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Abstract: By means of equal channel angular pressing (ECAP) technique, an Mg–12Gd–3Y–0.5Zr alloy was processed at 300°C for one and four passes. A bi-ultrafine (BU) microstructure of Mg-alloy, consisting of matrix grains and the second-phase particles with sizes smaller than 1μm, was obtained after the fourth pass of ECAP. The formation of this BU microstructure originated from the complex interactions between the matrix grains and second-phase particles during the ECAP processing at elevated temperature. A simultaneous improvement of both strength and ductility was achieved in the sample with the BU microstructure, compared with the original sample. The increase in strength is resulted from the refinement of both matrix grains and second-phase particles, while the improved ductility is attributed to the considerable plasticity provided by both dislocation mechanism and profuse shear bands in the BU microstructure. An enhanced plasticity induced by the shear-band mechanism is proposed for the magnesium alloy with the BU microstructure. [Copyright &y& Elsevier]

Abstract: The evolution of grain boundaries (GBs) and shear bands in Cu bicrystals during one-pass equal-channel angular pressing (ECAP) was systematically investigated by various techniques. Four Cu bicrystals were designed to make GBs with angles of 0°, 45°, 90° and 135° with respect to the intersection plane (IP) of the ECAP die. After ECAP, the shear bands and the GB orientations in the four bicrystals displayed distinct behaviors due to the difference in the initial GB directions and the special crystallographic orientation of the component grains. Based on the experimental results, it is suggested that the initial GBs have a remarkable influence on the shear deformation behaviors of the adjacent regions, and the deformation regions far from the GBs are mainly controlled by the crystallographic orientations. The present investigations further demonstrate that shear deformation along the normal of the IP plays an important role in the deformation of Cu bicrystals. [Copyright &y& Elsevier]

A coarse-grained Cu–16wt.% Ag alloy was processed by equal channel angular pressing (ECAP) for one pass to reveal the shear deformation behaviors by scanning electron microscope and electron backscattering diffraction. The evolution of shear bands was found to be affected by both the parallel and vertical shearing to the intersection plane (IP) of the ECAP die, indicating that, besides the shearing along IP, the shearing vertical to it also plays an important role during the plastic deformation. [Copyright &y& Elsevier]

Macroscopic yielding is essentially important in structural application. In this study, a wisdom semi-empirical model, based on the rule of mixture (RoM), has been proposed to predict the yielding strength in in-situ metallic glass matrix composites (MGMCs). The present model not only successfully predicts the yielding strength of a variety of dendrite-reinforced MGMCs, but also can be verified in bimetal alloys or composites with different systems. A quantitative relationship between the specific strength and shear velocity in metallic materials was established, which provides a guidance for the development of light-weight and high-strength MGMCs. Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract The Al 0.3 CoCrFeNi high-entropy alloys were successfully coated with the Ni P amorphous films through electroless plating. The room-temperature mechanical behavior under uniaxial tension was systematically investigated. There are good adhesive strengths between the films and substrates. And the surface hardness of the plating for 5 h is around 381HV with dramatically increasing by 121%, as compared to the uncoated substrate of 172HV. As a result, the yield strength and ultimate tensile strength (UTS) of coated samples were significantly increased as the film thickness rises. Specially, the specimen plating for 5 h, the yield strength (σ y) is 400 MPa with a 62% improvement, and UTS is 732 MPa, increased by 16% in comparison to the substrate (σ y = 246 MPa and UTS = 630 MPa). It can be attributed to the appearance of the repulsive image stress that will counteract the shear applied stress for the generation and propagation of dislocations during the elastic-deformation stage. Owing to the back stress inhibiting local plasticity and promoting the nucleation of cleavage cracks in the substrate, the cracks originated from the brittle film propagate into the substrate, resulting in the ductility losses. A deformation map is proposed to account for the transition of deformation modes from shear banding to cracking. These findings shed light on the fact that the surface coating is an effective means to enhance the mechanical properties of HEAs. [ABSTRACT FROM AUTHOR]

