G.L. Chen

Investigation of shear bands under compressive testing for Zr-base bulk metallic glasses containing nanocrystals

Granular flow of nickel particles down a vertical pipe from a hopper is shown to be retarded by a horizontal ac electric field applied to a local region along the pipe. The particles are released from the hopper by pulling out a stopper in the hopper. Two sequences of experiments with different initial flow conditions are performed. In the first sequence, a dilute flow in the pipe is created after a fixed voltage V (less than or equal to4.8 kV) is applied across two short, vertical copper electrodes. The steady-state flow rate Q remains practically constant for V V-1, the flow becomes dense; Q decreases with a power law, Qsimilar toV(-1). In the second sequence of experiments, V is first set at 4.8 kV; the flow is allowed to start, and soon becomes a dense flow; then, V is reduced to the desired voltage. The new, steady-state Q vs V curve coincides with the previous Q(V) curve of the first sequence, except for V-2 V-2) to a dilute flow (V<V-2). Our results show that a large enough ac electric field can decrease the flow rate of a dilute or dense flow; the critical voltage that can reduce a dense flow, V-2, is less than that for the dilute flow, V-1

Ordered clusters and free volume in a Zr–Ni metallic glass

The atomic arrangement of a model metallic glass Zr2Ni was studied by extended x-ray absorption fine structure and x-ray scattering experiments combined with reverse Monte Carlo simulation imposed an additional potential constraint. By an approach to calculating the free volume (FV) on atomic level, we have found a connection between the coordination number and FV, and then revealed that the atomic structure of Zr2Ni metallic glass is essentially an association of the ordered clusters and FV. The ordered clusters about 1.5nm consist of a densely packed core (i.e., icosahedral or fcc-type packing) and the surrounding loosely packed clusters with large FV.

Microstructure and ductile–brittle transition of as-cast Zr-based bulk glass alloys under compressive testing

Abstract This paper investigates mechanical properties and fracture mechanisms of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 alloys with various volume fractions of quenched-in crystalline. The alloys with various volume fractions of quenched-in crystalline were prepared by controlled oxygen content of alloys and overheating of the pouring. The phase structure, particle size and volume fraction of all samples were identified by X-ray diffraction, differential scanning calorimeter (DSC) curves and scanning electron microscopy (SEM) photographs. The mean sizes of crystalline increased from 0.3 to 1.3 μm with increasing volume fraction of crystalline from 4 to 13%. The compressive mechanical tests show a ductile–brittle transition with significant decrease in the fracture stress and ductility. Detailed observations in the flow deformation and fracture surface illustrate the relationship between the quenching-in crystalline and the mechanical behavior. The full bulk amorphous Zr-based alloy exhibits typical ductile deformation and fracture behavior. The torn shear bands form the typical vein patterns on the fracture surface. The effects of quenching-in crystalline on the flow deformation and fracture behavior depend on the nature, size, volume fraction and distribution. The particle size of the crystalline in the sense of the width of shear bands is critical. When the size is larger than the width of the shear bands the particles induce an obvious inhomogeneity of the flow deformation and more microcracks by the separation of the interfaces. Nano-scale particles, on the other hand, may increase the viscosity of the flow but do not form microcracks, resulting in particle strengthening of the metallic glass. Increasing the volume fraction of large-scale particles is favorable to leaking the microcracks and brittle fracture. With increasing particle size and volume fraction up to two times the width of the shear band and 10% vol., respectively, the ductile fracture of bulk amorphous alloy completely transforms to brittle fracture under compressive testing.

A preliminary study on the creep behavior of Ti–45Al–10Nb alloy

The creep response of a nearly-lamellar Ti–45Al–10Nb at 760°C and 207 MPa suggests that high Nb additions greatly improve the creep resistance of TiAl alloys. The alloy exhibited a minimum creep rate of 6×10−9 s−1 and the time to 0.5% strain of 80 h. These properties are comparable to that of the most creep resistant wrought and cast TiAl-base alloys.

Investigation on the 1000, 1150 and 1400 °C isothermal section of the TiAlNb system

Abstract The 1000, 1150 and 1400 °C isothermal sections of the TiAlNb system have been determined using a diffusion couple technique. A series of ternary alloys in the single-phase or multi-phase regions has been employed to identify the phase relations shown in these isothermal sections. A new ternary phase γ1 has been identified. Several of the binary phases were found to have extensive solubilities, which increased with increasing temperature. The binary compounds TiAl3 and NbAl3 form a continuous solid solution in the ternary system. Determination of these phase relations was accomplished through the use of electron microprobe analysis (EPMA), X-ray diffraction (XRD) and microstructure observations.

