Study On The Effect Discipline Of TiC Particles And TiB Whiskers On Residual Stresses Of Ti Matrix Compsites

Discontinuous reinforced titanium matrix composites have bright application prospect because of their excellent mechanical properties.However,when cooling from elevated temperature to room temperature,residual stresses are generated in composites due to the mismatch of thermal expansion between reinforcements and Ti,which have significant impact on mechanical properties of composites.Currently,there are numerous researches on residual stresses in one inclusion reinforced Ti matrix composites.But it needs a further discussion about effect of multiple reinforcements on residual stresses in Ti matrix composites.In this study,in-situ synthesized TiC/Ti?TiB/Ti and(TiC+TiB)/Ti composites are produced by vacuum melting with Ti,C and TiB2.The discipline of effect of TiC and TiB on residual stress and mechanism of effect of residual stresses on composites' yield strength are studied,and superposition relationship of residual stresses in multi-inclusions reinforced Ti matrix composites is revealed.(1)In the in-situ synthesized Ti matrix composites,TiB presents short fiber,distributes randomly;TiC present two morphologies,fine equiaxe-shaped eutectic TiC and dentrite-shaped primary TiC,respectively.After forging and annealing,TiB distribute along with forging direction,aspect ratio of which deceases;TiC still have two shapes,equiaxed and ellipsoidal,in which ellipsoidal TiC is generated with dentrited primary TiC fracturing axially in the forging process.(2)Residual stresses in the cast 5vol%TiC/Ti,5vol%TiB/Ti,5vol%and 10vol%(TiC+TiB)/Ti composites are 113MPa,-78MPa,15MPa and-2MPa,respectively.Because coefficient of thermal expansion of Ti is in the middle of those of TiC and TiB,the thermal residual stress is in tension in TiC/Ti composites,which is under compression in TiB/Ti composites,and these two opposites stresses exist simultaneously and counteract in the(TiC+TiB)/Ti composites.Based on the separation equations of residual stresses,superposition equations of stresses and Eshelby model,we establish residual stresses fitting formula of(TiC+TiB)/Ti composites,With calculation,residual stresses caused by TiC and TiB satisfy this superimposed relationship in q=1.5.(3)After forging and annealing,residual stresses perpendicular to the forging direction in 5vo%TiC/Ti,5vol%TiB/Ti,5vol%and 10vol%(TiC+TiB)/Ti composites are 27MPa,-21 MPa,-4MPa and-19MPa,respectively.Same with cast composites,there are two opposite residual stresses,which counteract with each other,in annealed(TiC+TiB)/Ti composites.Modeling residual stresses in annealed(TiC+TiB)/Ti composites with fitting formula above,stresses generated by addition of TiC and TiB are in superposition relationship with q=2.(4)Tensile and compressive properties of Ti and Ti matrix composites are tested.The difference between compressive and tensile yield strength of pure Ti is only 4MPa,while that is quite large in composites.When in tension,external load superimposed on the residual tensile stress and Ti matrix yielded easily;when in compression,matrix yields after overcoming tensile residual stresses.Therefore,the phenomenon that yield strength of composites in compression is greater than that in tension is caused by residual stresses in matrix of composites.