In this study Electroless plating is used to deposit amorphous Ni-P films on the surface of the Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (Vit.1) bulk metallic glasses (BMGs) with different pH values and film thicknesses. The special amorphous/amorphous structure exhibited large plasticity. The plastic strain is 10.6% for the coated pillars with a coating thickness of 86.5 μm, which is 5.9 times of 1.8% of the bare Vit.1 pillars. The amorphous Ni-P films provide high radial confinement stress, which leads to more homogeneous formation of a high density of smaller shear bands at the Ni-P/Vit.1 interface. The technique provides an effective way to enhance the plasticity without obvious sacrifice of the yielding strength for BMGs at room temperature. [ABSTRACT FROM AUTHOR]

Using electrodeposition and chemical plating, Ni film and Ni–P film were coated on the surface of Zr-based amorphous alloy of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (Vit.1). The compression behavior of the bare Vit.1 pillars and the Vit.1 pillars with the Ni-coating and Ni–P/Ni-coating was studied. The Ni–P/Ni/Vit.1 pillars with “amorphous–crystalline–amorphous” sandwiched structure formed a “hard–soft–hard” composite, and the Ni/Vit.1 pillars formed a “soft–hard” composite. The experimental results show that the surface coating increases the density of shear bands in the Vit.1 pillars, resulting in the improvement of the compression ductility. The Ni–P/Ni/Vit.1 pillars have the largest engineering plastic strain of 21.3±1.0% for the “uniform plastic deformation”, about 11.2 times of 1.9% of the bare Vit.1 pillars. The Ni–P/Ni-coating limits the propagation of shear bands, which leads to the initiation and formation of multiple shear bands. The technique developed in this work provides a new route to enhance the compression ductility of amorphous alloy at room temperature. [ABSTRACT FROM AUTHOR]

Abstract: Burst sizes in different amorphous alloys are predicted by . When the diameter of tested samples is decreased, the burst size is greatly increased. And the critical diameter, corresponding to shear failure, could be predicted. The present prediction could be as a criterion to guarantee steady serrations free of failure. [Copyright &y& Elsevier]

The Ni–Mo–P amorphous films were successfully deposited on the CoCrFeNi high-entropy alloys by electroless plating. This work will focus on reviewing and discussing the ambient-temperature mechanical properties, plastic deformation, and cracking mode of the coating-substrate system. The yielding strength (σ y) and ultimate tensile strength (UTS) of the specimen plating for 1 h were 260 and 760 MPa, respectively, with 44.4% (σ y) and 14.2% (UTS) improvement contrast to the uncoated HEAs (σ y and UTS are 180 MPa and 560 MPa, respectively). It is attributed to the back stress and repulsive image stress to suppress the nucleation and mobility of dislocations. Due to the substrate confinement, the deformation model was gradually changed from cracking (with homogeneous flow) to shear banding (with homogeneous flow). Eventually, fully-homogeneous flows occur with reducing the film thickness, and the deformation mechanism map is developed to expound the transition of the deformation mode. The brittle fracture with a high velocity originating from amorphous films leads to cleavage cracking on the surface of the HEA substrate. The thicker film has higher crack velocity and causes deeper cracks penetration into the substrate, reducing the substrate ductility. In the coating-substrate system, with enhancing the film thickness, the substrate confinement effect and the adhesion are reduced, which causes that the ratio of the elastic energy in the amorphous film transferred to shear banding or cracking is decreased, which implies the size dependence of the deformation mode for Ni–Mo–P amorphous films in tension. The deformation mode of amorphous coating changes from fully-homogeneous to shear banding (with homogeneous flow), and eventually, the cracking (with homogeneous flow) dominates with increasing the coating thickness. Image 1 • The strength mechanism of Ni–Mo–P/CoCrFeNi system was proposed. • A deformation map was delineated to reveal transition of deformation mode. • The fracture of amorphous films induced the cleavage fracture in substrates. [ABSTRACT FROM AUTHOR]