Glass-Forming Ability, Mechanical Properties And Microstructure Of Ternary Ti-Cu-Ni Alloys

Over the past few decades, bulk metallic glasses (BMGs) have attracted extensive interests due to their unique physical, chemical and mechanical properties, associated with the high yield strength, large elastic limit, corrosion resistance and wear resistance, etc. Much more attention was paid to Ti-based amorphous alloys not only because of the advantages mentioned above but also their low density and relatively low cost. Aiming to explore new and practical Ti based bulk metallic glasses, the glass-forming ability (GFA), thermal stability, mechanical properties and microscopic morphology of ternary Ti50Cu50-xNix(x=5, 8, 11) alloys have been investigated. The results are shown as follows:(1)A kind of new and low cost Ti-based BMG Ti50Cu42Ni8 with a diameter of 2mm has been fabricated by copper-mould casting method. XRD and DSC have been employed to investigate the glass-forming ability(GFA). Compared with Ti50Cu45Ni5 and Ti50Cu39Ni11 alloys, Ti50Cu42Ni8 alloy has been proved to be a prominent glass-former. The high and observable glass transition temperature Tg(659K), high crystallization temperature Tx(716K),remarkable supercooled liquid regionΔTx=Tx-Tg (57K) and large reduced glass transition temperature Trg=Tg/Tm (0.59) suggest that the alloy is a good glass former with good GFA and thermal stability.(2)For the rod samples with a diameter of 2mm, the XRD patterns indicate that the Ti50Cu42Ni8 alloy is fully amorphous and Ti50Cu45Ni5 and Ti50Cu39Ni11 are partially amorphous with a significant fraction of crystalline phases. The crystalline phases were identified as TiCu and TiNi for the Ti50Cu45Ni5 and Ti50Cu39Ni11 alloys, respectively.The Ti50Cu42Ni8 alloy has a low liquidus temperature Tl and composition close to ternary eutectic point, which can avoid the precipitation of the primary TiCu phase in the Ti50Cu45Ni5 alloy with the smaller amount of Ni (5at.%) or the primary TiNi phase in theTi50Cu39Ni11 alloy with the larger amount of Ni (11at.%). The local atomic structure of the liquid, tending to form local clusters consisting of Ni-Ti atomic pairs in Ti50Cu42Ni8 alloy can lead to an increase in the degree of dense random packed atomic configurations and make the rearrangement of the constituent elements on a long-range scale difficult. This favors the stabilization of the liquid phase with respect to competing crystalline phases during solidification. Thus, the structure of the liquid of the Ti50Cu42Ni8 alloy is more stable than those of Ti50Cu45Ni5 and Ti50Cu39Ni11 alloys which exhibit a high Tl and off-eutectic composition attributed to the precipitation of the primary TiCu and TiNi phase during solidification, respectively, resulting in the improvement of GFA and the forming of BMG.(3)Fracture behaviors under monaxial compression at room temperature of ternary Ti50Cu50-xNix(x=5, 8, 11) alloys have been investigated. The sample of Ti50Cu42Ni8 alloy shows outstanding mechanical properties: high fracture strength is 2008MPa which is larger than that of Ti50Cu45Ni5 and Ti50Cu39Ni11(1500Mpa,1400Mpa )and low Young's modulus of 100 GPa, indicating that very high values of elastic energy can be stored in the material. Microscopic morphology of fracture of Ti50Cu42Ni8 alloy has been observed. Under compressive loading the fracture takes place along the maximum shear plane, inclined by about 45°to the direction of the applied load indicating dominance of shear stress in the failure process. Further observations show that the fracture surface showed a well-developed vein pattern. Also it can be noticed that local melting as evidenced by the formation of"liquid droplets"takes place during shear deformation. This well-developed vein and local melting pattern is consistent with the observations in most metallic glasses, which is attributed to local softening or melting induced by the decrease of viscosity or local adiabatic heating within the shear band.(4)By analyzing the microscopic morphology of Ti50Cu50-xNix(x=5,8,11) alloys, no visible crystalline phase was found from the edge and central region of the cross-section of the 2 mm rod sample of the Ti50Cu42Ni8 alloy ,whereas, a great deal of crystalline phases were found from the surface of both Ti50Cu45Ni5 and Ti50Cu39Ni11 alloys. Ti50Cu42Ni8BMG separates out strip and club crystalline phase after be annealed at 480℃and 650℃, respectively.From above results, one can see that the ternary Ti rich-Cu rich-Ni alloy system is a good candidate for developing new bulk metallic glasses. It is expected that an addition of additional elements to the Ti-Cu-Ni alloys will improve the glass forming ability even further.