates.

(2) The constitutive equation of TiBw/Ti60 composites is established. The apparent activation energy is calculated to be 681 kJ/mol in the temperatures ranging

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (NSFC) under Grant nos. 51471063 and 51271064, the High Technology Research and Development Program of China (863) under Grant no. 2013AA031202, the Fundamental Research Funds for the Central Universities (Grant no. HIT.BRETIII.201401) and China Postdoctoral Science Foundation funded project (Grant no. LBH-TZ0506).

References (21)

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