Hua Hou

First-principles study on structural, elastic and thermal properties of γ-TiAl and α2-Ti3Al phases in TiAl-based alloy under high pressure

The structural, elastic and thermal properties of γ-TiAl and α2-Ti3Al phases in the TiAl-based alloy under pressure were reported using CASTEP program based on the density functional theory. The calculated equilibrium parameters and elastic constants are in good agreement with experimental and the available theoretical data. The results indicate that under the same pressure, the α2 phase in the direction along a-axis is easier to be compressed than the γ phase, while the compression along c-axis of γ phase is larger than that of α2 phase; when the pressure is below 20 GPa, both the two phases are elastically stable, but the γ phase have higher shear modulus and Young’s modulus, and the α2 phase has better ductility and plasticity. Debye temperature, bulk modulus, thermal expansion coefficient and heat capacity of the γ phase and α2 phase under high pressure and high temperature were also successfully calculated and compared using the quasi-harmonic Debye model in the present work.

Effect of applied pressure on microstructure and mechanical properties of Mg–Zn–Y quasicrystal-reinforced AZ91D magnesium matrix composites prepared by squeeze casting

Abstract The Mg–Zn–Y quasicrystal-reinforced AZ91D magnesium matrix composites were prepared by squeeze casting process. The effects of applied pressure on microstructure and mechanical properties of the composites were investigated. The results show that squeeze casting process is an effective method to refine the grain. The composites are mainly composed of α-Mg, β-Mg 17 Al 12 and Mg 3 Zn 6 Y icosahedral quasicrystal phase ( I -phase). With the increase of applied pressure, the contents of β-Mg 17 Al 12 phase and Mg 3 Zn 6 Y quasicrystal particles increase, further matrix grain refinement occurs and coarse dendritic α-Mg transforms into equiaxed grain structure. The composite exhibits the maximum ultimate tensile strength and elongation of 194.3 MPa and 9.2% respectively when the applied pressure is 100 MPa, and a lot of dimples appear on the tensile fractography. Strengthening mechanisms of quasicrystal-reinforced AZ91D magnesium matrix composites are chiefly fine-grain strengthening and quasicrystal particles strengthening.

A first-principles study on interfacial properties of Ni(001)/Ni3Nb(001)

Abstract The Ni (001) surface, Ni 3 Nb (001) surface and Ni (001)/Ni 3 Nb (001) interfaces were studied using the first-principles pseudopotential plane-wave method. The adhesion work, thermal stability and electronic structure of Ni/Ni 3 Nb (001) interfaces were calculated to expound the influence of atom termination and stacking sequence on the interface strength and stability. Simulated results indicate that Ni and Ni 3 Nb (001) surface models with more than eight atomic layers exhibit bulk-like interior. The (Ni+Nb)-terminated interface with hollow site stacking has the largest cohesive strength and critical stress for crack propagation and the best thermal stability among the four models. This interfacial Ni and the first nearest neighbor Nb atoms form covalent bonds across the interface region, which are mainly contributed by Nb 4d and Ni 3d valence electrons. By comparison, the thermal stability of Ni/Ni 3 Nb (001) interfaces is worse than Ni/Ni 3 Al (001) interface, implying that the former is harder to form. But the Ni/Ni 3 Nb interface can improve the mechanical properties of Ni-based superalloys.

Preparation of semi-solid ZL101 aluminum alloy slurry by serpentine channel

Abstract Semi-solid slurry of ZL101 aluminum alloy was prepared using serpentine channel. The influences of the pouring temperature, the number of curves and the serpentine channel temperature on the microstructure of semi-solid ZL101 aluminum alloy were investigated. The results show that, satisfied semi-solid slurry of ZL101 aluminum alloy was prepared with pouring at 630-680 °C. The morphology of primary α(Al) grains transforms from rosette to spheroid with the decrease of pouring temperature. At the same pouring temperature, increasing the number of curves can improve the morphology of primary α(Al) grains and decrease the grain size. Qualified slurry can be attained with lowering the pouring temperature when the serpentine channel temperature is higher. The alloy melt has the effect of “self-stirring” in the serpentine channel, which can make the primary nuclei gradually evolve into spherical and near-spherical grains.

First-principles Investigation of the Structural, Electronic and Elastic Properties of Al2Ca and Al4Sr Phases in Mg-Al-Ca（Sr） Alloy

First-principles calculations have been carried out to investigate the structural stabilities, electronic structures and elastic properties of Mg17Al12, Al2Ca and Al4Sr phases. The optimized structural parameters are in good agreement with the experimental and other theoretical values. The calculated formation enthalpies and cohesive energies show that Al2Ca has the strongest alloying ability, and Al4Sr has the highest structural stability. The densities of states (DOS), Mulliken electronic populations, and electronic charge density difference are obtained to reveal the underlying mechanism of structural stability. The bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio are estimated from the calculated elastic constants. The mechanical properties of these phases are further analyzed and discussed. The Gibbs free energy and Debye temperature are also calculated and discussed.

Effect of Zr, Hf, and Sn additives on elastic properties of α2-Ti3Al phase by first-principles calculations

First-principles calculations within density functional theory have been carried out to investigate α2 phase in the Ti3Al based alloy with Zr, Hf, and Sn (6.25at%) elements doped. The lattice constants, total energies and elastic constants were calculated for the supercells. The formation enthalpies, bulk modulus, shear modulus, Young’s modulus, and intrinsic hardness were investigated. The ductility of the doped α2 phases was analyzed by the Cauchy pressure, G/B and Poisson’s ratio. The results show that the substitution of Ti(6 h) by Zr, Hf, and the substitution of Al(2n) by Sn can make the doped α2 phase more stable. The inflexibility and hardness of α2 phase can be enhanced by doping with Zr and Hf, while Sn brings the opposite effect. