Liangshun Luo

Well-aligned in situ composites in directionally solidified Fe–Ni peritectic system

Well-aligned in situ composites obtained in directionally solidified Fe–Ni peritectic system are formed by nonisothermal cellular coupled growth instead of isothermal coupled growth because of morphological instabilities. Peritectic coupled growth, with a plane interface like eutectic coupled growth, is always unstable due to the influence of peritectic reaction around the liquid/δ∕γ trijunctions. However, cellular nonisothermal peritectic coupled growth, in which one of the two solid phases bulges towards the liquid ahead of the other one, can reach a steady state and produce well-aligned in situ composites under proper growth conditions and sample composition.

Producing well aligned in situ composites in peritectic systems by directional solidification

Recently, it was found that cellular peritectic coupled growth (CPCG) can be a candidate method to grow well aligned in situ composites in peritectic alloys. In this letter, we experimentally show that there is a narrow growth region in which CPCG can be stable to avoid the influences of peritectic reaction around the trijunctions and the sidebranching instability to produce well aligned in situ composites. A simplified model was developed to predict the growth region of stable CPCG. Good agreement was obtained between the theoretical predictions and the experimental observations.

Numerical simulation of intermediate phase growth in Ti/Al alternate foils

To investigate the diffusion reaction between Ti/Al solid diffusion couple, Ti/Al alternate foils formed by hot pressing were annealed at 525, 550, 575 and 600 °C for time ranging from 1 to 40 h. The experimental results show that TiAl3 was the only observed phase at Ti/Al interface. The interface thermodynamics favored the preferential formation of TiAl3 in Ti/Al couple. The growth of TiAl3 layer occurred mainly towards Al foil side and exhibited a parabolic law. Using the interdiffusion coefficients calculated based on the contribution of grain boundary diffusion, the growth of TiAl3 was simulated numerically with the finite difference method, and the simulated results were in good agreement with the experimental ones.

First Phase Selection in Solid Ti/Al Diffusion Couple

Abstract Ti/Al diffusion couples fabricated by hot pressing were annealed at 525, 550, 575 and 600 °C. TiAl3 was the only observed phase at the Ti/Al interface when the unreacted Al foils remained. TiAl3 grew towards Al foil side. Few Al atoms were detected in Ti foils. The first formation of TiAl3 is explained on the basis of solubility limits of terminal solid solution, lattice mismatch among Al, Ti and TiAl3, and the increasing interfacial energy caused by newly formed interface. The first saturation of Al (Ti) solid solution due to the little solubility of Ti in Al, and the slight misfit among the close-packed planes of Al, Ti and TiAl3, advance the nucleation of TiAl3. TiAl3, rather than other compounds, has the lowest increasing interfacial energy, indicating its preferential formation. The formation of other titanium aluminides is suppressed due to their growth which is kinetically unstable.

Fabrication of wavy γ-TiAl based sheet with foil metallurgy

Abstract A 0.7 mm-thick wavy γ-TiAl sheet with fully lamellar microstructure was fabricated by hot pressing Ti/Al alternate foils with heat treatment of 640°C, 15 h+850°C, 35 h+1350°C, 2 h. The intermetallic compounds formed during heat treatments were identified by scanning electron microscopy (SEM) and X-ray diffraction (XRD). TiAl 3 was the only observed phase at the Ti/Al interface when Al foils were not consumed. After being annealed at 850°C for 35 h, the microstructure was composed of α-Ti, α 2 -Ti 3 Al, γ-TiAl and TiAl 2 . A fully lamellar microstructure formed after annealing at 1350°C. Most of the angles between the lamellar interface and the sheet plane are below 30°. Using thinner starting foils is favorable to produce sheets with fine microstructure.

Microstructure and mechanical properties of ZrNbMoHfV high entropy alloy

Abstract A novel high entropy alloy, ZrNbMoHfV with equiatomic concentrations, was prepared by vacuum arc melting. This alloy exhibits a typical dendrite and interdendrite structure and contains three phases: two body centred cubic phases and a Laves phase. Owing to the formation of Laves phase, this alloy possesses high hardness and compressive strength while it is fragile. At 500°C, ZrNbMoHfV alloy remains the high compressive strength and fragile. At 1000°C, the ductility of this alloy increases significantly, while the compressive strength decreases dramatically.

