Twinning Behaviours In Titanium, Zirconium And TiAl Intermetallic Compound

The mechanical behaviours in titanium, zirconium and TiAl compound have been studied under the different conditions of temperature and load type in terms of the combination of twinning deformation with mechanical properties, and the correspondent relationship among testing conditions, twinning behaviours as well as mechanical properties have been discussed in this thesis.The results of tensile tests of commercial purity titanium at different temperatures showed that the strength and ductility increased simultaneously with the decreasing of temperature, and the hardening characteristic of tensile curves and fracture morphology changed obviously from RT to - 196℃ . The observation of mi-crostructure showed that at RT slipping was the principal deformation mode while at - 196℃ twinning was. By comparing the feature of twinning deformation and martensite transformation, the "twinning induced plasticity" was put forward to explain the tensile property in - 196℃.The results of compressive tests of commercial purity titanium and zirconium showed that with the decreasing of temperature, the hardening characteristic of compressive curves had no obvious variation while the amount of twins increased, indicating that the twinning behaviour has a little influence on the compressive properties of titanium and zirconium.The impact tests of commercial purity titanium and zirconium at different temperatures were carried out and the results showed that with the decreasing of temperature, the amount of twins increased but the impact properties decreased, Twins, as a kind of crystal defact, acting as a crack initiation source and crack propagative path, had a negative influence on the impact properties.The cyclic tests of commercial purity titanium at different temperatures were carried out and the results showed that at - 196℃ the cyclic stress-strain curve was above the tensile stress-strain curve, indicating the characteristics of cyclic hardening. The amount of twins increased with the increasing of cyclic strain amplitude or cyclic numbers, and the interaction between twins and twins, twins and disloca-tions, twins and grain boundaries had an important effect on cyclic hardening at -196TC The cyclic deformation with pull - pull condition at - 70C demonstrated a less amount of twins compared with cyclic deformation at - 196'C . The cyclie deformation at RT showed that it took place cyclic hardening only when the cyclic strain amplitude was >1.5% and not only the dislocation evaluation, grain boundary breaking but also the twins had a contribution to its cyclic hardening.The fatigue life-plastic strain amplitude curves at - 196'C and RT were obtained and the data was regressed in terms of Coffin-Manson relationship as following:-196C: A￡p/2 = 0.21(2Nf)-°-4520C: Aep/2 = 0.083(2N{)-°-42It was found that fatigue life at - 196C was longer than that at RT and the effect of twinning on fatigue life was analysied.The fracture morphology of fatigue at - 196'C was examined and the results exhibited that at higher cyclic strain amplitude, the fatigue cracks initiated on the surface of specimen and propagated in a mode of tearing corrugation while at lower cyclic strain amplitude, the fatigue cracks initiated on the surface or subsurface and propagated in the form of fatigue strip. At the centre of fracture surface a large amount of plastic hollow was discovered.The cyclic deformation of Zr~4 alloy at - 196'C was carried out, comparing with that at RT and high temperature, it was discovered that from - 196'C to 600C , the prismatic slipping 11010( was the primary deformation mode, with the pyramidal slipping 112111 and basal slipping |0002( at higher temperature and twinning at RT and — 196C .The twin interface relationship among lamellar laths in lamellar microstructure of Ti - 48A1 — 2Mn — 2Nb alloy was analysed and in-situ fatigue testing was carried out on this microstructure. It was found that lamellar microstructure with twin relationship had a great influence on fatigue crack propagation, by making the crack grow path be more tortuous and propagate by the connection of uncontinuously mi-crocracks.Cyclic deformations with different stress mode were carried out. The results showed that the twin number in the compressive cyclic were much more than that in tensile cyclic deformation, and high compressive stress cycling deformation were much more than that in low stress level. By analysing the ordered symmetry of LIqstructure and deformation mode, the experiment phenomenon were explained.The thermal cycling tests between - 196X3 and 100X3 with no extra restrain were performed on commerial purity titanium and zirconium as well as Ti - 48A1 -2Mn~2Nb alloys, experimental results showed that there appeared twins in all of the above materials after thermal cycling with different cyclic number. The further ?observation in titanium after 300 numbers of thermal cycling showed that the thermal cycling twin distributed inhomogeneous and its type was found to be 11011 f o...