Fabrication And High Temperature Properties Of In-situ Synthesized Ti₂AlN/TiAl Composites

In this paper, in situ Ti₂AlN/TiAl composites were designed and fabricated.Microstructure and synthesis mechanism of Ti₂AlN/TiAl composites weresystematically investigated by scanning electron microscope（SEM）, transmissionelectron microscope（TEM）, X ray diffraction（XRD）, differential scanningcalorimetry（DSC） and so on. Properties of Ti₂AlN/TiAl composites were tested bythree-point bending, oxidation, hot compression and friction and wear, relatedfactors were also analyzed.Ti, Al and TiN particles were raw materials, used hot-pressing technique tofabricated Ti₂AlN/TiAl composites with20%and50%reinforcing phase volumefraction. the synthesis process of Ti₂AlN/TiAl composites were divided into fourstages: the first stage began with Al completely melted（>665℃）, TiAl3layer formedat Ti particles surface were the main new phase; in the second stage, Al powderparticipated in synthesis reaction and were depleted, multilayer Ti-Al compoundsbounding Ti particles, minor amounts of small-grained Ti₂AlN particles formed atTiAl3layer edge by liquation reaction; the third stage was at temperature1100℃to1300℃, the base material transformed into TiAl（the core were Ti3Al） at hightemperature, at this time, TiN particles closely contacted with TiAl; at the fourthstage, kept the temperature at1300℃for1h, TiN completely reacted with TiAl andformed many Ti₂AlN particles, the base material homogenizing transformed intoTiAl phase, finally obtained Ti₂AlN/TiAl composites.The base TiAl of Ti₂AlN/TiAl composites could transformed to full lamellar,duplex and near γ phase microstructure after heat treatment. For Ti₂AlN/TiAlcomposites that with full lamellar microstructure TiAl base, the hardness and elasticmodulus increased with the increase of Ti₂AlN volume fraction; at high temperature（800℃）, bending strength increased at first and then decreased with Ti₂AlN volumefraction increased, the maximum bending strength of the composites happenedwhen Ti₂AlN volume fraction was20%, too high Ti₂AlN volume fraction wasunfavorable to bending strength.Oxidation test was carried out at800℃to900℃in air. At900℃, the typicaloxidation process of Ti₂AlN/TiAl composites were divided into two stages: initial stage was fast oxidation stage of phase （or grain） boundary and surface layer, thencarried out the oxide layer continuously thickened oxidation stage, which wascontrolled by diffusion, the oxidation rate decreased with the thickness of the oxidelayer increased. At900℃in air, from outer to inner, the oxide layer were dividedinto two layers, they were TiO2layer and （Al2O3+TiO2+Ti5Al3O2） mixed oxide layer.Hot compression test results showed that, compressive strength of TiAl alloyand Ti₂AlN/TiAl composites were all decreased with temperature increased, andwere increased with strain rates increased; at high temperature high strain rates andlow temperature low strain rates, compressive strength of composites increased40%⁵0%compared with TiAl. Besides sliding, hot compressive deformationmethod of Ti₂AlN/TiAl composites also contained grain boundary sliding oflamellar microstructure, lamellar bending, interlamellar sliding, twin crystal andrecrystallization and so on. In low Ti₂AlN volume fraction composites, Ti₂AlNparticles that were at grain boundary could hinder grain boundary sliding, this washelpful to enhance material strength; In high Ti₂AlN volume fraction composites,Ti₂AlN that were connected to hard frame could effectively carrying strength, thatsignificantly enhanced the compressive strength of Ti₂AlN/TiAl composites.In friction and wear test, room temperature friction process of Ti₂AlN/TiAlcomposites-nickel base alloy friction pair were divided into two stages: Ⅰ stage wasunsteady state friction, nickel base alloy was the wore side, adhesive wear of nickelbase alloy adhered on composites wear face was the wear type; Ⅱ stage was steadystate friction, nickel base alloy still was the main wore side, wear type was abrasivewear of Ti₂AlN/TiAl composites to nickel base alloy, at the same time, Ti₂AlN/TiAlcomposites was adhesive wear. At room temperature, coefficient of friction andwear loss of friction pair increased with load increased; with sliding speedincreased, total wear loss of composites increased, coefficient of friction and wearrate of Ti₂AlN/TiAl composites decreased.At high temperature, the friction pair happened oxidation reaction, Ni alloyoxide to NiO, Ti₂AlN/TiAl composites oxide to Al₂O₃and TiO₂. Friction process ofTi₂AlN/TiAl composites-nickel base alloy friction pair were divided into threestages: Ⅰ stage was unsteady state-coefficient of friction increase stage, occurredadhesive wear of nickel base alloy; Ⅱ stage was unsteady state-coefficient offriction decrease stage, wear type of nickel base alloy changed from adhesive wearto abrasive wear, wear type of composites mainly were adhesive wear; Ⅲ stage was steady state-coefficient of friction stable stage, nickel base alloy was the main woreside, abrasive wear was the wear type. At high temperature, with the increase ofsliding speed, coefficient of friction and wear rate decreased, and total wear loss ofcomposites increased; with temperature increased, coefficient of friction and wearloss of composites were all decreased. No matter at room temperature or hightemperature friction test, the wear loss of composites were lower than that of TiAlbase alloy, with the increase of Ti₂AlN volume fraction, the wear loss of compositesdecreased.