Study On The Strengthening And Toughing Mechanism Of Amorphous Alloys And Heterostructure Metals

Homogeneous materials always have the inverted relationship between strength and plasticity,which severely limits the engineering applications,such as room temperature brittleness and strain softening problems caused by the unique crystal structure of amorphous alloys.In response to these problems,the introduction of crystal structures with significant physical property differences to significantly overcome the property inversions,which has become one of the research hotspots in the field of materials in recent years.For amorphous alloys,the crystal second phase is formed by the internal or external method to block the shear band,overcome the early damage and improve the ductility.For crystal materials,the layered,bi-modal and gradient materials are prepared by surface mechanical grinding or cumulative rolling to form heterogeneous induced strengthening and hardening,which effectively overcomes the inverted relationship between strength and plasticity.In this paper,three main aspects are investigated for two heterogeneous materials,amorphous composites and layered materials:2D finite element simulations of the synergistic effects of TRIP effects and microstructural inhomogeneities on the ductility enhancement of amorphous composites(BMGc),atomic-scale molecular dynamics simulations of amorphous alloys and 3D finite element simulations to investigate the origin of“shear bands”(i.e.“strain concentration bands”)in layered heterogeneous materials.TRIP effect can effectively improve the room temperature brittleness and strain softening behavior of bulk amorphous alloys,but its hindrance mechanism to shear bands has not been quantified.Firstly,the TRIP effect enhanced ductility in crystalline materials is usually attributed to retarded necking due to increased work-hardening rates,whereas amorphous composites with this sub-stable second phase never experience necking failure.Secondly,the load distribution between austenite,martensite and amorphous parent phase can be explored by in-situ diffraction technology,but the mechanism of preventing or alleviating the transition from shear band to failure has not been revealed.In this paper,the propagation behavior of shear bands in amorphous matrix during loading is simulated by finite element method based on free volume theory,and the strain-driven TRIP model is calibrated by neutron diffraction.The role of geometric factors is studied from the perspective of micromechanics,and the synergistic effect of microstructure inhomogeneity and TRIP effect on plasticity and toughness are analyzed,such as seepage,core-shell and inverse core-shell distributions.By adjusting the research parameters,it is shown that the strength of the BMG matrix must be between the soft austenite phase and the hard martensite phase in order to effectively hinder the expansion of the shear band near the second phase and reduce the maximum shear strain,thereby improving the tensile plasticity.Furthermore,since the observation of nanoscale shear band evolution and mechanical behaviour during deformation of amorphous alloys by experimental testing is very difficult,this paper investigated the effects of temperature,strain rate,notch size,nanoindenter size and nanoscale second phase on the mechanical behaviour and shear band evolution of amorphous alloys by molecular dynamics simulations.The results show that higher strain rates and temperatures create more shear bands by promoting atomic thermal upheaval,which favors ductility.The self-organized critical behavior is demonstrated from a simulation perspective by analyzing the serrated flow behavior and elastic energy density distribution.Next,according to the shear strain probability distribution,stress steep drop(Dt=t_y-t_s,need to distinguish from Serrated flow avalanche)and strength normalized difference(Dt/t_y),it is shown that the notch length is more favorable to the formation the main shear band than the width,which produces a greater weakening effect on the amorphous alloy.In contrast,the formation of the main shear band is promoted and then weakened by increasing notch width,i.e.,the three-directional stress state induced by a larger notch width favors the formation of multiple shear bands.Meanwhile,it is revealed that the blocking effect of the nanoscale second phase on the shear band depends heavily on the geometry,and the reason for the absence of severe distortion in the Cu crystalline phase with a diameter of 4 nm is attributed to the synergistic reinforcement effect of dislocations and stacking fault.Furthermore,it is demonstrated that smaller indenter size benefits the shear band spreading sufficiently,while smaller downward pressure rate induces more small-sized shear bands.The connection between micromechanical behavior,shear band evolution,and macroscopic mechanical properties is established by serrated flow behavior and stress increment distribution.Finally,hetero-deformation induced(HDI)strengthening is regarded as the key driver to overcome the strength and plasticity trade-off of the heterogeneous materials.However,whether it is inside the bimodal structure or the surface layer of layered or gradient materials,the strengthening and toughening mechanism are usually accompanied with“shear bands”or strain localizations either inside(e.g.,for dual phase,harmonic structures)or on the surface(e.g.,for layered or gradient materials).On the one hand,the geometrically necessary dislocations(GNDs)form forward and backward stresses,which applies to micro size,most likely fails in the macroscale by the lack of origin and arrangement mechanisms in macroscopic“shear bands”.On the other hand,the shape and number of"shear bands"obtained from DIC experiments are highly random and uncertain,and depend mainly on the cross-sectional ratio of the specimen.Therefore,the synergistic effects of heterogeneous strengthening of soft and hard layers and surface perturbation mechanisms are quantitatively investigated by3D finite element simulations in this work.For surface macroscopic perturbations to improve the ductility of heterogeneous layered materials,the parameter variation results show that the perturbation structure(e.g.,wavelength,initial amplitude,phase difference)is the key factor of the three“shear bands”arrangements types,including I,X and W types.Moreover,the wave vector angle and wavelength exhibit the self-selection behavior by the mechanical simulations.Even without considering the GND and HDI mechanisms,the formation of“shear bands”on the surface of layered heterogeneous materials can be explained by continuum mechanics and surface perturbations,and the angles of the“shear bands”obtained from simulations using actual parameters are consistent with experimental results in the literature.