Preparation And Properties Of In-Situ TiC_x-Ni₃?Al,Ti?/Ni Functionally Graded Composites

During recent years,much attention has been paid to metal-based functionally graded materials due to their remarkable mechanical behaviors,high toughness,high strength,high hardness,and good thermal stress relaxation.However,with the increasingly active demand for the improvement of mechanical performances and service life in the critical heat-resistent parts in the advanced aerospace engines,a novel metal-based functionally graded materials combined high hardness in external with high toughness in internal should be designed accordingly.Herein,this thesis focuses on preparing a novel TiCx-Ni3(Al,Ti)/Ni functionally graded materials by in-situ hot press sintering technology with the starting materials of Ni and Ti3AlC2 powders so as to satisfied the aforementioned conditions,to.The microstructure characterization and phase compositions of the TiCx-Ni3(Al,Ti)/Ni functionally graded materials were analyzed by means of scanning electron microscopy(SEM),X-ray diffraction(XRD),and energy dispersive spectroscopy(EDS).Meawhile,the relationship between microstructure and volume fraction of Ti3AlC2 was also analyzed.In addition,the mechanical properties of the gradient composites were investigated,and the effects of gradient structure and Ti3AlC2 content on the mechanical properties of the gradient composites were investigated.Finally,the reaction process of the gradient composites and the relationship between microstructure and mechanical properties were summarized.The experimental results were showed as following:(1)In this paper,a novel type of functionally graded material was successfully fabricated from pure Ni and Ti3AlC2 powder mixtures by hot-press sintering route at 1200? and 17 MPa for 60min.The prepared FGM exhibits uniform interlayer transition,achieving an advantages balance of high hardness and toughness from external to internal area.(2)Ti3AlC2 particles transformed into TiCx phase at high temperature,while the additional Al-Ti atoms decomposed from Ti3AlC2 diffused into the Ni matrix,giving rise to the formation of ?'-Ni3(Al,Ti).Part of the submicron TiCx and Ni3(Al,Ti)particles were uniformly distributed in the Ni matrix,and some of the TiCx maintained the original Ti3AlC2 plate-like morphology.As the volume content of Ti3AlC2 particles increases,TiCx particles in the matrix are distributed more uniformly,and the interspace between TiCx particles becomes smaller and smaller.(3)The prepared TiCx-Ni3(Al,Ti)/Ni functionally graded materials have excellent mechanical properties,and the flexural strength and Vickers hardness gradually increase with the increase of Ti3AlC2 volume content.The overall fracture toughness and flexural strength reached 21.09 MPa·m1/2 and 1329±34 MPa,respectively.From the 10Ti3AlC2/Ni layer to the 60Ti3AlC2/Ni layer,the hardness value is gradually increased up to 7.12 GPa,and the hardness of the transition zone of the two layers is continuously changing,without step-type mutation.Furthermore,the transition layer has excellent shear strength,and the maximum shear strength value is 590.14 MPa.(4)The prepared TiCx-Ni3(Al,Ti)/Ni functionally graded materials perform excellent thermal shock resistance.When the thermal shock temperature is lower than 1000?,the bending performance does not decrease significantly after 10 thermal shock cycles.When the thermal shock temperature reach 1200?,the bending performance decreases with the increase of the number of thermal shock cycles,which decreases from 1329 MPa before the thermal shock to 843 MPa.This is mainly due to the increase of thermal shock temperature during thermal shock,which causes more thermal stress inside the gradient composite.(5)There was no interlaminar fracture in the transition layer after thermal shock,and there was no large crack in each layer of composite.This is mainly due to the native attribute of gradient structure.The gradient of the internal composition of the gradient material could alleviate part of the thermal stress.Increasing the thermal shock temperature and the number of thermal shock cycles resulted in the increase in thermal stress,hence,brought about the great formation of pores and defects.