A Study On Hot Deformation Behaviour, Microstructure And Mechanical Properties Of ZrTiAlV Alloy

Spacecraft was affected from in the extreme environments during the process of long-term service in space, which resulted in the damage or failure of a large number of the key parts. Therefore, the structural materials with long-term dimensional stability in an irradiation environment, high strength, good plasticity and toughness, corrosion resistance and high temperature resistant performance were required for the development of space technology. In order to meet the high strength requirements in aeronautical industries, a series of new Zr Ti Al V alloys have been recently developed as advanced structural materials. In this present work, an attempt has been made to study the hot deformation behavior and the phase transformation of Zr Ti Al V alloys, and the effect of stress-induced martensite transformation on the microstructure and tensile properties of Zr Ti Al V alloys.The flow curves can be classified into three types during the hot deformation of the47Zr-45Ti-5Al-3V alloys with a coarse grain structure in the b phase field, marked as A,B and C. Type A curves occurred at high strain rates and low deformation temperatures and only dynamic recovery took place in this case. Type B curves exhibited a load drop and secondary yield point and followed by dynamic recovery and dynamic recrystallization processes, which occurred at high deformation temperatures and a strain rate of 10^-1 s^-1. Type C curves showed typical characteristics of dynamic recrystallization,which occurred at low strain rates and high deformation temperatures. The activation energy of deformation at different strains was determined to be in the range of 155.8 to186.6 k J/mol.For the hot deformation behavior of the 47Zr-45Ti-5Al-3V alloy with diiferent initialβ grain sizes, at low temperatures and high strain rates, the flow curves of the fine-grained and coarse-grained alloys exhibited a pronounced stress drop at the very beginning of deformation with increasing strain. The magnitude of the stress drop for the coursegrained alloy was higher than that for the fine-grained alloy. Dynamic recovery only occurred during hot deformation. At high temperatures and low strain rates, the flow curves exhibited typical characteristics of dynamic recrystallization. The peak stressincreased with decreasing deformation temperature and increasing strain rate. The peak stress for the coarse-grained alloy was higher than that for the fine-grained alloy at a given deformation temperature and strain rate. Based on the hot processing maps of the finegrained and coarse-grained alloy at different strains, the optimum processing parameters for hot working was at the temperature of 850?C and strain rate of 10-3 s^-1 for the fine-grained alloy, while the optimum processing parameters for hot working was at the temperature of 950?C and strain rate of 10-3 s^-1 for the coarse-grain alloy.The hot deformation behavior of the 47Zr-45Ti-5Al-3V alloy with initial lamellar αstructure was studied. In the α+β phase field, the flow curves exhibited a continuous flow softening. The extent of flow softening first decreased, and then increased with further increasing strain rate. The mechanism was the globularization of lamellar α phase at high deformation temperatures and low strain rates, and the flow localization at low deformation temperatures and high strain rates, respectively. In the single β phase field,the flow curves exhibited a pronounced stress drop at low temperatures and high strain rates. The magnitude of the stress drop increased with decreasing deformation temperature and increasing strain rate. At high temperatures and low strain rates, the flow curves exhibited typical characteristics of dynamic recrystallization. The deformation mechanism was the dynamic recovery at high deformation temperatures and low strain rates, and dynamic recrystallization of β phase at low deformation temperatures and high strain rates,respectively.The transformation of α â†' β phase occurred for the 47Zr-45Ti-5Al-3V alloy with initial lamellar α structure during hot deformation. At the deformation temperature of550?C, the volume fraction of β phase increased with increasing strain rate. At 600?C and650?C, the volume fraction of β phase first decreased with increasing strain rate, and then increased. The volume faction of β phase after hot deformation is higher than that before hot deformation, indicating the hot deformation is beneficial to the transformation of α â†'β phase for the 47Zr-45Ti-5Al-3V alloy with initial lamellar α structure.The 51.1Zr-40.2Ti-4.5Al-4.2V alloy is a metastable alloy retaining a single β phase structure when quenched from above the β transus temperature. The stress-induced martensite（SIM） transformation can occur upon deformation at room temperature. Theamount of stress-induced martensite decreased with increasing strain rate and initial βgrain size. The SIM transformation affected strongly the deformation behavior of the51.1Zr-40.2Ti-4.5Al-4.2V alloy. The triggering stress decreased with decreasing strain rate and initial β grain size, while the ultimate tensile strength increased above the transus temperature. The curve of work-hardening rate vs. true strain was divided into three stages.At a given strain rate, the work-hardening rate at stage ΙΙ or ΙΙΙ increased with decreasing initial β grain size and strain rate.