Formation And Oxidation Resistance Of Porous NiAl-based Intermetallics Prepared Using Thermal Explosion Reaction

Ni-Al intermetallic compounds have the advantages of low density,high melting point,good oxidation resistance and good high-temperature stability,which has received extensive attention and exploration in high-temperature structural and functional materials.Meanwhile,Ni-Al power compacts can form a continuous porous structure during sintering process and are expected to be developed and applied in new fields such as filtration,heat insulation and catalysis.Therefore,it is necessary to explore a simple,fast and green preparation process to obtain this new type of Ni Al porous materials.Ni powder??75?m,99.8%purity?and Al powder?38-75?m,99.9%purity?were used as raw materials in this thesis.The atomic ratios of Ni/Al powder mixture were chosen to be Ni-25Al at.%,Ni-50Al at.%,and Ni-75Al at.%and then the mixture powders were pressed into cylindrical disc.The compacts were heated with a heating rate of 10?/min to the final sintering temperature of 700?and holding for 30 min under a high purity argon atmosphere.The effect of atomic ration on the combustion behavior,macro morphology,pore structure and oxidation resistance of the samples were investigated.Results shown that the obvious thermal explosion occurred in these three groups of samples and the sample temperature was ignited at 638?,620?and642?,then rapidly increased to 1381?,1613?and 1172?within the reaction time of 1.57 s.0.21 s and 0.72 s,respectively.XRD and SEM analysis results showed that three porous materials of Ni₃Al,Ni Al and Ni Al₃ were synthesized.Ni Al₃ has a maximum open porosity of 41.5%with interconnected pore channels,while the open porosity of Ni Al was only 8.6%with many closed pores.The cyclic oxidation test was performed at 650?for 96 h,three porous products showed excellent antioxidant performance,especially for Ni Al,in which the weight gain was only 0.06%.At the same time,the combustion temperature exceeds the melting point of the Ni-Al product due to the large amount of heat released during the thermal explosion reaction,causing the sintered product to be melt and undergo the severe deformation phenomenon.The Ni-50Al at.%powders was selected as raw materials,combined with the thermal explosion reaction and space holder method to obtain high porosity porous Ni Al with controllable macro-shape and adjustable micro-pores.Replication pores were formed after Na Cl removal,which significantly increased the open porosity of the samples?up to 77%?.Meanwhile,the increase in porosity increased the sample ignition temperature from 620?to 650?,and the combustion temperature showed the opposite trend,decreasing from 1613?to 1377?.When the amount of Na Cl added above 30 vol.%,the sample maintained its original shape and no crack deformation occurred.Three pores with different sizes and shapes existed in the final products,which are the replication pores from Na Cl particles,small pores generated by the thermal explosion reaction,and the micropores precipitated by the solidification of liquid phase from the skeletons.These pores formed an interconnected pore channels during the product particles.The introduction of the pore-forming agent enables the heat generated by the sample during the thermal explosion and the original gas inside the sample to escape through these channels,thereby maintaining its original macroscopic shape.In addition,Ni-50Al at.%green compact was preheated at 550?for 15 min and then heated up to 700?for 30 min to obtain pure porous Ni Al intermetallic.It can be seen from the temperature-time and DSC curve,the preheating treatment would not affect the occurrence of TE reaction in the subsequent reaction process,but the maximum combustion temperature dropped to 1533?.BSE and XRD analysis showed that there was an intermediate phase Ni₂Al₃ around Ni particles after pre-heating process,resulting in a decrease in the heat released by the thermal explosion reaction,which was the main reason for controlling the macroscopic shape of the sample.When further heating the sampe to 700?and holding for 30 min,porous Ni Al materials with uniform pore distribution,good pore structure were obtained,and the open porosity was up to 56.59%.The mass gain rate of porous Ni Al in the antioxidant performance test was 0.46%,demonstrating good oxidation resistance at high temperature and pore structure stability.This paper includes 43 figures,5 tables and 112 references.