Study On Mechanical Properties Of TiAl-based Alloy At High Temperature

TiAl-based alloy is characterized with short-range order and long-range disordered structure,so it has both the metal plasticity and high temperature strength of ceramic.It also has low density,high temperature than strength,high temperature oxidation resistance and corrosion resistance.Compared with the traditional titanium alloy,it has a great advantage in the use of high temperature?900⁸00?and is widely used in aerospace,marine and automotive fields.However,the development and application of Ti Al based alloys have been hindered by low ductility at room temperature.And that's difficult to improve ductility at low temperature by traditional ways such as rolling,forging,drawing and mechanical processing.PM?Powder Metallurgy?has a great advantage in refining the grain size and reducing defects caused by casting.So it can provide researchers with a new idea to resolve the question of structure defects and low ductility when combined with hot processing.From the fine crystalline of view,TiAl-based alloy was prepared by PM in this paper.Then the mechanical properties of samples at high temperature were measured by compression and tensile testing machines to study the effect of strain rate and defoemation temperature on the mechanical properties and microstructure.Firstly,high-energy ball milling was used to prepare the Ti-45Al-5Nb composite powders.The speed of ball milling is 550r/min and milling time is 50h.Then 25h-milled,40h-milled and 50h-milled powders were sintered respectively by vacuum sintering furnace.30Mpa was given to the powder when it was heated to 1200?,then the pressure and temperature were held for 1h.Scanning electron microscope?SEM?,transmission electron microscope?TEM?and X-ray diffraction?XRD?were used to analysize the phase and structure of sintering sample.Property of high temperature temsile and compression was tested by multi-purpose electronic testing machine?DDL50?and Gleeble-1500 thermal simulator.During the process of high temperature compression,the temperature range is from 1100?to1200?and the range of strain rate is from 1×10^-2s^-11 to 1×10^-4s^-1.In the experiment of high temperature tensile,the temperature range is from 950?to 1100?and the range of strain rate is from 1×10^-4s^-11 to 5×10^-5s^-1.The datas returned from the,etc.were analysized.The results show that:?1?The particle size of composite powder Ti-45Al-5Nb?at.%?that was milled for 25h,40h and 50h by high-energy ball milling is less than 500nm.In the range of 5h⁴0h,particle size is refined from original 50?mto 0.5?m with obvious effect of refinement and becomes more and more uniform.50h-milled powder appears obvious agglomeration.During the process of milling,new phases TiAl and Ti₃Al₅ intermetallic compounds are generated,which means mechanical alloying happens.?2?Density of the sintering alloy of composite powder milled for 25h,40h and are94.0%and 96.4%and 96.3%respectively.The sintering structure of 40h-milled powder has the most equiaxed microstructure and finer grains whose size is about 200nm.Structure of the three sintering alloys is equiaxed grain with double phases of TiAl and Ti₃Al.The grain size of TiAl phase is less than 500nm and the grain size of Ti₃Al phase is in the range of 1nm¹00nm.?3?The flow stress is sensitive to the temperature and strain rate in the process of compression test at elevated temperature.When the temperature is constant,the flow stress will decrease with the reductionof strain rate and when the strain rate is constant,the flow stress will decrease with increasing temperature.When the temperature is 1200?and strain rate is 1x10^-4s^-1,the peak flow stress is only 30MPa.Porosity of sintering structure decrease obviously by hot compression,and will decrease with the increasing temperature of compression.Microhardness of the sintering structure is 1024.47HV,and after compression,the hardness is above 1300hv.Maximum hardness is 1435.5HV.?4?During hot deformation,the deformation temperature,the flow stress and the strain rate about Ti-45Al-5Nb alloy are satisfied with the Arrhenius hyperbolic sine relationship.It is indicated that the high temperature thermal deformation is controlled by heating,and the thermal deformation of at the temperature of 1100-1200?(0.01-0.0001s^-1).Ti-45Al-5Nb alloy strain sensitive coefficient m is 0.55,the high temperature deformation activation energy Q is 623.298kJ·mol^-1.The constitutive equation for the high temperature thermal deformation of ultrafine grained TiAl alloy by powder metallurgy?mechanical ball milling and vacuum hot pressing sintering?was established:?=e^41.69[sinh?.0019??]¹.83exp?-623298/RT??5?In the process of tensile test at elevated temperature,when the temperature is constant,the engineering stress decreases with decreasing strain rate and when the strain rate is constant,the engineering stress decreases with increasing temperature.There should be also a similar relationship between the strain rate,temperature and porosity rate.When the deformation temperature is 1000?and the strain rate is1×10^-4s^-1,plasticility and tensile strength of the sintering alloy milled for 40h are higher than that of the sintering alloy milled for 25h.Grain refinement can improve the ductility and strength of TiAl-based alloys.The microstructure after tensile at high temperature exists dislocation network obviously.After high temperature tensile,grain size is mainly in the range of100nm¹?m and part grains of a₂-T₃Al phase reaches nanometer nanoscale.