J.W. Qiao

Tensile softening of metallic-glass-matrix composites in the supercooled liquid region

A Ti-based metallic-glass-matrix composite exhibits tensile softening (necking) in the supercooled liquid region, accompanied by a large tensile ductility and a fragmentation of dendrites. Subjected to high temperatures, concurrent crystallization does not occur, suggesting a good thermal stability of the glass matrix. The presence of high-volume-fractioned dendrites lowers the rheology of the viscous glass matrix at high temperatures, which results in an absence of super elongation as monolithic bulk metallic glasses (BMGs). A tensile strength of 970 MPa is higher than those of most BMGs under varying strain rates, ascribing to the retardation of softening by the dendrites.

An improved tensile deformation model for in-situ dendrite/metallic glass matrix composites.

With regard to previous tensile deformation models simulating the tensile behavior of in-situ dendrite-reinforced metallic glass matrix composites (MGMCs) [Qiao et al., Acta Mater. 59 (2011) 4126; Sci. Rep. 3 (2013) 2816], some parameters, such as yielding strength of the dendrites and glass matrix, and the strain-hardening exponent of the dendrites, are estimated based on literatures. Here, Ti48Zr18V12Cu5Be17 MGMCs are investigated in order to improve the tensile deformation model and reveal the tensile deformation mechanisms. The tensile behavior of dendrites is obtained experimentally combining nano-indentation measurements and finite-element-method analysis for the first time, and those of the glass matrix and composites are obtained by tension. Besides, the tensile behavior of the MGMCs is divided into four stages: (1) elastic-elastic, (2) elastic-plastic, (3) plastic-plastic (work-hardening), and (4) plastic-plastic (softening). The respective constitutive relationships at different deformation stages are quantified. The calculated results coincide well with the experimental results. Thus, the improved model can be applied to clarify and predict the tensile behavior of the MGMCs.

Exponential decay of shearing stress during jerky flows in a Zr-based bulk metallic glass

Presently, an exponential decay of shearing stresses with the time during jerky flows in a Zr-based bulk metallic glass is revealed. The flow stresses with the time satisfy the following equation: σ = A · e−B · x, where A = −158.24*exp ( − N/47.60) + 1939.63 (N = 1, 2, 3…), and the mean value of obtained B is −0.13 with a Normal distribution. During jerky flows, the viscous shearing layer is formed, and the exponential decay of shear banding is present. The evolvement of shear bands is explained by the consumption of free volumes during flows.

Crystallization pathways of liquid-bcc transition for a model iron by fast quenching.

We report simulations on the local structural evolution in the liquid-bcc transition of a model iron. Fourteen main Voronoi polyhedra are chosen as the representatives of short-range orders (SROs) and their transformations during crystallization are also investigated. Thus, the crystallization pathways for the main SROs are drawn. Our results also show that the transformations between two SROs in the crystallization pathways can be classified into two categories, first the enlargement of coordination number, second the transformation of local symmetry from five-fold to four-fold. The former reduces the potential energy while the latter increases it. It is found that the potential energy cannot decease monotonously whatever crystallization pathway is chosen to transform the icosahedral SRO to bcc SRO. Therefore, the latter transformation might provide the energy barrier of crystallization. We propose two transformation styles among SROs. All the transformations in the crystallization pathways can be achieved according to the styles. Moreover, the two transformation styles indicates that the bcc structure is more similar to liquid than other crystals. That might be the reason why the first phase nucleated during a rapid cooling process should be bcc crystal.

Nanoindentation characterised plastic deformation of a Al0.5CoCrFeNi high entropy alloy

Abstract The plastic deformation of a high entropy alloy Al0.5CoCrFeNi was investigated by instrumented nanoindentation over a broad range of strain rates at room temperature. Results show that the creep behaviour depends on the strain rate remarkably. In situ scanning images showed a significant pile up around the indents, demonstrating that a highly localised plastic deformation occurred in the process of nanoindentation. Under different strain rates, contact stiffness and elastic modulus basically remain unchanged. However, the hardness decreases as indentation depth increases due to indentation size effect. For the same maximum load, serrations became less prominent as the loading rate of indentation increased. Similar serrations have been observed in the current alloy upon quasi-static compression.

