Research On The Behavior And Mechanism Of Short-time High Temperature Creep And Stress Relaxation Of Fine Grain Tc21Titanium Alloy

Fine-grain TC21titanium alloy(Ti-Al-Sn-Zr-Mo-Cr-Nb-X)is a newly developedα+β titanium alloy, with high strength, high toughness and damage tolerance. Fortitanium alloy, the application of titanium plate is more extensive, and the limit oftitanium alloy sheet formability is mainly the springback, wrinkling and tearing,however, at high temperature thermal activation can improve the plasticity, reduce theflow stress and internal stress, and at the same time improve the forming precision, sothe hot forming technology has become the effective solution. At present, many researchhave been done on the hot sheet forming technology of titanium alloy and matureTechnological Theory and schemes have been made, but mechanism of hot sizingremains to be explored. Thermal deformation mechanism for the stress relaxation ofsizing form process, and the deformation mechanism of the stress relaxation is materialcreep deformation. Combined with the property of fine grain TC21with the advantagesof hot sheet metal forming process, short-time creep and stress relaxation experiment onthe fine-grain TC21titanium alloy(2μm,4μmand7μm) are conducted at high temperature,respectively.Mechanical behavior and microstructure are obtained in the short-term creepexperiment at different temperature (600~750℃) and different stress (40~160MPa).Main results are shown as follows: With increasing temperature, steady creep rate isaccelerated, especially at temperature higher than700℃, the larger nominal activationenergy is gained, the stress exponent is3or so; with the increase of temperature andstress, greater changes of the creep mechanism have taken place, from creep(dislocation slip and small amounts of climbing) mechanism to diffusion creepmechanism of grain boundary sliding shift, and creep constitutive equation isestablished in700℃.Mechanism of creep changes from slipping of large amount ofdislocation and climbing of a small amount dislocation at low temperature and stress toslipping of grain at high temperature and stress..Tress relaxation at different temperature (600~750℃) and the different holdingtime (0~60min), is investigated in this article. With increasing temperature, stressrelaxation speed faster, limiting stress are decreased; Along with increasing in holdingtime, the transformation of elastic strain into the plastic strain increases gradually,anelastic strain continually decreased; By comparing short–time creep microstructure atthe same temperature, similar morphology characteristics are found.By comparing the mechanical behavior of short-time creep and stress relaxation, the result shows that elastic-plastic strain conversion rate in the stress relaxation processequals to certain steady creep rate during the creep process at the same temperature andstress.According to deformation mechanism of short-time creep and stress relaxation, hotsizing stage in bend forming test of fine-grain TC21titanium alloy sheet is conducted,the reduction of spring back is evident at elevated temperatures. The spring back valuegradually decreases with increasing holding time and stress relaxation has the similarchange pattern. Forming precision of parts gradually increases with increasingtemperature and holding time.