Constitutive Modeling for Hot Working Behavior of SP-700 Titanium Alloy
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JMEPEG https://doi.org/10.1007/s11665-019-04355-x
Constitutive Modeling for Hot Working Behavior of SP-700 Titanium Alloy Amir Hossein Sheikhali, Maryam Morakkabati, and Seyed Mahdi Abbasi (Submitted November 17, 2018; in revised form July 12, 2019) In this paper, the constitutive modeling and hot deformation behavior of SP-700 titanium alloy was investigated by performing isothermal hot compression tests in the temperature range of 700-950 °C and strain rates of 0.001, 0.1, and 1 s21. The flow curves of the alloy indicated that the yield point phenomenon occurs at strain rates of 0.1 and 1 s21. The friction, adiabatic heating, and stroke-rate controlling effects were eliminated from the raw data. Then, three types of phenomenological constitutive models, considering the compensation of strain, were used to predict the flow stresses of the alloy. The correlation coefficients (R2) of the measured and predicted results for the power-law, hyperbolic-sine law, and dynamic softening models were found to be 0.9790, 0.9883, and 0.9877, respectively. The corresponding average absolute relative errors (AARE) were 6.3, 5.9, and 7.45%, respectively. The flow softening behavior of SP-700 alloy was modeled using the combined hyperbolic-sine law and dynamic softening model approach. The predicted flow stresses were in a good agreement with the experimental data, indicated that the developed models can accurately characterize flow behavior of the alloy. In addition, the strain-rate sensitivity (m) distribution map of the alloy was obtained by plotting the m-value against temperature and strain rate in the form of a contour map. There was the maximum of m-value (0.36) in the temperature ranges of 700-715 and 837-875 °C, and strain rates of 0.001-0.006 s21. The microstructure of hot deformed at 700 °C mainly consists of globularized and lamellar a phase, while dynamic globularization was completed at 800 °C. The hot deformation activation energy of the alloy in a/b region (305.5 kJ mol21) was higher than that in singlephase b region (165.2 kJ mol21) due to the globularization of lamellar a. Keywords
activation energy, constitutive model, globularization, hot compression, SP-700 titanium alloy
1. Introduction Ti-4.5Al-3 V-2Mo-2Fe (wt.%) is a beta-rich titanium alloy introduced in 1989 by the Japanese company NKK (Nippon Kokan) in order to develop superplastic forming at lower temperatures compared to Ti-6Al-4V (Ref 1). The alloy was named as SP-700, due to its superior superplastic properties at 700 C (Ref 2). The SP-700 alloy shows superior fatigue strength, tensile properties, and fracture toughness compared to Ti-6Al-4V (alpha-rich) (Ref 3). Although many investigations have been carried out on superplastic forming of SP-700 alloy in the last decades (Ref 1, 4-7), only a few papers are available on the characterization of hot deformation behavior of the alloy (Ref 8, 9). Basic subjects such as flow stress characterization, microstructural evolutions, hot deformation mechanisms, dynamic material model for hot working of the alloy hav
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