Investigation On Solid Phase Transformation Of Pure Titanium Under High Pressure

Pure titanium experiences phase transformation from body centered cubic(bcc)β to hexagonal closely compacted(hcp)α during cooling from high temperature.Such transformation can be realized in a diffusional or diffusionless manner according to the cooling rate.However,both of them are unable to lead to effective strengthening and hence pure titanium is assumed a metal that is unhardable via heat treatment.This is because unlike martensite in steel,carbon supersaturation does not exist during the phase transformation for pure titanium and the volume change is relatively small.Recent investigations show that high pressure is able to enhance the free energy change during solid phase transformation for Fe and is able to lead to super strengthening.Consequently,it is expected to strengthen pure titanium via solid phase transformation under high pressure.The unique behaviors for the solid phase transformation under high pressure for tatianium together with the microstructural evolution and the strengthening methanisms are worthy of being systematelly investigated.Here we chosen pure titanium as the experimental materials to subject to thermal cycle under atmospheric pressure and high pressure of 1-6 GPa(holding at 1200℃ for 30 min followed by cooling to room temperature with the cycling water).Vickers hardness was used to evaluate the strengthening behavior,and optical microscope,X-ray diffraction,scanning electron microscope and transmission electron microscope were used to characterize the induced microstructures.Computer programmes were complied by using Matlab software to analyze the variant pairing,variant identification and misorientation calculation etc.The microstructural evolution during high pressure phase transformation was analyzed and the hardening memchanisms were discussed,and the following conclusions were reached:(1)AMatlab-based computer programme was developed and succesussfully applied to the data analysis of the pure titanium after phase transformation under atmospheric and high pressure.This programme can read EBSD data,by which the automatically identification of variant,the calculation of the distribution of misorientation can be realized.(2)Under 1-6 GPa high pressure,solid phase transformation from β to α for pure titanium was realized in a diffusional manner,during which the parent phase and daughter phase obey the classical Burgers relationship:{110}β//{0001}α and<111>β//<1120>α.High pressure does not effectively refine the prior β grains and the microstructure of α phase,but leads to significantly enhanced dislocation density.Compared to the quenched sample under atmospheric pressure,high pressure samples show higher dislocation density by more than one order of magnitude.(3)High pressure phase transformation leads to effective hardening in pure titanium.The average hardness for the 1-6 GPa samples is 307 HV,312 HV,319 HV,309 HV,297 HV and 280 HV,respectively,being a factor of 3 compared to the starting sample.The significant hardening was attributed to the enhanced dislocation density that contributes the strengthening via Taylor model.