Fabrication Of New Composite Inoculants And Their Effects On Microstructure And Mechanical Properties Of Titanium Alloys

Titanium and its alloys are extensively used as structural and engineering materials in aerospace,vehicle,petroleum chemical industry,medical equipment and other fields,due to their low density,high specific strength and specific modulus at high-temperatures,excellent corrosion behavior,and attractive fracture related properties.With the rapid development of science and technology,the higher requirements on the mechanical properties of titanium alloy have been put forward.However,the as cast titanium alloy often accompanied by large size of the grain,poor ductility,low toughness and fatigue resistance,what severely limit its application.Grain refinement by the addition of inoculants is acknowledged as an economical industrial technology and an effective route to prodigiously improve the mechanical properties of cast metal,which has been widely used in aluminum alloys,magnesium alloys,and steel materials.But up to now,litter work has been done for the research and application related to the inoculation refining in titanium alloys,and the exact grain refinement mechanism is unclear.Therefore,the development of effective inoculants refining in titanium alloys and its related techniques,obtainment the as-cast titanium alloy with fine-grain structure have an application prospect and realistic significance.In present study,new composite inoculants?in situ Ti₅Si₃/Ti composite inoculants,in situ?TiB+TiC?/Ti composite inoculants and in situ?TiB+TiN?/Ti composite inoculants?with in situ strengthening phases,were successfully prepared by vacuum arc melting combining with vacuum rapid solidification technique.The phase composition and the microstructure of the master alloy and the ribbon inoculants were studied,and the refining and reinforcing effects of the new composite inoculants on pure Ti and Ti6Al4V alloy were analyzed.After vacuum rapid quenching treatment,the microstructure of the inoculant mater alloy and the size of the in situ strengthening phase are obviously refined.The microstructure of the in situ Ti₅Si₃/Ti composite inoculants is the inerratic isometric-shaped cellular structure with the mean particle size is about 70-90 nm and the average grain size of the Ti₅Si₃ strengthening phase is refined to 12.3 nm.At the same time,the sizes of the TiC,TiN particles and TiB whisker in the in situ?TiB+TiC?/Ti composite inoculants and in situ?TiB+TiN?/Ti composite inoculants were refined into nano-scale.When adding into titanium alloy,the large quantities of nano-sized in situ strengthening particles and whisker in new composite inoculants can work as the cores for heterogeneous nucleation as well as the strengthening phases.So can improve the heterogeneous nucleation rate and restrain the growth of the crystal nucleus in solidification,contributing to modify the titanium alloy by fine-grain strengthening and particle strengthening.The well-banded particle/matrix interface ensures the good wettability between the nano-sized ceramic particles and the matrix titanium alloy melt and significantly promotes the heterogeneous nucleation effect.The new composite inoculants have excellent modifying and reinforcing effects on pure Ti and Ti6Al4V alloy.The grain size of the Ti6Al4V alloy can be refined from a grade of 790±52?m to about 109±26?m with a very small in situ Ti₅Si₃/Ti composite inoculants adding ratio of 0.6%in weight.With the adding of the inoculant,the mechanical properties of the titanium alloy,including tensile strength,microhardness,impact properties,and resistant properties were improved obviously,but the toughness was slightly declined.When the addition of in situ Ti₅Si₃/Ti composite inoculants have reached 0.4 wt.%,the Ti6Al4V alloy showed the best mechanical properties of tensile strength,reaching 1105 MPa,which represented an increase of 12.6%compared to Ti6Al4V,where the elongation decreased by 13.1%.At the same time,the microhardness increased from 91.1±1.4 HRB to 112.9±1.0 HRB,the impact toughness decreased by 50.1%and the tendency of brittleness increased.The size of the Ti₅Si₃particle in Ti6Al4V alloy modified by in situ Ti₅Si₃/Ti composite inoculants was less in20 nm,while the size of the TiN particles in Ti6Al4V alloy modified by the in situ?TiB+TiN?/Ti composite inoculants was about 20-30 nm and the diameter of the TiB whisker was about 5 nm.The nano-sized reinforcements distributed homogeneous in the grain,result in a significant intracrystalline reinforcing effect in the titanium alloy.The reinforcement effect of the new composite inoculants was better than that of the traditional in situ strengthening phases.