Nanometer-scale gradient atomic packing structure surrounding soft spots in metallic glasses

The hidden order of atomic packing in amorphous structures and how this may provide the origin of plastic events have long been a goal in the understanding of plastic deformation in metallic glasses. To pursue this issue, we employ here molecular dynamic simulations to create three-dimensional models for a few metallic glasses where, based on the geometrical frustration of the coordination polyhedra, we classify the atoms in the amorphous structure into six distinct species, where “gradient atomic packing structure” exists. The local structure in the amorphous state can display a gradual transition from loose stacking to dense stacking of atoms, followed by a gradient evolution of atomic performance. As such, the amorphous alloy specifically comprises three discernible regions: solid-like, transition, and liquid-like regions, each one possessing different types of atoms. We also demonstrate that the liquid-like atoms correlate most strongly with fertile sites for shear transformation, the transition atoms take second place, whereas the solid-like atoms contribute the least because of their lowest correlation level with the liquid-like atoms. Unlike the “geometrically unfavored motifs” model which fails to consider the role of medium-range order, our model gives a definite structure for the so-called “soft spots”, that is, a combination of liquid-like atoms and their neighbors, in favor of quantifying and comparing their number between different metallic glasses, which can provide a rational explanation for the unique mechanical behavior of metallic glasses.Metallic glasses: gradient atomic packing and plasticityLiquid-like atoms in a gradient atomic packing structure might determine plasticity in metallic glasses. A team led by Yanqing Su at the Harbin Institute of Technology in China and Robert Ritchie at the University of California, Berkeley, in the USA, used molecular dynamics simulations to classify atoms in different metallic glasses according to their stacking, thereby accounting for the amorphous medium-range order in metallic glasses. They found that local atomic structures gradually transitioned from a loose to a dense stacking of atoms via liquid-like, transition, and solid-like regions, and that liquid-like atoms and their neighbors were equivalent to ‘soft spots’ and associated with initiation of local irreversible atomic arrangements. Modeling this gradient atomic packing structure may help us better understand and design plasticity in glassy alloys.

Study on in situ Al-Si functionally graded materials produced by traveling magnetic field

Abstract The aim of this contribution was to investigate the microstructure of in situ Al-Si functionally graded materials produced by traveling magnetic field. The research shows that the composition and associated microstructural feature of Al-Si alloys processed by this method changes from the outer of samples to the inner, respectively, from Al-Si hypereutectic with particles of primary Si to Al-Si eutectic to hypoeutectic composition with a great number of primary Al dendrites. Moreover, the hardness, the wear resistance of samples and the volume fraction of primary Si particles all have obviously gradient characteristics in the samples.

Effect of traveling magnetic field on gas porosity during solidification

Abstract The effects of traveling magnetic field on degassing of aluminum alloys were investigated, and the critical radius of the pores was calculated. The results show that the critical radius of the pores decreases with increasing the magnetic density linearly when the traveling magnetic field is applied during solidification, and the use of traveling magnetic field promotes the heterogeneous nucleation of pores. After the gas dissolved in the metal liquid accumulates to form large bubbles, the traveling magnetic field forces the bubbles to the surface of molten metal, so the gas is easy to separate from the melt in the liquid stage. The number of pores in the sample decreases with increasing the intensity of traveling magnetic field.

Stability of remelting and solidification interfaces of triple-phase region during peritectic reaction at lower speed

Abstract Peritectic reaction was studied by directional solidification of Cu-Ge alloys. A larger triple junction region of peritectic reaction was used to analyze the interface stability of the triple junction region during peritectic reaction. Under different growth conditions and compositions, different growth morphologies of triple junction region are presented. For the hypoperitectic Cu-13.5%Ge alloy, as the pulling velocity ( v ) increases from 2 to 5 μm/s, the morphological instability of the peritectic phase occurs during the peritectic reaction and the remelting interface of the primary phase is relatively stable. However, for the hyperperitectic Cu-15.6%Ge alloy with v =5 μm/s, the nonplanar remelting interface near the trijunction is presented. The morphological stabilities of the solidifying peritectic phase and the remelting primary phase are analyzed in terms of the constitutional undercooling criterion.