Martensite Transformation And Strain Recovery Characteristics Of TiB/Ti-Ni-Hf Shape Memory Alloy Composite With Network Structure

Ti-Ni-Hf alloys are the most promising high temperature shape memory alloys due to its various advantages such as:adjustable transformation temperature within a higher temperature range,good workability,relatively lower price and so on.However,the lower matrix strength and smaller recoverable strain restrict their extensive application.In the present study,TiB/Ti-Ni-Hf composites containing network structure prepared by combining lower energy balling and hot-pressed sintering,which possess higher transformation temperature,higher strength and larger recoverable strain.The microstructure,martensitic transformation behaviors,mechanical properties and strain recovery characteristics of TiB/Ti-Ni-Hf composites with network structure are investigated systematically by X-ray diffraction(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM),differential scanning calorimetry(DSC),characterization of mechanical properties and strain recovery features,and the mechanisms of achieving superior functional properties in TiB/Ti-Ni-Hf based composites with network structure are clarified.TiB2,B,B4C and LaB6 ceramic particles are attached at the surface of Ti-Ni-Hf alloy powder by lower energy balling(200 r/min,300 min,ball-to-powder ratio=5:1,under high pure Ar atmosphere),and sintered at the optimum sintering process(sintering temperature and time 1100?/1h;pressure of 60MPa),in-situ TiB,TiB and HfB2,TiB and HfC,TiB and La2O3 phase form respectively,and the TiB/Ti-Ni-Hf based composites with network structure are successfully prepared.Experimental results reveal that the amount of in-situ reinforcements at the surface of Ti-Ni-Hf alloy powder increases with the increasing of ceramic particle,making the evolution from quasi-continuous network structure to continuous network structure.The TiB/Ti-Ni-Hf based composites with network structure prepared by adding TiB2 or LaB6 ceramic particle are in B19' martensite state at room temperature;while the B powder and B4C ceramic particle react with both Ti and Hf for network structure TiB/Ti-Ni-Hf based composites fabricated by adding B powders and B4C ceramic particle,which leads to the transition from B19' martensite to B2 austenite.In addition,consumption of excessive Ti and Hf induces the presence of high density of nanoscale coherent H phase,when 1.4wt.%B powder and 1.0wt.%B4C ceramic particle are added,respectively.It is revealed that the stress field with a gradient distribution around the in-situ reinforcements of quasi-continuous network structure in TiB/Ti-Ni-Hf based composites induces transition from preferentially orientated martensite to self-accommodation martensite.Moreover,the degree of selective orientation of martensite heightens with the in-situ reinforcements showing a quasi-continuous network structure increasing.When the volume fraction of in-situ reinforcement forming the quasi-continuous network structure is less,the preferred martensites are dominated by the favorable martensite;however,the preferred martensite around the network structure constructed by major in-situ reinforcements is featured with single martensite variant.The stress concentration and plastic deformation occur at local zone around the continuous network structure owing to the constraint of continuous network structure in TiB/Ti-Ni-Hf based composites;single-oriented and fine martensite variant pair presents at the area slightly far from the network structure in order to reduce total energy of the whole system;plastic deformation zone can be avoided by inducing larger numbers of coherent H precipitates in the matrix.It is found that TiB/Ti-Ni-Hf based composites with network structure show one-stage B2(?)B19' martensite transformation.The reaction between the ceramic particle and Ti(or Hf)element in Ti-Ni-Hf alloy powder results in the decrease of(Ti+Hf)/Ni ratio,which is the main reason for the descrease of martensite transformation temperature.The formation of preferentially orientated martensite in TiB/Ti-Ni-Hf based composites with a quasi-continuous network structure results in the decrease of elastic strain energy,which should be responsible for the smaller transformation temperature hysteresis;However,the existence of plastic deformation zone around continuous network structure increase the dissipated energy(?2.67J/m2)during martensite transformation,resulting in the increase of transformation temperature hysteresis of TiB/Ti-Ni-Hf based composites with continuous network structure;the presence of nanoscale coherent H precipitate can further enhance the matrix strength,which can avoid the occurrence of plastic deformation zone and reduce the transformation temperature hysteresis.Both the mechanical properties and shape memory effect of TiB/Ti-Ni-Hf based composites with network structure firstly increase and then decrease with the increasing of content of ceramic particle.When 0.6wt.%LaB6 ceramic particles are added into Ti-Ni-Hf alloy powders and the size of network structure is fixed as 115?m,TiB/Ti-Ni-Hf composite with a quasi-continuous network structure exhibits the outstanding mechanical properties with a fracture stress of>3GPa and fracture strain of>25%.In addition,the maximum completely recoverable strain of 8%containing 3.50%shape memory effect strain can be obtained in martensitic state for TiB/Ti-Ni-Hf composites with network structure.The higher strength and larger complete recoverable strain can be attributed to the larger partial stress concentration factor of in-situ reinforcement showing network structure,which makes the larger load-bearing capability of reinforcements and larger elastic spring back of martensite during unloading.TiB/Ti-Ni-Hf composites containing the network structure and nanoscale coherent H precipitates show the perfect superelasticity with a fully recoverable strain of 10%in austenitic state,which is improved by 67%than the reported Ti-Ni-Hf shape memory alloys.