Deformation And Fracture Behavior Of TiBw/TA15 Titanium Matrix Composite With Network Reinforcement Architecture At Elevated Temperatures

With the development of the technology,there are increasing demands for materials with higher strength and higher service temperature from a wide industrial sectors.Consequently,as an important member of the composite family,metal matrix composites,such as titanium matrix composites(TMCs),have attracted much attention in the past half century.In particular discontinuously reinforced titanium matrix composites have been attracting considerable interest in recent year due to their excellent properties including high specific stiffness,high specific strength and low cost.In the context of TMCs,a novel structure with three-dimensional quasi-continuous network distribution of in situ reinforcing TiB whiskers in TA15(Ti-6Al-2Zr-1Mo-1V)titanium matrix has been developed to improve the mechanical properties and service temperature.However,there is a lack of understandings of high temperature deformation and fracture behavior of TiBw/TA15 composites with a network structure,it is therefore of significant interest to investigate the high temperature mechanical properties for further improvement and application.In this thesis,the deformation behavior of a 3vol% TiBw/TA15 with quasicontinuous network microstructure fabricated by reaction hot pressing was investigated by using in-situ SEM tensile test at 700?.The microstructure and fracture morphology of the fracture edge were observed by SEM after fracture to infer the deformation behavior of composites at room temperature.The strain maps were calculated using digital image correlation(DIC)method and compared with the results obtained from numerical simulations to investigate the role of discontinues network reinforcement architecture during deformation.Key research findings are shown below.(1)Firstly,at room temperature,there was almost no cracks occurred in matrix and cracks only occurred in TiB whisker reinforcement without any multiple cracks.In most cases,only one crack propagated in reinforcement along zigzag path or split the reinforcement.There is also slip band occurred in in the matrix ? phase.(2)At 700?,microcracks almost simultaneously initiated in TiB whisker and in ? phase of TA15 and would be inhibited when they propagated into ? phase of TA15.With the increase of deformation,multiple fractures appeared in TiB whisker and ? phase and slip bands were observed in ? phase.This demonstrates the strengthening effect of the TiB has been fully exploited and the plastic deformation of ? phase played a vital role in retarding cracks propagation.As the microcracks begun to accumulate,these cracks would joint together and propagated along slip bands in ? phase leading to the final fracture.The fracture mechanism of the composites is predominantly controlled by TiB whisker fracturing or debonding followed by fracture of ? phase and plastic deformation of ? phase.The macroscopic fracture surfaces exhibit a honeycomb-like feature and it illustrates the cracks mainly propagated along the network boundary.(3)For the strain maps calculated by DIC using the SEM images at 700? and OM images at room temperature,it can be seen that,at 700?,in the elastic stage,the larger strain area and smaller strain area were adjacent to each other around the reinforcement and in the plastic stage,the strain was prone to ±45° distribution with respect to the loading direction,and concentrated in the area near the microcracks.At room temperature,the strain was uniform in elastic stage and concentrated in the middle of sample in plastic stage as expected.Cracks mainly propagated along the network boundary leading to the final fracture.(4)The results of 2D numerical simulations indicate that,for the discrete reinforcement model within the network region,the network structure has little influence on the stress distribution of the composite material,but it can change the strain distribution,so that the strain is prone to ±45° distribution with respect to the loading direction,the strain distribution is relatively uniform,and the tendency of catastrophic fracture is reduced;for the continuous model,a network structure has a great influence on the stress distribution.In the 2D model,the network section parallel to the tensile direction plays the role of bearing stress,while the network section perpendicular to the tensile direction only starts bearing stress at the later stage of deformation,which has a great influence on the generation of cracks.