Microstructure And Properties Of TiAl-based Alloy Prepared By Powder Metallurgy

Intermetallic compound γ-TiAl has the character of long-range order and thecoexistence of metallic bond with covalent bond. Crystal which is formed by covalentbond has the properties of high strength, high hardness and low coefficient of thermalexpansion, as a consequence, TiAl based alloy has the properties of high strength, highmodulus, high creep resistance and high temperature oxidation resistance, in addition,the density of TiAl based alloy is between3.7~4.3g/cm3,which is lower than Ti basedalloy’s,it’s lower than the half of Ni based super alloy. The character of excellent high-temperature behavior and low density make TiAl based alloy be identified as a goodsubstitute of Ni based super alloy, which is applied to aerospace and automotive field.Nevertheless, some imperfections impede its engineering application,such as the lowductility, low forming property and some casting flaws. In this study, TiAl based alloyis prepared by TiAl pre-alloyed powder through the process of Spark Plasma Sinteringand cold pressing technology.The average size of powder is105.80μm. XRD analysis, SEM observation andTEM analysis conducted on the powders in different size (38-75μm、75-106μm、106-150μm) indicate the powder is mostly formed by2phase, with the increase of the sizeof powder, a small quantityof γ phase exists. XPS analysis indicates that different sizedpowde is oxidated in different level, Al2O3、AlxO、TiO、TiO2、Y2O3、YOxoxides isformed. The bigger the powder is, the lower the oxidation degree is. The ageingtreatment shows the relationship between ageing temperature and finally structure ofTiAl based alloy. Near gamma structure is obtained when ageing temperature is nearfrom tetemperature, duplex is obtained when ageing temperature is in the middle of(γ+) phase area, nearly lamellar and fully lamellar is obtained when ageingtemperature is below and above t. Nanoindentation indicates the powder(38-75μm) hasthe highest nanohardness, the hardness drops after ageing treatment.Cold pressed TiAl based alloy is prepared by using cold hydro-mechanicalpressing apparatus, after that ageing treatment is conducted at1260℃for2h. Its densityachieves95%of SPSed TiAl based alloy. SEM observation shows that the structure isinhomogeneous, which is formed by near gamma, duplex, fully lamellar structures.Meanwhile,2phase is spread on the surface layer of some powders, this kind ofstructure weaken the mechanical property of cold pressed TiAl based alloy. TiAl pre-alloyed powder is sintered at1050℃,1100℃,1150℃,1200℃,1250℃by SPStechnology. XRD analysis and SEM observation show near gamma structure is obtainedafter sintered at1050℃,1100℃. Duplex, nearly lamellar and fully lamellar structure isobtained after sintered at1150℃,1200℃,1250℃. Microhardness test indicates near gamma structure has the highest hardness:480.1,468.9HV. Duplex, nearly lamellar,fully lamellar structures’ hardness is spread around410~420HV. Friction and wearexperiment indicates that Duplex structure has the best abrasive resistance, near gammastructure is secondary. Compression test and tensile test performed at room temperature,400℃,600℃,700℃,800℃,900℃indicate near gamma structure has the lowestmechanical property and ductility. Duplex, nearly lamellar, fully lamellar possess thebest mechanical property and ductility. Tensile test performed at900℃indicates allkinds of TiAl based alloys’ tensile strength become much higher than the test performedat600℃,700℃,800℃, meanwhile, all of the TiAl based alloys possess good ductilitywhich is spread around8-11%.