J. Tan

Correlation between internal states and plasticity in bulk metallic glass

We report a close correlation between the internal states and plasticity in a bulk metallic glass (BMG) and discover that the optimization of copper-mold casting current can induce large plasticity stably in an otherwise brittle BMG. It is possible to confirm that larger plasticity corresponds to the internal states with more average free volume (FV) as revealed by lower density, higher enthalpy change, and higher Poisson’s ratio. The enhanced plastic deformation mechanism is interpreted based on the FV model of BMGs, and our results may have some implications for understanding the role of the FV during plastic deformation of BMGs.We report a close correlation between the internal states and plasticity in a bulk metallic glass (BMG) and discover that the optimization of copper-mold casting current can induce large plasticity stably in an otherwise brittle BMG. It is possible to confirm that larger plasticity corresponds to the internal states with more average free volume (FV) as revealed by lower density, higher enthalpy change, and higher Poisson’s ratio. The enhanced plastic deformation mechanism is interpreted based on the FV model of BMGs, and our results may have some implications for understanding the role of the FV during plastic deformation of BMGs.

Correlation between the microstructures and the deformation mechanisms of CuZr-based bulk metallic glass composites

The variation of the transformation-mediated deformation behavior with microstructural changes in CuZr-based bulk metallic glass composites is investigated. With increasing crystalline volume fraction, the deformation mechanism gradually changes from a shear-banding dominated process as evidenced by a chaotic serrated flow behavior, to being governed by a martensitic transformation with a pronounced elastic-plastic stage, resulting in different plastic deformations evolving into a self-organized critical state characterized by the power-law distribution of shear avalanches. This is reflected in the stress-strain curves by a single-to-“double”-to-“triple”-double yielding transition and by different mechanical properties with different serrated flow characteristics, which are interpreted based on the microstructural evolutions and a fundamental energy theorem. Our results can assist in understanding deformation behaviors for high-performance metastable alloys.

Stable fracture of a malleable Zr-based bulk metallic glass

We report a stable fracture phenomenon during the compression of a malleable Zr-based bulk metallic glass. In the process, the shear band along which the sample fails is constrained by the machine crosshead, thus causing a slow release of the stress and the elastic energy by small steps in the stress-strain curve. A novel and unique fishbone-like pattern was observed on the fracture surface after the final rupture instead of the typical vein-like pattern usually found upon catastrophic failure for metallic glasses. The formation of this pattern might be a result of the modest temperature rise during failure and the frustration of the meniscus instability in the crack tip due to stress redistribution in the constraint. This fracture behavior, where the crack propagation is at a much lower speed and the elastic energy is released in a stable way, might be suitable for studying the crack propagation process and the fracture mechanism in metallic glasses.

Correlation between atomic structure evolution and strength in a bulk metallic glass at cryogenic temperature.

A model Zr41.25Ti13.75Ni10Cu12.5Be22.5 (at.%) bulk metallic glass (BMG) is selected to explore the structural evolution on the atomic scale with decreasing temperature down to cryogenic level using high energy X-ray synchrotron radiation. We discover a close correlation between the atomic structure evolution and the strength of the BMG and find out that the activation energy increment of the concordantly atomic shifting at lower temperature is the main factor influencing the strength. Our results might provide a fundamental understanding of the atomic-scale structure evolution and may bridge the gap between the atomic-scale physics and the macro-scale fracture strength for BMGs.

Deformation behaviour of hot extruded Mg alloy AZ31 during compressive deformation

Abstract Deformation behaviour of a hot extruded Mg–3Al–1Zn alloy subjected to uniaxial compression at room temperature was investigated. Microstructure evolution showed that the amount of lenticular shaped extension twins increased with the increase in true strain when it was lower than ∼4% but decreased when it was higher than ∼4%. Previous studies indicated that detwinning occurred. However, we have demonstrated in the present paper that no detwinning occurred, but a process of twin growth and coalescence, leading to an analogous detwinning phenomenon observed in the optical micrographs because most of the matrixes were consumed. A strong texture evolution occurred in this process, and the reorientation produced by strong twinning was mainly responsible for strain hardening when compressed along the extrusion direction.

Effect of Fe on Crystallization Process of Zr-Co-Al-(Fe) Bulk Metallic Glasses

The change in the internal states of Zr56Co28Al16 bulk metallic glass (BMG) upon minor substitution of Co with Fe was investigated for alloys with different compositions of Zr56Co28-xAl16Fex (x = 0, 1 and 2, respectively). Results exhibited that the ductile Zr-Co-Al-Fe BMGs were obtained and showed better glass-forming ability (GFA) via a small amount of Co partial replacement by Fe. In addition, the addition of a small amount of Fe enhanced the crystallization process and reduced the activation energy. The micro-alloying with Fe reduced the heat of mixing, which made the rearrangement of atoms easier during the crystallization process.

