Study On Heat Deformation Behavior And Effect Of Pre-aging On Superplasticity Of TB8 Titanium Alloy

TB8 is a kind of metastable β Titanium alloy with ultra high strength, which is widely used for structural components with temperature requirements in the aerospace field. Microstructure after hot deformation effects property of the structural components directly, therefore it is important to research heat deformation behavior to obtain structural components of excellent microstructure and performance. Low superplasticity of TB8 Titanium alloy is determined by high β stabilizing elements, whether better superplasticity of TB8 Titanium alloy can be obtained by types of treatment before superplastic deformation, to make this kind of material more widely applied in the field of superplastic deformation.Based on hot compression simulation experiment of TB8 titanium alloy, hightemperature plastic deformation behavior and microstructure evolution were researched at temperature of 750 ～ 900 ℃, strain rate from 0.001 to 10 s^-1.And processing map was established based on hot compression simulation experiment to obtain better thermal processing parameters.Different parameters of pre-aging treatment to TB8 titanium alloy were carried before the superplastic tensile test, to obtain disciplinarian of superplastic elongation and the microstructure of different parts,and than getting the pre-aging process parameters when maximum elongation. The main research work is as follows:（1） According the data of the thermal simulation compression test to establish TB8 titanium alloy flow stress constitutive equation model, the result show that activation energy for heat deformation of this alloy is 284.65 KJ/mol in these deformation parameters, and the accuracy of this model is higher after verification.The microstructure evolution of this alloy was researched, the results show that the band structure occurs after the dynamic recovery whatever strain rate when deformation temperature in the two-phase zone. There is a fine grain and uniform equiaxed microstructure after obvious dynamic recrystallization only when heat deformation in the single-phase region 、 a low strain rate. The grain is crushed stretched to perpendicular to deformation direction when heat deformation in the single-phase region、the high strain rate.（2） Processing map of TB8 titanium alloy based on Murty and Prasad criterias what forecast instability deformation zone in the two-phase region and high strain rates. The instability deformation zone of TB8 titanium alloy are in the ranges of 750～780 ℃, 0.03～10s^-1 and 780～900℃,0.35～10s^-1. The microstructure characteristics of instability deformation zone are band structure 、flow localization and unequal deformation of β grain. Stable deformation zone is in the single-phase region and low strain rate conditions. The preferable ranges of stability region is in the ranges of 815～885℃、0.03～0.1s^-1, the microstructure of stability region is equiaxed structure after dynamic recrystallization.（3） The effect of superplastic elongation and microstructure of fracture site after superplastic tensile test by different pre-aging temperature and time were summarized, got the pre-aging process parameters: 520 ℃, 40 min which induced the best superplasticity of TB8 titanium alloy, the superplastic elongation up to 362% at this point. Compared to the same stretching temperature 840 ℃, strain rate at a lower magnitude of 10-4 s^-1 conditions（not pre-aging treatment）, higher elongation and superplastic forming efficiency was improved, and smaller microstructure grains of fracture site were obtained.