Transformation Induced Plasticity Of Ti-based Bulk Metallic Glass Composites

Bulk metallic glasses(BMGs)have attracted much attention due to their excellent properties.However,during deformation,it is easy to induce highly localized shear bands in the glassy matrix due to the lack of dislocation and other defects during deformation,resulting in the room-temperature brittleness and work-softening.In order to improve the room-temperature mechanical performance of BMGs,the fabrication of BMG composites should be one effective method.In particular,BMG composites with transformation induced plasticity are one of the best candidates.However,transformation induced plasticity effect was only found in CuZr-based or Be-bearing BMG composites.It is necessary to apply this concept to other alloy systems.In this paper,TiCu-based and TiNi-based alloy systems are adopted and the corresponding BMG composites with transformation induced plasticity are prepared by microalloying method and rapid solidification technology.Firstly,the as-cast Ti45.5Cu37.5Ni7.5Zr2.5Hf3Si1Sn3,Ti46.5Cu37.5Ni7.5Zr2.5Si1 Sn5,Ti43.5Cu37.5Ni7.5Zr2,5Hf3SiiSn5 and Ti42.5Cu37.5Ni7.5Zr2.5Hf5Sn5 rods with a diameter of 2 mm were prepared by vacuum arc suction-casting device.The experimental results show that the microstructure of TiCu-based BMG composites is composed of B2(Ti,Zr)(Cu,Ni)crystal phase and amorphous phase as well as a small amount of Ti2Cu intermetallic compounds at the interface between B2 crystals and the glassy matrix.During deformation process of TiCu-based BMG composites,transformation induced plasticity can be observed,where martensitic transformation occurs within B2 crystals and multiple shear bands can be induced,leading to a certain room temperature plasticity and work-hardening effect.Although it is found that the samples show good mechanical properties when the content of B2 crystal phase is higher than 50%,the ductility of TiCu-based BMG composites is worse than that of CuZr-based systems.According to the nanoindentation and TEM measurements,the main reason for the poor plasticity can be attributed to the large difference between Young's moduli of B2(Ti,Zr)(Cu,Ni)crystals and the glassy matrix and the precipitation of some Ti2CU intermetallic compounds at the B2 crystal boundaries,which aggravates the shear banding instability.It also means that high Cu content is not beneficial for the enhancement of the plasticity of Ti-based BMG composites.Secondly,in order to further develop novel Ti-based BMG composites,high-strength and high-toughness TiNi-based BMG composites were developed by reducing the Cu content and increasing the Ni content.We have also studied the martensitic transformation behaviors and mechanical properties of B2 TiNi crystal phase in TiNi-based alloys,and the glass-forming ability(GFA)and mechanical properties of TiNi-based BMG composites.The corresponding results are described as follows:(1)The as-cast Ti50-x/2Ni50-x/2Hfx(x = 0,2,4,6,8,10 or 12 at.%)rods with a diameter of 1.5 mm were prepared by rapid solidification are mainly composed of B2(Ti,Hf)Ni phase and B19'(Ti,Hf)Ni phase as well as(Ti,Hf)2Ni intermetallic compounds precipitating in the interdendritic region.With increasing number of thermal cycles,the martensitic transformation temperature(Ms)of Ti-Ni-Hf alloy decreases.For Ti50-y/2Ni50-y/2Siy(y = 1,2,3,5,7 or 10 at.%)alloys,the microstructure mainly includes B2 TiNi austenite phase,B19' TiNi martensite phase and Ti4Ni4Si7 intermetallic compound.The Ms temperature decreases with increasing number of thermal cycles when the content of Si is below 3 at.%.For more than 3 at.%Si,the Ms temperature remains almost constant.During the deformation of the as-cast rods mentioned above,the significant martensitic transformation and detwinned process occur,which significantly improve the plasticity and strength of alloys.As a result,the yield strength tends to increase with increasing Hf or Si content,but it is still relatively low because there is no any amorphous phase in the samples.(2)For the melt-spun TiNi-based ribbons,the Ti50-x/2Ni50-x/2Hfx(x = 2,4,6,8,10 or 12 at.%),Ti50-y/2Ni50-y/2Siy(y = 1,2,3,5,7 or 10 at.%)and Ti50-z/2Ni50-z/2Zrz(z = 1,5,6,8,or 10 at.%)alloy systems show a good GFA and the GFA increases with increasing the micro alloying element content.Besides,the precipitations in the glassy matrix for the ribbons is mainly B2 TiNi phase.Based on the influences of Hf and Si on the GFA of TiNi-based ribbons,TiNi-based BMG composites can be successfully achieved in the as-cast Ti46.5Ni46.5Hf2Si5 rod.Furthermore,a small amount of Nb,Al and Sn elements are also introduced in order to further enhance the GFA.Although the content of the amorphous phase in the present rods is still less,it can significantly increase the yield strength of the alloys and retain good plasticity.The alloys indeed exhibit excellent mechanical properties.For example,the yield strength,the fracture strength and the plastic strain of the as-cast((Ti46.5Ni46.5Hf2Si5)98Nb2)98Al1Sn1 BMG composites are 1099 ± 20 MPa,2519 ± 35 MPa and 13.3 ± 0.3%respectively.