Dynamic Mechanical Properties Of The TiB Whiskers Reinforced Titanium Matrix Composites With A Quasi-continuous Network Reinforcement Distribution

Fast spreading of high-end technologies in aerospace,military and civil industries brings many challenges in composite materials.In the context of metal matrix composites,TiB whiskers(TiBw)reinforced Ti6A14V titanium matrix composites(TMCs)with a quasi-continuous network reinforcement distribution has been developed to resolve the notorious problem of room-temperature brittleness in TMCs.Such a composite exhibit excellent room-temperature mechanical properties and much improved high-temperature strength compared to Ti6A14V matrix alloys.Considering the potential dynamic applications for TMCs and scarcity of the knowledge of dynamic behaviour of TiBw/Ti6A14V composites with a network architecture,it is of significant interest to investigate their dynamic mechanical properties.In this thesis,the split Hopkinson pressure bar(SHPB)was used to measure dynamic mechanical behaviour of TiBw/Ti6A14V composites with the strain rate range of 1000?3500s-1.Optical microscope(OM),scan electron microscope(SEM),tansmission electron microscope(TEM)and electron backscatter diffraction(EBSD)were adopted to identify useful microstructural information such as dislocations,twins and grain misorientation distribution.The classic Johnson-Cook constitutive model was modified to predict and explain mechanical properties of composites under compressive loading.Key findings are shown below.(1)During the dynamic compressive mechanical testing of the TiBw/Ti6A14V composites,a much higher yield stress of 1435 MPa at 3500s-1 strain rate was obtained demonstrating typical strain rate effect comparing with 1121 MPa yield stress at 0.001s-1 starin rate.This is further validated by the strain rate sensitivity which is 3.7 times larger than that under quasi-static loading.Dynamic loading also enhances the yield stress,yield strain,true plastic strain and plastic work with increasing strain rate.(2)The temperature rising from plastic work during dynamic loading was calculated and used in the modified Johnson-Cook constitutive model.A corrected heat-effect parameter was introduced to the Johnson-Cook model to take account of the thermal effect on the interface of composites,which remains as the major discrepancy between classic models and obtained experimental data.(3)The microstructure of deformed TiBw/Ti6A14V composites after dynamic loading was characterised.Tangled dislocations and twins were observed near the interface between TiB whiskers and matrix under strain rate of 1500s-1,which strengthens the the TiBw/Ti6Al4V composites.Twins are believed to have caused the abnormal increasing of strain hardening rate.A higher density of tangled dislocations and dislocations across the TiB whiskers were formed under strain rate of 3000s-1.,Stress distribution map from EBSD indicates that TiB whiskers act as the main stress bearable phase.Adiabatic shear band(ASB)was observed in addition to internal cracks due to coalescence extension of voids therein.It should be noted that ASB did not appear in samples tested at strain rates up to 2000s-1.(4)A four-stage deformation mechanism was proposed to explain the microstructural evolutions of TiBw/Ti6Al4V composites with increasing strain during dynamic loading.In the first stage,motion and tangling of dislocations contribute to the strengthening of the composites whereas twins cause the abnormal increase of strain hardening rate at relatively low strain rates up to 2000s-1 during the second stage.In the third stage,the deformation of composites reaches to 0.06 plastic strain get the same stress state due to the main stress bearable phase of TiB whiskers.Adiabatic shearing is observed in the scenarios of strain rates at 3000s-1 and 3500s-1 which led to the decrease in flow stress of the composites during the fourth stage,which is typical different failure mechanisms from that under quasi-static loading.Thermal softening effect works during the whole plastic deformation.