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Research on the Microstructrue and Properties of PM TiNiAl Alloys

Author: LiuBoLu
Tutor: LiuZiLi
School: Nanjing University of Aeronautics and Astronautics
Course: Materials Processing Engineering
Keywords: TiNiAl Alloys Al Content Sintering Temperature Ti/Ni Atom Ratio Two-stepSintering Microstructure Properties
CLC: TG146.23
Type: Master's thesis
Year: 2013
Downloads: 7
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Abstract


TiNiAl alloys endow with high strength at the room and high temperature, good oxidationresistence and low density, which can substitute Ni-based superalloys to be used as new intermetallicstructural materials in aerospace applications. In the present work, Ti50-x/2Ni50-x/2Alx（x=0,3,6,9）alloys were prepared by hollow cathode plasma sintering and the two-step sintering route（900℃/2h+1050℃/2h）, and the optimum composition of Ti47Ni47Al6was obtained; the mechanicalactivation of Ti47Ni47Al6powders was conducted and followed by sintering at900℃for3h with slowheating from room temperature to900℃; Ti50-x/2Ni50-x/2Alx（x=0,6） and Ti47+xNi47-xAl6（x=0,0.5,1）alloys were prepared by the two-step sintering process with the slow heating. Effects of Al content,mechanical activation, sintering temperature and Ti/Ni atom ratio on the microstructure and propertiesof the alloys were investigated by using many analysis and testing means, such as optical microscope（OM）, X-ray diffractometer （XRD）, scanning electron microscope （SEM） with energy dispersivespectrometer （EDS）, differential scanning calorimeter （DSC）, microhardness and flexural strengthtesting, polarization and cyclic voltammogram measurement.The results show that the microstructure of the alloy without Al mainly consisted of NiTi matrix,strengthening phases Ti2Ni, Ni3Ti and some pores. With the increase of Al content, the amount ofTi2Ni（Al）, the number and size of pores increased, while the amount of Ni3Ti（Al） decreased and asmall amount of strengthening phase Ni2TiAl was also formed in Ti45.5Ni45.5Al9. The flexural strengthof the alloys increased with the increasing Al content and reached the maximum of296.3MPa at theAl content of6%, while the hardness of alloys increased with the increase of Al content and thehardness of Ti45.5Ni45.5Al9was295.6Hv.With the conduct of the mechanical activation, the lattice strain of Ti47Ni47Al6powders and thelattice constant of Ti, Ni increased. Ti47Ni47Al6powders ball milled for20h formed the lamellarTi-Ni-Al ternary powders and the phases consisted of Ti, Ni, Al without the formation of new phase.In Ti47Ni47Al6alloy prepared by sintering powders ball milled for1h, the strengthening phasesTi2Ni（Al） and Ni3Ti（Al） distributed with the lump shape, while the pores are close to spherical shapewith some large pores. In Ti47Ni47Al6alloy prepared by sintering powders ball milled for20h,Ti2Ni（Al） dispersed with the decreased size and increased amount, Ni3Ti（Al） still distributed with thelump shape and its amount increased, while the number of large pores decreased and the relativeensity increased The hardness and flexural strength of Ti47Ni47Al6alloy prepared by sintering1hpowders were273Hv and291MPa separately, while these of Ti47Ni47Al6alloy prepared by sintering20h powders increased to369Hv,356MPa respectively. Thermal explosions of Ni-Al and Ti-Ni were avoided by the slow heating from room temperatureto900℃and thesubsequent hold for1h at900℃, which contributed to the increase of the relativedensity. With the sintering temperature increasing to1180℃and1230℃, Ni4Ti3（Al） appeared inTi47Ni47Al6alloys, the size of pores decreased significantly firstly and then changed insignificantly,while the relative density, microhardness and flexural strength increased dramatically firstly and thenchanged unobviously.In the3.5%NaCl solution, Ti50Ni50alloy sintered at1080℃exhibitedpassivation behavior andthe etch pits dispersed with small size. The crevice corrosion easily occured at sites of pores and thecorrosion also tended to occur at interfaces of Ti2Ni, Ni3Ti phases. After Al addition, the alloypresented active dissolution behavior. The size of etch pits and the corrosion area increased,decreasing its corrosion resistance. With the sintering temperature increasing to1180℃, Ti47Ni47Al6transformed to passivation behavior again. The size of etch pits and the corrosion area dramaticallydecreased, resulting in the best corrosion resistance.With the increase of Ti/Ni atom ratio, the amount of Ti2Ni（Al） increased, while the acicularNi4Ti3（Al） phase transformed from dispersing to be distributing along grain boundaries; the amount ofthe persistent liquid increased and contributed to the filling of pores, decreasing the size and numberof pores; the hardness and flexural strength increased, and these of Ti48Ni46Al6were535Hv and687MPa respectively.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Non - ferrous metals and their alloys > Light non-ferrous metals and their alloys > Titanium
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