Fatigue Crack Growth Behavior And Microstructure Evolution Mechanisms Of Hybrid-reinforced Titanium Matrix Composites At High Temperature

Hybrid-reinforced titanium matrix composites（TMCs）present good mechanical properties at room temperature and high temperatures,and they have been widely used in aerospace fields,such as aero-engines and high-speed aircraft.The coupling effect of heat and force at high-temperature service environment makes fatigue fracture become an important failure mode of TMCs.It is of great significance to investigate the fatigue fracture of TMCs at high service temperature,which can promote the wider application of TMCs in the aerospace field.Adding hard ceramic phases into Ti6Al4V alloy such as short TiB fibers and TiC particles is expected to increase the service temperature by 100～200℃.However,at present,there are few studies on the high-temperature fatigue of TMCs,and the change rules of high-temperature fatigue crack growth rate（FCGR）,and the effects of reinforcements on fatigue crack growth（FCG）have not been fully understood,thus,it is difficult to evaluate the remaining life of TMCs containing defects.In this paper,the Ti6Al4V matrix composites reinforced with unit TiC particles and hybrid（TiB+TiC）are taken as the research objects to study the FCG behavior and microstructure evolution mechanism of hybrid-reinforced TMCs.The major findings are as followed:（1）The effect of the volume fraction ratio of short TiB fibers and TiC particles on the mechanical properties of in-situ TMCs was elucidated.When the total volume fraction of reinforcement was 8%,the aspect ratio of TiB decreased with the decreasing TiB content;the TiC diameter increased with the increasing TiC content.After forging and heat treatment,the rod-likeα-Tiprecipitated from the C-poor TiC particles,which aggravated the stress concentration inside the TiC particles,resulting in the formation of microvoids and microcracks within the TiC during deformation,thus,reducing the strengthening effect of the TiC particles.When the volume fraction ratio of short TiB fibers and TiC particles was 6:2,the short TiB fibers had good load-bearing effect,and the TiB and TiC brought significant dislocation strengthening effect,so that the TMCs had excellent room temperature and high-temperature mechanical properties,to achieve a balance between high strength and plasticity at room temperature.The tensile strength at 450℃exceeded 750MPa,and the tensile strength at 550℃reached 630MPa.（2）It was found that the high-temperature FCGR（da/d N）of TMCs was not sensitive to the change of matrix microstructure,but da/d N increased with the increasing stress intensity factor amplitudeΔK and temperatures.Among the TMCs with different volume fraction ratios of short TiB fibers and TiC particles,（6vol.%TiB+2vol.%TiC）/Ti6Al4V had the most excellent fatigue crack resistance at allΔK and high temperatures,and its Paris formula was da/d N=7.19×10^-10（ΔK）^1.902,and that of Ti6Al4V alloy was da/d N=2.55×10^-10（ΔK）^2.310,whenΔK≤10MPa?m^1/2,the FCGR of TMCs was higher than that of Ti6Al4V alloy;withΔK further increasing,ΔK≥15MPa?m^1/2,the FCGR of（6vol.%TiB+2vol.%TiC）/Ti6Al4V was significantly lower than that of Ti6Al4V alloy.It was also found that the logarithm of the high-temperature FCGR of TMCs(lg（da/d N）had a linear growth relationship with the temperatures（T）,which was helpful to quickly predict the high-temperature FCGR of TMCs.（3）The effects of matrix microstructure,ΔK and temperatures on the high-temperature FCG behavior of TMCs were studied.WhenΔK<20MPa?m^1/2,the high-temperature fatigue cracks all grew discontinuously in the damage accumulation mode,and the fatigue striation spacings were smaller than the high-temperature FCGR.The fatigue cracks in the TMCs with equiaxed microstructure mainly propagated along the primaryα/βboundaries and within the primaryαphases;and fatigue cracks in the TMCs with duplex microsturcture mainly grew within theβtransition microstructure;and those in the TMCs with basketweave microstructre grew through the lamellarαphases.With the increase ofΔK,the high-temperature fatigue cracks of TMCs evolved from invading the reinforcements and growing along a straight line to deflecting at the interface or the end of the short TiB fibers,frequently showing an"n"-shaped propagation path.As the temperature increases,the strain incompatibility between the matrix and reinforcements at 550°C was weakened.When the temperature was lower than 450°C,the fatigue fracture was characterized by brittle fracture,the crack propagation path was tortuous,and there were many secondary cracks;however,the fatigue fracture at 550°C was characterized by ductile fracture,and the fatigue cracks mainly propagated at the TiB end,and there were less secondary cracks.（4）The crystallographic characteristics of high-temperature FCG of TMCs,the damage behavior of short TiB fibers and TiC particles,and the mechanism of FCG were revealed.When loading along the long axis of short TiB fibers,the surrounding grains around TiB and TiC had hard orientations of basal or prismatic slip,and the prismatic and pyramidal slip led to high-temperature FCG,mainly pyramidal slip.During cyclic loading,the rod-likeα-Tiin the TiC aggravated the stress concentration inside the particles,forming lots of microvoids and microcracks,and accelerating the FCGR.At low temperatures,the serious internal stress concentration resulted in lots of microvoids forming in the short TiB fibers.The microvoids decreased but oxidation occured with the increasing temperature,and the strength of short TiB fibers decreased.As theΔK increased,the size of the plastic zone was gradually larger than that of TiB/TiC,the deflection effect induced by TiB/TiC on fatigue cracks was enhanced,which reduced the driving force of FCG,resulting in roughness-induced crack closure effect,which slowed down the FCGR.（5）The microstructure of TMCs after fatigue test at 450℃was characterized,and the microstructure evolution mechanisms of hybrid-reinforced TMCs during high-temperature fatigue was revealed.Oxygen in high-temperature air diffused to theβphase at the crack tip.The elements diffusion such as O and V led to the nano-αphase nucleating at dislocations.The dislocation-induced many nano-αvariants preferentially nucleated and grew up.Different variants were beneficial to the deflection of the fatigue cracks as it propagated within theβphase,thereby slowing down the FCGR.The dislocations in the primaryαphase in the high-temperature fatigue crack tip plastic zone decomposed under high stress state,resulting in{101?2}twins nucleating at the primaryα/βboundaries,TiB interface,TiC interface and primaryαphase.The deformation twins were likely to nucleate in the grains around the reinforcements,which had hard orientations of prismatic/basal slip.The twins hindered the dislocation movement,coordinated the plastic deformation of hard-oriented grains around the reinforcements,deflected the fatigue crack,thereby improving the high-temperature fatigue performance of TMCs.