Corrosion Behavior of Ti-Based In Situ Dendrite-Reinforced Metallic Glass Matrix Composites in Various Solutions

The electrochemical corrosion behaviors of Ti40Zr24V12Cu5Be19in situ dendrite-reinforced metallic glass matrix composites (MGMCs) were investigated by potentiodynamic polarization experiments and electrochemical impedance spectroscopy in acidic, salty, and alkaline solutions. Ti40Zr24V12Cu5Be19in situ dendrite-reinforced MGMCs have an impressive corrosion resistance in strong acidic environment, while their performance was not so great in strong alkaline environment. Further immersion test in same solutions revealed similar chemical corrosion behaviors. XRD and SEM examinations were conducted to check the structure and surface modification of the material during the corrosion process. EDS test indicated that the amorphous matrices, which show excellent corrosion resistance, have a considerable composition variation from its crystalline dendrites counterpart.

Structural disorder in metallic glass-forming liquids

We investigated structural disorder by a new structural parameter, quasi-nearest atom (QNA), in atomistic configurations of eight metallic glass-forming systems generated through molecular dynamics simulations at various temperatures. Structural analysis reveals that the scaled distribution of the number of QNA appears to be an universal property of metallic liquids and the spatial distribution of the number of QNA displays to be clearly heterogeneous. Furthermore, the new parameter can be directly correlated with potential energy and structural relaxation at the atomic level. Some straightforward relationships between QNA and other properties (per-atom potential energy and α-relaxation time) are introduced to reflect structure-property relationship in metallic liquids. We believe that the new structural parameter can well reflect structure disorder in metallic liquids and play an important role in understanding various properties in metallic liquids.

Twinning-induced plasticity (TWIP) and work hardening in Ti-based metallic glass matrix composites

The present study demonstrates that Ti-based metallic glass matrix composites (MGMCs) with a normal composition of Ti43Zr32Ni6Ta5Be14 containing ductile dendrites dispersed in the glass matrix has been developed, and deformation mechanisms about the tensile property have been investigated by focusing on twinning-induced plasticity (TWIP) effect. The Ti-based MGMC has excellent tensile properties and pronounced tensile work-hardening capacity, with a yield strength of 1100 MPa and homogeneous elongation of 4%. The distinguished strain hardening is ascribed to the formation of deformation twinning within the dendrites. Twinning generated in the dendrites works as an obstacle for the rapid propagation of shear bands, and then, the localized necking is avoided, which ensures the ductility of such kinds of composites. Besides, a finite-element model (FEM) has been established to explain the TWIP effect which brings out a work-hardening behavior in the present MGMC instead of a localized strain concentration. According to the plasticity theory of traditional crystal materials and some new alloys, TWIP effect is mainly controlled by stacking fault energy (SFE), which has been analyzed intensively in the present MGMC.

Superior Mechanical Properties of AlCoCrFeNiTi x High-Entropy Alloys upon Dynamic Loading

High-entropy alloys with composition of AlCoCrFeNiTix (x: molar ratio; x = 0, 0.2, 0.4) under quasi-static and dynamic compression exhibit excellent mechanical properties. A positive strain-rate sensitivity of yield strength and the strong work-hardening behavior during plastic flows dominate upon dynamic loading in the present alloy system. The constitutive relationships are extracted to model flow behaviors by employing the Johnson-Cook constitutive model. Upon dynamic loading, the ultimate strength and fracture strain of AlCoCrFeNiTix alloys are superior to most of bulk metallic glasses and in situ metallic glass matrix composites.

Scattering mechanical performances for brittle bulk metallic glasses

Scattering mechanical performances of brittle La- and Mg-based BMGs are found in the present study. Upon dynamic loading, there exist largely scattered fracture strengths even if the strain rates are under the same order, and the BMG systems are the same. The negative strain rate dependence for La- and Mg-based BMGs is obtained, i.e., a decreased fracture strength is dominating from quasi-static to dynamic compression. At cryogenic temperatures, distinguishingly low fracture strengths are available for these two brittle BMGs, and decreased tolerance to accommodate strains makes BMGs more and more brittle. It is concluded that the scattering mechanical performances of brittle BMGs should be carefully evaluated before actual applications.