Deformation mechanisms in a high-Nb containing γ–TiAl alloy at 900°C

Abstract Extensive twinning, the glide and climb of ordinary dislocations, and much activity of 〈011] and 1/2〈112] superdislocations were observed in the γ phase of Ti–45Al–10Nb alloy after deformation at 900°C at a strain rate of 5×10 −4 s −1 . The presence of moderate density of superdislocations is likely to arise from a high-Nb alloying in γ phase. Two important effects of high-Nb alloying in γ phase were identified: the increase of the critical resolved shear stress (CRSS) for ordinary slip, and the decrease of the stacking-fault energy (SFE), as a result of Nb solid solution and/or low Al concentration in high-Nb alloying γ phase. The CRSS and the SFE were estimated approximately to be on the magnitude of 180 MPa and ∼20 mJ/mm 2 , respectively. The high CRSS and the difficulty of dislocation climb due to low SFE are believed to be the major factors responsible for the enhanced high-temperature strength retention of Ti–45Al–10Nb alloy.

Deformation mechanism at large strains in a high-Nb-containing TiAl at room temperature

Abstract This paper investigates the deformation behavior and nano-scale structural characteristics after large deformation (29%) at room temperature of a Ti–45Al–9Nb–2.5Mn (at.%) alloy with duplex microstructure. Dislocation glide is the main deformation mode at low strains (such as 2%). The interaction of moving dislocations reduces the mobility of dislocations by pinning, and when deformation strain reachs 5%, both dislocation glide and mechanical twining are active. With further increase in strain, deformation twining becomes the dominant deformation mode, dividing the original γ grains and lamellar structure into nano-scale. The characteristic feature of the deformation twin in γ grains after large deformation at room temperature is the bent deformation twin (DT or DT′). There are many sub-structures in the DT, which makes the (111)T not exactly straight. High resolution TEM observations showed that (111)DL plane of the DT twins exhibits a large misorientation angle with the (111)M plane of the γ matrix. The orientation relationship between the matrix and the DT or DT′ is actually not the expected true twin relationship. The existence of numerous 1/3[111] Frank partial dislocations was regarded as accommodating the bending of the boundaries of DT. Local zigzag bending of the barrier twin boundaries can be observed due to twin intersections. Intersection induced lattice distortion in the intersection area for type I twin intersection is more severe than that for type II intersection. As results of the relaxation of the heavy lattice distortion, nano-scale subgrains boundaries may form together with a rectangular region near the intersection area. Nanotwins could be observed in the rectangular region for type I intersection. The nanotwins are formed with a homogeneous 1/6[11 2 ] twinning dislocation glide mechanism. For type II twin intersection, the intersection induces the formation of secondary twinning ST in the barrier twin. No region with nanotwins can be observed near the intersection area. The local stress concentration in type II twin intersection may be relaxed by the emission of dislocations from the intersection area.

Icosahedral ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass

This paper presents a computational evidence of icosahedral short and medium range ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass using ab initio molecular dynamics simulation. It is found that 1551, 1541, and 1431 types of bond pairs are pronounced in both the liquid and glass states, resulting in icosahedral coordinate polyhedra at low temperatures. By linking the individual icosahedra through vertex-, edge-, face-, and intercross-shared atoms, icosahedral medium range ordering is formed. The predicted homogenized structure factor and pair correlation function of the glass structure have been confirmed to be in agreement with the experimental results.

Preparation, microstructure and mechanical properties of Zr-based bulk amorphous alloys containing tungsten

Abstract A new Zr-based bulky amorphous alloys containing W are designed and prepared by using the combined technique of jet and water-cooled copper mold casting. XRD, DSC and SEM analysis are conducted to investigate the microstructure, the supercooled liquid region Δ T x (= T x − T g ), and thermal stability of the new bulk metallic glass system. The effects of the addition of W and the increase of Cu content on the glass forming ability (GFA) and thermal stability are discussed. Compressive experiments were also carried out to study the Youngs modulus, compressive fracture strength and elongation of the new alloys. The fracture characterization and mechanism of the new bulk amorphous alloys are discussed in detail.

The 1100 °C isothermal section of the Ti-Ni-Si ternary system

The isothermal section of the Ti-Ni-Si ternary system at 1100 °C was constituted. Twenty-one single-phase areas were determined. The binary Ti3Si phase proposed to occur at 1100 °C was not found in the investigation. A new phase, with the composition of NiTi4Si4 named H, was detected. Some of the ternary compounds show noticeable composition ranges, especially G′-Ni3Ti2Si.