Laws And Mechanisms For The Effects Of The Addition Of Ceramic Of Nb And Ta And The Elements Of Cr And Mo On The Compression Properties Of NiAl Intermetallics

Due to the desirable combination of the high melting temperature, high-temperatureoxidation resistance and low density, the NiAl intermetallic compound is regarded as one ofthe most promising candicates for high temperature applications. Howerver, the actual use ofthe NiAl alloy has been limited by the low ductility at room temperature and the inadequatecreep resistance at high temperatures. There are many methods have been developed for thiscompound, including powder processing, casting, arc melting, directional solidification andself-propagating high temperature synthesis. The methods metioned above have somedefects, such as the tough melting condition, high cost, and composition segregation. As toimproving the defects of the methods metioned above, high-temperature combustion reactionand hot press was used for fabricationg the NiAl and NiAl-matrix composite.High-temperature combustion reaction and hot press based on the combustion reaction.When the reation oucurs, a axial force is added. The characteristic of the method is easy, lowcost and the fine grains, and so on.In this paper, the Ni-Al-Nb-B₄C system and the Ni-Al-Me （Me=Mo, Cr and B） are usedfor fabricationg the NiAl matrix composite and NiAl alloys by the high-temperaturecombustion reaction and hot press method. The effect of the kind and the size of the ceramicpartuculates and the kind and the content of the alloying elements on the room-temperaturecompression properties are researched in the paper. The strengthen mechanism of theceramic particulates and the alloying elements for the NiAl was discussed.The main results are as follows:（1） Ni-Al, Mo/Cr-Ni-Al, Ni-Al-Nb-B₄C systems were used for fabricating the NiAlalloy, NiAl-xMo（x=5,10and15） alloys, NiAl-xCr（x=25,30and35） alloys andNiAl-matix composite with Nb₂C, NbB₂and NbC by the high-temperature combustion reaction and hot press. All the grains of the NiAl alloys were refined.The grain size of the alloys was about60-80μm,10-20μm and10-30μm,respectively. The alloys were formed in instant, which leads to the rapid cooling.The size of the nucleation reduced, and the alloys were refined. When the reactionoccurs, a aixl force is applied. It can produce a lot of dislocations.（2） The mechanism of the fine garins in the NiAl, NiAl alloys and NiAl-matrixcomposite was discussed.a) The mechanism of the pure NiAl: The reaction occurs and finished at one sight,and the cooling degree was very low, which can make the size of the criticalnucleation little. The probability of the nucleation improved, and the grains werefine.b) The mechanism of the NiAl-Mo/Cr alloys: The addition of the high meltingpoint metals Mo （2623℃） and Cr （1863℃） made the max combustion temperaturedecreased obviously, which can make the grain fine.c) The mechanism of the NiAl with Nb₂C, NbB₂and NbC: The misfit between the（0001） of Nb₂C and NbB₂and （100） of NiAl are6.1%and6.2%, repectively. So theceramic particulates of Nb₂C and NbB₂were regarded as the nucleation for NiAl. Theamount of the nucleation increased, and the grain became fine.（3） The NiAl, NiAl-Mo/Cr alloys and the NiAl-matrix composite have goodroom-temperature compression properties and high work-hardening capacity at thestrain rate of1.0×10^-4s^-1.a) The ture yield strength, fracture strength, the fracture strain, the work-hardeningcapacity and the hardness of the NiAl are419MPa、1005MPa、21.6%、1.40and360HV, respectively.b) The room-temprature compression properties were improved obviously with theaddition of5,10and15at.%Mo. The compression properties of the NiAl-15Moalloy are the best, and the ture yield strength, fracture strength, and the hardness are747MPa,1619MPa and621HV, respectively. While the ture fracture strain andthe work-hardening capacity are little, lower than those of the pure NiAl.c) The addition of Cr can improve the compression properties of NiAl alloy.Among the NiAl-xCr alloys, the NiAl alloys with30at.%Cr has the best properties.The ture yield strength, fracture strength, the fracture strain, the work-hardeningcapacity and the hardness of the NiAl-30Cr alloy are809MPa,1909MPa,21.4%,1.35and667HV.390MPa,904MPa and307HV, respectively improve the trueyield strength, the fracture strength and the hardness. The ture fracture strain and the work-hardening capacity is as high as those of the pure NiAl alloy.d) The compression properties of the NiAl-matrxi composite with5wt.%Nb₂C,NbB₂and NbC are the best among the different content and kind ceramicparticulates reinforced NiAl-matrix composites. The true yield strength, fracturestrength, and the hardness of the composite are653MPa,1497MPa and466HV,repectively. The fracture strain and the work-hardening capacity decreased lightly.（4） It was found that the NiAl alloy, NiAl-Mo/Cr alloys have strain rate sensitivity.When the strain rate decrease from1.25×10^-2s^-1to1.0×10^-4s^-1,a) For NiAl, the fracture strain and the work-hardening capacity are improved by8.4%and0.62, respectively.The true yield strength and the fracture strength aredecreased by151MPa and15MPa, respectively.b) For NiAl-30Cr alloys, the true fracture strength, fracture strain andwork-hardening capacity improved by118MPa,2.7%and0.65, especially thework-hardening capacity, which is improved by93%, while the yield strengthdecreased245MPa.c) For NiAl-matrix composite with5wt.%Nb₂C, NbB₂and NbC, both of thestrength and the work-haedening capacity are improved by6.4%and0.46,respectively. The true fracture strength did not changed obviously, and the yieldstrength decreased149MPa.When the compression strain rate decreased from1.25×10^-2s^-1to5.0×10^-4s^-1, thetrue yield strength, the frcacture strength, the fracture strain and thework-hardening capacity of the NiAl-15Mo alloy were improved by46MPa,250MPa,13.7%and0.2.（5） The mechanism of the strengthening of the NiAl, NiAl-Mo/Cr, NiAl-matrixcomposite with Nb₂C, NbB₂and NbC was discussed.a) The mechanism for NiAl, The NiAl fabricated by the high-temperaturecombustion reaction and hot press was refined, and the strength and the fracturestrain are improved. There were many dislocations distributed in the NiAl matrix,and the lattice of the NiAl distorted. The amount of dislactions increased during thecompression processing, from5.08×1011cm-2to5.92×1011cm-2, which canimprove the work-hardening capacity. Therefore, the mechanism for NiAl isfine-grained strengthening.b) The mechanism of the NiAl-Mo/Cr alloys, Mo and Cr exsit in NiAl as threetypes. A proportion of Mo/Cr solutes to the NiAl, which leads to the latticedistorted. A proportion of Mo/Cr precitipitation in NiAl, and the size is about20-30 nm. As Mo and Cr is soft than NiAl, they exsit in NiAl as a soft phase. Anotherproportion of Cr/Mo distributed at the grain boundaries. The strengtheningmechanisms are fined-grain strengthening, solution strengthening and the secondphase strengthening.c) The mechanism of the NiAl-matrix composite with5wt.%Nb₂C, NbB₂andNbC: The size of the ceramic paticulates in the grains is about1-2μm, while thesize of the ceramic particulates at the grain boundaries is about40μm. The amountof dislactions increased during the compression processing, from8.28×1011cm-2to1.20×1012cm-2, which can improve the work-hardening capacity. Therefore, themechanism for NiAl-matrix composite with Nb₂C, NbB₂and NbC is fine-grainedstrengthening and the second phase strengthening.In summary, the effect of the addition of the ceramic and alloying elements on thecompression properties of NiAl was studied in this paper. It can be benifical to theimprovement for the compression properties of NiAl.