Structural Manipulation And Its Effects On Mechanical Properties Of Ti-and Zr-based Bulk Metallic Glass Composites Ductilized By The B2-structured Phase

Bulk metallic glasses(BMGs)have shown outstanding mechanical properties such as high strength and large elastic limit.However,room-temperature brittleness and strain softening remain the critical challenges for their structural applications.Unlike the crystalline materials,BMGs do not have the "dislocation"type crystalline defects and therefore the related work-hardening mechanisms due to the lack of the long-range ordered atomic packing structure.The plastic deformation of BMGs is limited in very localized regions(i.e.,shear bands),leading to the catastrophic brittle failure.It was found that introducing crystalline phases into the BMG matrix is an effective way to promote multiplication of shear bands,inhibit propagation of shear bands and thus improve the plasticity.Particularly,design of TRIP(transformation-induced plasticity)-reinforced bulk metallic glass composites(BMGCs)via introducing a metastable crystalline phase which could transform into a more stable phase during loading was considered to be one of the effective ways to simultaneously overcome room temperature brittleness and strain softening of BMGs.However,the morphology characteristics of the reinforcing phases in the TRIP-reinforced BMGCs is hard to control,and especially in the TRIP-reinforced Ti-and Zr-based BMGCs,the metastable parent crystalline phases are prone to coarse and thus distribute inhomogeneously.All these dramatically deteriorate the ductilization effect of the crystalline phases and lead to large variation of the mechanical properties of BMGCs.In order to overcome the aforementioned issues,we have attempted to modulate the morphology characteristics of the metastable crystalline phases by microalloying to control the in-situ B2 precipitation and introducing ex-situ transformable media.The relationship between the microstructures,martensitic transformation and the mechanical performance under different loading modes were investigated.The main findings of this thesis are as follows:(1)To address the issue of inhomogeneous distribution of the metastable B2 crystalline phase,which deteriorates the TRIP effect and the resultant mechanical properties in Ti-Cu-Ni-Zr TRIP-reinforced BMGCs,we added 0.2?0.3 at.%of tantalum in a typical Ti-Cu-Ni-Zr alloy and successfully achieved the homogenous precipitation of B2 phase and simultaneously improves the strength and plasticity.It was found that microalloying of Ta could not only promote nucleation of B2 phase,but also induce precipitation of the disordered A2 phase embedded in the B2 phase.The large amount of A2 nanoparticles inhibits the coarsening and rapid growth of the B2 phase.During quasi-static deformation,the A2 nanoparticles facilitate the nucleation of martensites with multiple orientations inside the B2 phase,and inhibit the rapid propagation of the transformed martensite plate,thus homogenizing the plastic flow and enhancing the mechanical properties.The quasi-static compressive fracture strength,plasticity and working hardening exponent of Ti44.7Cu41Ni9Zr5Ta0.3 are?3050 MPa,?25%and 0.11,respectively.More strikingly,it also exhibited plastic strain of nearly 1%under quasi-static tension.(2)In order to solve the poor structural reproducibility of the in-situ B2 phase-reinforced BMGCs,the ex-situ metastable phase was added.We found that introducing ex-situ TiNi fibers into Zr47Ti13Cu11Ni10Be16Nb3 BMG could greatly improve consistency of mechanical properties.With addition of 50 vol.%TiNi fibers,the Zr47Ti13Cu11Ni10Be16Nb3 BMGC exhibits a combination of simultaneously enhanced fracture strength of 2134±46 MPa(30%higher than that of the base alloy),improved plasticity of 13.4± 1.0%and strong work-hardening ability under quasi-static compression.Moreover,the newly developed BMGCs show ductility of 0.8±0.2%under tension.The introduction of ex-situ TiNi fibers not only incorporates the TRIP effect as expected,but also coordinates the plastic deformation of the TiNi fibers and the amorphous matrix,thus enhancing the mechanical performance.By introducing the TiNi fibers,structure can be much easier to be controlled and the reproducibility of mechanical properties was greatly improved.(3)In view of the possible usage of metallic glassy materials under extreme conditions,the high-strain-rate deformation of TRIP-reinforced BMGCs was systematically studied,and effects of the martensitic transformation of the B2 phase on hardening characteristics at high strain rates were also clarified.It is showed that the TRIP-reinforced Cu46 25Zr48Al4Ag1Sn0.75 BMGC characterized with outstanding dynamic mechanical properties,i.e.,with a unique combination of ultimate strength of?1550 MPa,fracture strain of?15%and unusual two-stage work-hardening behavior upon dynamic loading.Additionally,the newly developed TiNi fibers-reinforced Zr47Ti13Cu11Ni10Be16Nb3 BMGCs also achieve fracture strains of about 15%under dynamic loading.At high strain rates,the optimal volume fraction of crystalline phases is around 40%for effective ductilization of the current TRIP-reinforced BMGCs,much lower than that of traditional dendrite-reinforced BMGCs,which avoids the substantial decrease of strength.In addition,we found that the deformation-induced martensitic transformation can also take place at high strain rates,effectively releasing the local stress concentration,retarding rapid propagation of shear bands and promoting formation of multiple shear bands,and eventually homogenizing the plastic deformation.Moreover,it was found that the competition between the TRIP effect and adiabatic heating determines the fracture behavior as the strain rate increases.On one hand,the martensitic transformation would be stimulated,which is beneficial for the TRIP effect.On the other hand,the local adiabatic heating is also aggressively intensified with the strain rate,which gives rise to strain localization.It should be noted that the TRIP effect is still the dominant mechanism on the strain-rate hardening and the enhancement of dynamic mechanical properties.As elaborated above,the TRIP-reinforced BMGCs show outstanding dynamic properties,which enable them great potential for engineering applications in the high-speed field.