High-temperature Strengthening Mechanism Of NiAl-based Alloy And Combined Sintering Forming Of Tubular Components

With the development of aerospace industry,the requirements for the hightemperature structural materials with considerable performance increase.In order to solve the problem of high density and insufficient high-temperature performance for the widely used nickel based superalloys,it is urgent to develop the next-generation hightemperature structural materials.The intermetallic compound NiAl,which possesses advantages such as high melting point,low density and high thermal conductivity,is possible to become the attractive candidate of the next-generation high-temperature structural materials.At present,the method for improving the insufficient strength at elevated temperature and the proper forming technique are the hot topics for the reserachers.In this dissertation,the NiAl-based alloy with network-like reinforcements was prepared by rapid solidification and hot-press sintering process in order to increase the high-temperature strength.Meanwhile,based on the near net forming of the method,a combined sintering process for brittle and hard-to-deform NiAl-based alloy was utilized to prepare its closed-section components.According to the researches of NiAl alloy,the structural optimization method of NiAl-Cr(Mo)eutectic alloy was proposed to strengthen the alloy.The process is as follows: the original powders with nanoscale metastable Cr(Mo)lamella are prepared by rapid solidification at first.Then,the powdres are compacted by hot-press sintering process.The microstructure and properties of the alloy are optimaized by changing the sintering parameters and the fully dense sample with the peak stress of 405.4 MPa at1000°C is obtained.Based on the experimental results,the relationship between the sintering parameters and the microstructure and the mechanical properties of the alloys was established.Furthermore,the nano-scale microstructure and fracture mechanism of the prepared alloys were investigated by microstructural observation and fracture morphology analysis after deformation.In order to reveal the high-temperature strengthening mechanism of NiAl-based alloy with network-like microstructure,the three-dimensional morphology characterization and the deformation mechanism at service temperature were studied.The tomography method was employed to observe the 3D morphology of micro and nano Cr(Mo)phases.The formation mechanism of network-like microstructure was further investigated by heat treatment of the original powders and the evolution of the network microstructure was discussed.Moreover,the microstructure of NiAl-based alloy with different strain was observed by TEM.It was found that the alloy deformed through the dislocation sliding and climbing at service temperature.Based on the deformation mechanism,the strengthening effect of the typical high-temperature strengthening mechanisms was evaluated.It is noted that the excellent high-temperature performance of the prepared NiAl-based alloy results from the integrated effect of the load bearing mechanism and the dislocation blocking mechanism by the high connectivity nano-sized network.To solve the problem of uneven density of closed-section components prepared using intermetallic compound and hot-press sintering in the height direction,the combined sintering method is employed,which divides the closed-section components into several parts and compact them step by step.In combined sintering,the densification and the connection can be realized simultaneously.The evolution of microstructure and properties of the samples prepared by combined sintering were analyzed.The results show that during combined sintering,the densification of the powders and the joining between the powders and the surface of the pre-sintered part can be achieved simultaneously,resulting from the forming of the sintering neck and thus obtaining an interface without defects.The tensile strength at HT and 3-point bending strength at RT of the interface are both in the range of the strength for the two base alloys,which indicates a good interface bonding.In addition,the effects of multiple sintering processes on the microstructure and properties of the interface and the pre-sintered part were analyzed by two-step combined sintering.It was found that two-step combined sintering of the obtained NiAl-based alloy would not lead to significant change of the microstructure and deterioration of the properties for the pre-sintered part or the interface.Based on the results of combined sintering,the process for preparing closed-section components using brittle and hard-to-deform NiAl-based alloy was studied,which aims to fabricate the tubular component with length thickness ratio of 12 and the flange cone component.The hollow mandrel was used to relieve the hoop stress caused by the difference of thermal expansion during cooling and eliminates the circumferential cracking defect of the component.According to the simulation and experimental results,the controlling parameters to manufacture the dense tubular component were obtained,and the pre-sintered parts with uniform density were prepared.Then,the combined sintering experiment was carried out.The results show that the properties of the obtained components are similar to the sintered samples with the same parameters,and the interfacial strength of the components is between the two base alloys,which indicates the good interface bonding.