Investigation of Slow Eutectoid Element on Tensile Properties and Superplasticity of a Forged SP700 Titanium Alloy

Dong Han,Yongqing Zhao,Junfeng Xiang,Weidong Zeng

doi:10.3390/met11111852

Abstract

The tensile properties and superplasticity of a forged SP700 alloy with slow eutectoid element (1.5%Cr) addition were investigated in the present paper. The results of the microstructures showed that slow eutectoid element Cr has a significant influence on stabilizing the β phase and the SP700Cr alloy showed a uniform duplex and completely globular microstructure after annealing at 820 °C for 1 h and aging at 500 °C for 6 h. The results of the tensile tests showed that the yield strength, ultimate tensile strength and elongation of the alloy with optimized microstructure were 1312 MPa, 1211 MPa and 10% at room temperature, and the elongation was achieved to 1127% at 770 °C. Compared with that of the SP700 alloy, the strain rate sensitivity of the SP700Cr alloy showed a higher value. The microstructures after elevated temperature tensile tests showed that the higher density of dislocations and twins exists in SP700 alloy and the lower density of dislocations favor distribution in SP700Cr alloy. Based on the above results, the tensile properties and superplasticity of the forged SP700 alloy with 1.5% Cr addition was analyzed. In addition, microstructure characteristics were investigated by the TEM and EBSD technologies.

Highlights

Titanium alloys have been widely used in the aerospace, automobile, chemical and other industries due to its desirable properties of higher strength, ductility and superplasticity, in which, SP700 alloy is attracting increasing attention for its excellent superplasticity [1,2,3,4,5,6]
Superplasticity refers to the capability of high tensile elongation before failure of polycrystalline materials, which is usually characterized by low flow stress and high plastic strain
The temperature of superplasticity deformation of titanium alloys is generally relatively high and it results in significant difficulties in many practical applications of the alloys

Summary

Introduction

Titanium alloys have been widely used in the aerospace, automobile, chemical and other industries due to its desirable properties of higher strength, ductility and superplasticity, in which, SP700 alloy (a kind of excellent β-rich α + β titanium alloy) is attracting increasing attention for its excellent superplasticity [1,2,3,4,5,6]. The temperature of superplasticity deformation of titanium alloys is generally relatively high and it results in significant difficulties in many practical applications of the alloys For this purpose, it is desirable to find methods to enhance the superplasticity at lower temperature or at higher strain rate. As known, refining the grain structures is favorable for superplasticity at elevated temperature, in which the equiaxed recrystallized grain with high angle grain boundaries or the non-recrystallized grain are two typical microstructures to obtain excellent superplasticity [8,9,10] To achieve this kind of grain refinement, severe plastic deformation (equal-channel angular pressing and high-pressure torsion, etc.) as an effective method have been widely used [3,11,12]. Some researchers concentrate on the methods to improve the superplasticity by optimizing the chemical composition through adding alloy elements that can control grain size or using β stabilizing elements that can promote the superplasticity at low temperature [13]

Results

Discussion

Conclusion