Effect of hydrogen content on microstructures and room-temperature compressive properties of TC21 alloy

Highlights

• The amounts of β and δ phases increase with the increase of hydrogen content.

• Ultimate compression of TC21 alloy increases by 30% after hydrogenation.

• Flow stress decreases about 150–200 MPa after hydrogenation.

• The optimum hydrogen content for cold forming of TC21 alloy is 0.9 wt.%.

Abstract

Mechanical tests were carried out at room temperature to reveal the effect of hydrogen content (0.2–1.2 wt.%) on the compressive properties of TC21 alloy. Results show that different hydrogen contents have distinct effects on microstructures and compressive properties of TC21 alloy. With the increase of hydrogen content, the amounts of α phase and acicular α′ martensite phase decrease, while the amounts of β phase and δ hydride increase, δ hydrides distribute along phase/grain boundaries first and then precipitate in β phase. As hydrogen content increases, the yield strength and microhardness decrease first and then increase, and the ultimate compressive strength decreases. The ultimate compression changes little first, and then increases up to a maximum, and finally decreases with the increase of hydrogen content. The optimum hydrogen content is about 0.9 wt.%, with which TC21 alloy exhibits higher plasticity and lower flow stress. Moreover, the fracture mode of TC21 alloys with hydrogen content less than 1.0 wt.% is a ductile fracture, while the fracture mode of the alloy with 1.2 wt.% hydrogen is a transgranular brittle fracture caused by hydride. Reasons for the distinct effect of hydrogen content on the room-temperature compressive properties are discussed.

Introduction

In recent decades, a generic class of titanium-based materials has been developed for a range of applications. Titanium and titanium alloys are currently widespread used in many industries due to their low density, high specific strength, excellent corrosion resistance and biochemical compatibility [1], [2], [3]. Nowadays, in order to meet the requirements of the damage tolerant design criteria in aircraft structural parts, many researches have focused more attention on the high damage tolerant titanium alloys [4]. TC21 alloy (Ti–6Al–2Mo–1.5Cr–2Zr–2Sn–2Nb) is a new kind of α + β damage tolerant titanium alloys with high strength, high fracture toughness and low crack propagation rate. The comprehensive mechanics performance of TC21 alloy is very similar to Ti–6–22–22S alloy [5], [6]. But similar to most titanium alloys, the high processing costs of these high damage tolerant titanium alloys induced by their poor machinability, low plasticity and high deformation resistance are the bottleneck of large scale application [7], [8].

Although high hydrogen content has a devastating effect on mechanical properties of vast majority of metals, the use of hydrogen as a temporary alloying element in titanium-based materials, so-called thermohydrogen processing (THP), is a particular kind of technology for titanium alloy which can refine microstructures, reduce β transus temperature, improve the plasticity, machinability and deforming limits [9], [10], [11], [12], [13]. Wang et al. [14] and Zhu et al. [15] studied the microstructural evolution and phase transformation of TC21 alloy after hydrogenation and found that hydrogen had a noticeable influence on its microstructure. Zong et al. [4] and Zhang et al. [16] found that suitable hydrogen addition decreased the flow stress significantly and improved the workability of TC21 titanium alloy at high temperature. Moreover, as a strong β stabilizer, hydrogen alloying stabilizes more ductile high-temperature body-centered cubic (BCC) β phase and decreases the critical rate [17], which can improve the capacity of cold plastic processing for titanium alloys theoretically. Lately, some work has been done on cold forming of the hydrogenated TC4 alloy [18], [19]. They found the plasticity of TC4 alloy increased after hydrogenation. However, treatments of specimens were complicated, which consisted of hydrogenation and extra heat treatments (solid solution and quenching) in order to stabilize the high temperature β phase to room temperature.

In the present work, compressive test was carried out at room temperature to reveal the effect of hydrogen content on the room-temperature compressive properties of TC21 alloy without extra heat treatment. Compressive test can measure the mechanical performance of TC21 alloy under compressive stress, which is closer to the actual forming process (extrusion, rolling, forging, etc.) of TC21 alloy. The effect of hydrogen content on microstructures and microhardness of TC21 alloy was investigated. Fracture surface morphologies of TC21-xH alloys were also investigated. The optimum hydrogen content for cold forming of TC21 alloy was determined. The plasticization mechanism was discussed. Results can play a favorable role in the cold forming of TC21 alloy.