In Situ TEM Investigation Of Size And Temperature Effect On Deformation Mechanisms In Titanium Alloys

Titanium alloys play an irreplaceable part in industrial applications because of the perfect strength and plasticity in both ambient and extremely cold temperature.As we have seen,the deformation mechanism determines the mechanical properties of the metal materials,while the mechanism is also affected by various factors such as alloy composition,ambient temperature and characteristic size.Therefore,to the R&D of the titanium alloy,the research on the deformation mechanism of titanium alloy in multi scales and at different temperatures would render fundamentally theoretical basis and technical guidance.In this paper,pure Ti and Ti-5 at.%A1 were studied on atomic,micro-meter and macro scales respectively.Combined with the in-situ TEM tensile tests and structural characterization on the atomic level,the article discovered the correlation between the multi-scale microstructures and mechanical properties.First of all,the a-Ti alloy,being in the hexagonal close packed crystal structure,was discovered to have a relatively low symmetry,which enabled itself to store dislocations,even in extremely small size.The infinite slip systems,which would slip most probably,facilitated the beginning of the multi-slip systems.As the result,the dislocation net with high density was formed,preventing the dislocation from escaping.The complicated interactions between the dislocations improved the velocity of the dislocation nucleation,higher than that of the dislocation escape,making the sample store as much dislocations,which made the material show powerful compressive strength and continual plastic flow.While the dislocation starvation existing in the body-centered-cubic and face-centered-cubic metal materials would not occur in this material.What1 s more,the vital effects of the alloy elements on the deformation mechanism were also demonstrated.At ambient temperature,the planar slip of dislocations was the dominant deformation mechanism of the pure Ti;while in the Ti-5 at.%Al,apart from the typical planar slip,cross slip events were also observed.However,at the extremely cold temperature,intensive cross slip events were observed in the pure Ti,indicating that the cross-slip mechanism dominated the plastic deformation;Whereas the planar slip mechanism played a dominant role in the deformation of Ti-5 at.%Al.Furthermore,the results of the tests on the atomic structure and chemical composition manifested the even distribution of Al atoms around the screw dislocation nucleation without ordered phase.Besides,according to the molecular-dynamics simulation,the reverse of the deformation mechanisms between pure Ti and Ti-5 at.%Al were resulted from the impact the temperature made on the stacking fault energy.Specifically,when the temperature changed,the stacking fault energy of the pure Ti varied slightly;Whereas the stacking fault energy of the Ti-5 at.%A1 showed a definite decline as the temperature descended,which led Al making totally different contributions to the solid solution strengthening at extremely cold temperature.