| More and more attention has been attracted by intermetallic compound,Ni3Al, due to its high hardness, high melting point, outstanding oxidation resistance and wear resistance, and the improvement of yield strength with temperature increasing below 950℉. In recent years, more and more researches have been carried on the use of Ni3Al as binder of cemented carbide and cermets for domestic and overseas. Aiming at producing cermets applying to elevated temperature condition, in this paper, mechanical alloying was adopted in preparation of Ni3Al intermetallic compound, and then Ni3Al powder was used as binder of cermet as a substitution of conventional binder Ni and/or Co.Ni and Al powders were used in formation of Ni3Al powder by mechanical alloying. During high-energy milling, Al diffused into Ni forming disordered Ni(Al) solid solution firstly by repeating crushing-cold welding and then the powder mixture was transformed to Ni3Al intermetallic compound on further milling. The addition of stearic acid (SA) can postpone reaction and refine powder, however, excess addition of SA would make the reaction not to be completed successfully. The results showed 0.5 wt% SA addition can make reaction accomplish smoothly, while the reaction cannot be completed with 1 wt% SA addition.The microstructure of Ni3Al-bonded Ti(C,N)-based cermets showed a weak core-rim structure particles embedded in binder phase, and the rim was incomplete. Transverse rupture strength (TRS) of cermets increased with the ascent of Ni3Al content and sintering temperature basically, reaching a peak value 1131±80.3 MPa, at 1450℃ with 30 wt% Bi3Al addition. And hardness (HRA) of cermets ascended firstly and then dropped down with the increase of binder content and sintering temperature, reaching a peak value in cermets sintered at 1450℃ with 30 wt% Ni3Al addition.The effects of WC addition on microstructure and mechanical properties of the TiC-Ni3Al system cermets were investigated in this paper. The results revealed that Ni3Al-bonded cermets showed a black core-white rim structure carbide particles embedded in Ni3Al binder. With WC content increasing, TiC cores were refined and the white rim became complete and got thicker gradually. The binder-rim interface showed two interface types, bonding directly and bonding with a disordered transitional layer. Interface between rim and core showed a completely coherent relationship. The rim closing to binder is rich in W, constituting an ideal coherence between hard phase and Ni33A1 binder phase. With WC content increasing, the densification of cermets was enhanced, and hardness and TRS were increased firstly and then reduced, reaching peak values 90.9 HRA (HV30 15 GPa) and 1629 MPa respectively in cermet N5 (25 wt% WC). Similarly, fracture toughness got a peak value (11.6 MPa·m1/2), at the composition with 20 wt% WC. The fracture mode of Ni3Al-bonded cermets was combination of transcrystalline fracture of hard phase and intergranular fracture in the interface between carbides and binder phase. With the increase of WC content, transcrystalline fracture of cermets transforms to intergranular fracture gradually, and when WC addition is over 25 wt%, it would fracture in rim under external pressure. Appearances of fracture of cermets showed cleavage fracture of hard phase and metal tearing ridge causing by the shedding of hard phase particles.The microstructure of cermets with different Mo content also showed core/rim structure particles embedded in binder and the carbides show black core-white rim structure. When Mo content is 15 wt%, the hard phase particles show grey outer rim-white inner rim-black core structure. During liquid-sintering, Mo tends to aggregate in the surface of undissolved carbide. With Mo content increasing, TRS and hardness of Ni3Al-bonded cermets increased firstly and then decreased, achieving peak values 1396 MPa and HRA 91.0, respectively at the composition with 8 wt% Mo.(Ti1-xWx)C solid solutions (x= 0.05,0.15,0.25,0.35) were synthesized by carbothermal reduction and then (Ti,W)C-Ni3Al cermets were prepared through powder metallurgy route. (Ti,W)C-Ni3Al cermets showed a weak core-rim structure carbide particles embedded in Ni3Al binder. As W content in (Ti,W)C increased, this core-rim structure of carbide particles got weaker and the contrast of particles lowered down in SEM morphologies. Furthermore, the densification of cermets was promoted and TRS and toughness were improved obviously with W content in solid solution increasing, reaching maximum values 1609 MPa and 13.0 MPa·m1/2 respectively. Hardness of cermets increased firstly and then decreased with the increase W content in solid solution, and got a peak value HV30 1093 as x= 0.15.High temperature TRS (HTTRS) of Ni3Al-bonded cermet was tested and the results showed that HTTRS was improved with temperature arising below 900℃, TRS of Ni3Al-bonded cermet can be enhanced 20% from room temperature to 900℃. High temperature oxidation experiment has been conducted and results revealed that mass gain of Ni3Al-bonded cermet is less than Ni-bonded cermet in different oxidation temperature and the mass gain is in accord with oxidation equation (Δm/S)=kn, t0.6, in which oxidation rate constant, kn, at 800℃,900℃ and 1000℃ is 0.157,0.912 and 2.27 respectively. Kinetic of high-temperature oxidation of Ni3Al-bonded cermet belongs to quasi-parabolic kinetic behavior. During high-temperature oxidation, Ni3Al binder would be oxidized, forming double oxide, NiAl2O4, which can restrain the diffusion of O2 in oxide, resulting in improvement of oxidation resistance of binder of cermet. The cross section morphology of oxidized Ni3Al-bonded cermet is comprised of oxide layer (OL), transformed layer (TL), and substrate. Obviously, some pores can be observed in OL and also slight pores exist in TL. Thickness of OL of Ni3Al-bonded cermet increased with oxidation temperature increasing and keeping time prolonging. In oxidation process, Al, Ti et al. would diffuse outwards to OL from inside of cermet and O2 would diffuse inwards into cermet forming oxygen permeability layer, namely transitional layer. |
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