Mechanical and corrosion properties of Ti-35Nb-7Zr- x HA composites fabricated by spark plasma sintering

Abstract To improve the bioactivity of Ti-Nb-Zr alloy, Ti-35Nb-7Zr- x HA (hydroxyapatite, x =5, 10, 15 and 20, mass fraction, %) composites were fabricated by spark plasma sintering. The effects of the HA content on microstructure, mechanical and corrosion properties of the composites were investigated utilizing X-ray diffraction (XRD), scanning electron microscope (SEM), mechanical tests and electrochemical tests. Results show that all sintered composites are mainly composed of β-Ti matrix, α-Ti and metal–ceramic phases (CaO, CaTiO 3 , CaZrO 3 , Ti x P y , etc). Besides, some residual hydroxyapatites emerge in the composites (15% and 20% HA). The compressive strengths of the composites are over 1400 MPa and the elastic moduli of composites ((5%–15%) HA) present appropriate values (46–52 GPa) close to that of human bones. The composite with 15% HA exhibits low corrosion current density and passive current density in Hanks solution by electrochemical test, indicating good corrosion properties. Therefore, Ti-35Nb-7Zr-15HA composite might be an alternative material for orthopedic implant applications.

Correlation Between Internal States and Strength in Bulk Metallic Glass

The internal states or local structures of bulk metallic glass (BMGs) can be well reflected from the changes of density, structural relaxation as well as the elastic constants. With the increasing free volume (FV) content, more local atomic clusters are capable of simultaneous plastic shear at different sites in the metallic glasses, inducing large plasticity. In this work, we report a close correlation between the internal states and strength in a BMG and discover that the yield strength can be changed by varying of the casting current, revealing that the yielding strength of BMGs is not only intrinsically associated with the glass transition, but also with the internal states, such as free volume and elastic properties. Such results may have some implications for understanding the correlations between the internal states and mechanical properties of BMGs.

Effect of calcium pyrophosphate on microstructural evolution and in vitro biocompatibility of Ti-35Nb-7Zr composite by spark plasma sintering

β-type Ti-35Nb-7Zr alloy has attracted considerable attentions as a bone implant material. The alloy, however, has poor bioactivity, which difficult to form a strong osseointegration between the bone tissues. Combining Ti alloy with a bioactive and biodegradable ceramic has been of interest to researchers. But the large difference in physicochemical property of high-melting metal and ceramic elements would bring the manufacturing restriction. In this work, Ti-35Nb-7Zr-CPP composites were fabricated with mechanical alloy of Ti, Nb, Zr and Nano calcium pyrophosphate (CPP) powders mixture followed by spark plasma sintering (SPS) routes. The effect of CPP ceramic on microstructural evolution and in vitro biocompatibility were investigated. As the addition of CPP (10-30 wt%), ceramic elements spreading towards the matrix, the generated metal-ceramic bioactive phases CaTiO3 are observed well consolidated with β-Ti matrix. With the CPP increasing, Ca and P atoms rapidly migrated to the β-Ti matrix to form granulated Ti5P3, which leads to the increasing porosity (10%-18%) in the composites. The results demonstrated that the favorable cell viability (the cell proliferation rates were higher than 100%) and growth inside the pores of the composites arise from the rough micro-porous surface and the release of bioactive metal-ceramic phase ions into the biological environment. The enhanced bioactivity and microstructural evolution behaviors of the Ti-35Nb-7Zr-CPP composites may provide a strategy for designing and fabricating multifunctional implants.

The Particle Shape of WC Governing the Fracture Mechanism of Particle Reinforced Iron Matrix Composites

In this work, tungsten carbide particles (WCp, spherical and irregular particles)-reinforced iron matrix composites were manufactured utilizing a liquid sintering technique. The mechanical properties and the fracture mechanism of WCp/iron matrix composites were investigated theoretically and experimentally. The crack schematic diagram and fracture simulation diagram of WCp/iron matrix composites were summarized, indicating that the micro-crack was initiated both from the interface for spherical and irregular WCp/iron matrix composites. However, irregular WCp had a tendency to form spherical WCp. The micro-cracks then expanded to a wide macro-crack at the interface, leading to a final failure of the composites. In comparison with the spherical WCp, the irregular WCp were prone to break due to the stress concentration resulting in being prone to generating brittle cracking. The study on the fracture mechanisms of WCp/iron matrix composites might provide a theoretical guidance for the design and engineering application of particle reinforced composites.