Microstructure and superplasticity of TA15 alloy

doi:10.1016/j.msea.2014.03.117

Materials Science and Engineering: A

Volume 606, 12 June 2014, Pages 401-408

https://doi.org/10.1016/j.msea.2014.03.117 Get rights and content

Abstract

Superplasticity of TA15 alloy was investigated by constant strain rate tensile method in this work. In order to enhance superplasticity, thermo-mechanical techniques were applied for refining the grains of the alloy first. The superplastic tensile tests were carried out on a SANS CMT4104 electronic tensile testing machine at temperatures ranging from 780 to 950 °C and strain rates from 3.3×10⁻⁴ to 1.1×10⁻² s⁻¹. The tensile elongation-to-failure values between 188% and 1074% were obtained. Microstructure evolution after superplastic deformation was also analyzed by optical microscope (OM) and transmission electron microscope (TEM). The micrographs show that the grains were coarsened after deformation, and α→β phase transformation took place at 950 °C, which resulted in the worst superplasticity. Extensive strain hardening stages were observed in the true stress–strain curves due to high dense dislocations in the thermo-mechanically processed alloy and dynamic grain growth during superplastic deformation. The strain rate sensitivity m and the activation energy values at various deformation conditions were calculated, respectively. Based on an analysis of the above studies, it may be inferred that grain boundary sliding (GBS) in TA15 alloy is accommodated by grain boundary diffusion at high temperatures and low strain rates, and the accommodation process involves dislocation glide creep at low temperatures and high strain rates.

Introduction

Titanium alloys are well known for their potential as superplastic materials for application in various areas including transportation, chemical and bio-medical fields, especially in aerospace field due to their lower density, excellent mechanical properties as well as good high heat-durability [1]. However, formability of these alloys is limited because they are sensitive to temperature in the hot forging process. Since superplastic deformation is characterized by low flow stress and high uniformity of plastic flow, superplastic forming (SPF) appears to be a promising way for fabricating the parts with complex geometry, and it also offers remarkable savings of cost and weight over other conventional manufacturing methods [2]. In the past few years, the superplastic properties of many titanium alloys have been investigated increasingly, whereby more efforts were concentrated on two-phase titanium alloys, especially Ti–6Al–4V alloy [3], [4], [5], [6], [7]. There are limited studies on the superplasticity of single phase titanium alloys in the open literatures, especially α and near α titanium alloys.

TA15 alloy as a kind of near α titanium alloys is usually used for manufacturing structural components of aircraft, such as gas turbine blades, nacelle center beam frame, large bulkheads, etc., due to its moderate strength at room and high temperature, good welding performance and excellent high thermal stability [8]. The plasticity of TA15 alloy is relatively inferior to that of two-phase titanium alloys because its microstructure mainly consists of single α phase [1]. It is difficult to form the complex geometry of large parts by conventional forging because of its high deformation resistance and narrow range of deformation temperature, which limits wide applications of the alloy in aerospace industry. Moreover, research reports on the superplasticity in TA15 alloy are very few at present. Therefore, a better understanding of the superplasticity in the alloy is important for the successful introduction of this material for wider industrial applications. In the present work, the superplastic mechanical properties and the influential factors of TA15 alloy are investigated, and possible superplastic mechanisms of the alloy are finally discussed based on related studies.

Section snippets

Experimental procedures

TA15 alloy used in the research was supplied by Beijing Institute of Aeronautical Materials. The nominal chemical composition of as-received material is given in Table 1 [9]. The β-transus temperature of the alloy was about 982 °C with quantitative metallography technique. In order to enhance the superplasticity of TA15 alloy, thermo-mechanical processing was applied to obtain the microstructure with fine and equiaxed grains, in which three-dimensional upsetting and cogging were performed to

Microstructure evolution during thermo-mechanical treatment

Fig. 2 shows the microstructure of as-received materials. As indicated in Fig. 2, in the initial state, the microstructure of TA15 alloy is very non-homogeneous, which consists of a number of coarse strip-shaped α phase and a small quantity of thin strip-shaped α phase on β-matrix. As shown in Fig. 3, severe plastic deformation (SPD) by thermo-mechanical treatment leads to significant microstructure refinement in the alloy. From Fig. 3(a), it can be seen that the microstructure is very fine and

Discussion

The obtained results indicate that superplasticity in TA15 alloy can be significantly enhanced by thermo-mechanical processing, and the tensile elongations from 188% to 917% were obtained at relatively low temperatures ranging from 780 to 850 °C and at high strain rates from 2.2×10⁻³ s⁻¹ to 1.1×10⁻² s⁻¹. Moreover, the superplastic behaviors also have several specific features.

The extensive strain hardening stage in the stress–strain curve is observed in TA15 alloy, and the strain hardening stage

Conclusions

1.
At all deformation temperatures ranging from 780 to 950 °C and strain rates from 3.3×10⁻⁴ s⁻¹ to 1.1×10⁻² s⁻¹, the fine-grained TA15 alloy exhibits better superplasticity, and at the optimum temperature of 900 °C and strain rate of 3.3×10⁻⁴ s⁻¹, the maximum elongation of 1074% is obtained.
2.
An extensive strain hardening of the true stress–strain curve at high temperatures and low strain rates is attributed to high dense dislocations in thermo-mechanically processed alloys and dynamic grain growth.
3.
With

Acknowledgment

This work is supported by the National Natural Science Foundation of China (Grant nos. 51075196 and 51164029). Also, the work is supported by the Open Fund of National Defense Key Discipline Laboratory of Light Alloy Processing Science and Technology of Nanchang Hangkong University (Grant No. gf201101002) and by Ph.D. Research Startup Foundation of Nanchang Hangkong University (Grant no. EA201303356).

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Microstructure and superplasticity of TA15 alloy

Abstract

Introduction

Section snippets

Experimental procedures

Microstructure evolution during thermo-mechanical treatment

Discussion

Conclusions

Acknowledgment

J. Mater. Proc. Technol.

Scr. Mater.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Int. J. Mech. Sci.

Int. J. Hydrog. Energy

Rare Met.

NanoStruct. Mater.

Int. J. Plast.

Trans. Nonferrous Met. Soc. China

Mater. Sci. Eng. A

Acta Mater.

Mater. Sci. Eng. A

Mater. Sci. Eng.

Materials Properties Handbook: Titanium Alloys