Effects Of Thermoplastic Deformation On Microstructure And Properties Of Nial Intermetallic Compounds

The rapid development in the aerospace field has led to more stringent requirements for high temperature structural materials.At present,the most widely used nickel-based superalloy is limited by its own melting point,and its use temperature cannot be further increased,so it is important to find materials with higher service temperatures and better high-temperature service performance.Ni Al intermetallic compounds with lower density and excellent high temperature properties are considered as one of the most promising high temperature structural materials.At room temperature,Ni Al alloys have less slip lines and poor plasticity and are difficult to process.However,studies have found that Ni Al intermetallic compounds have superplasticity above the ductile-brittle transition temperature.Therefore,the study of the changes of the material properties and microstructure under high temperature plastic deformation can provide a theoretical basis for the formation of Ni Al alloy at ultra-high temperatures.The Ni and Al element powders were used as raw materials to prepare Ni Al bulk materials by hot-press sintering process.The high-temperature compression in one direction of Ni Al was performed at a high temperature of 1300°C.As a result,the yield strength of the specimen increases as the deformation increases.Observing the change of grain size,grain boundary orientation gradient,texture,etc of specimens under different deformations through EBSD,the softening mechanism of the sample could be determined.The results show that the softening mechanism is mainly the transition mechanism from discontinuous dynamic recrystallization to continuous dynamic recrystallization.Throughout the deformation process,Grain size became smaller,the change in texture strength was slight,and there was no obvious anisotropy.The reason for the increase in yield strength is fine-grain strengthening.In order to further refine the crystal grains and improve the material properties,the Ni Al bulk material was subjected to multi-pass deformation in different directions,and its mechanical properties and microstructure were observed.Because the yield strength of the second pass sample is low and the crystal grain is coarse,the experiment was improved on the basis of the second pass experiment by reason analysis.The third pass sample has a relatively small proportion of small grains,and the grain refinement is obvious.However,at the same time,continuous dynamic recrystallization occurs in the third pass of compression,and the subgrain rotates continuously.Due to insufficient dynamic recrystallization,a more pronounced texture is formed.The existence of texture makes the third pass sample exhibit obvious anisotropy,the yield strength parallel to the compressive direction is poor,and the yield strength perpendicular to the compressive direction is better.Multi-pass thermoplastic deformation can significantly refine the grains,but in the experimental process,the sample heating time is too long,cooling too quickly greatly affect the performance and organization of the material.The introduction of a pulsed current into the high-temperature thermoplastic deformation allows for a rapid temperature rise of the sample and guarantees a constant temperature during the compression process.The Gleeble thermal compression simulation experiment was used to simulate the compression of Ni Al under different strain conditions at a strain temperature of 1300°C and a strain rate of 10-3 s-1.The flow stress-strain curve was obtained and the process of microstructure evolution of the sample was analyzed by EBSD.It is found that under this deformation condition,discontinuous dynamic recrystallization occurs mainly,and the first and second passes are not sufficiently recrystallized.In the third pass,the crystal grains air equiaxed as a whole,and the small grains accounted for most of them.Dynamic recrystallization is more abundant.Finally,electric heating was used to achieve the extrusion of Ni Al alloy at high temperature.This subject has changed the traditional power-on mode,raised the heating rate,and reduced the dependence on pulse power.The temperature of the sample was faster,and the temperature remained unchanged during the forming process.However,because the temperature is too high,the mold is cracked due to uneven heating,and it is not successfully formed,but it provides guidance and basis for forming large-scale ribs.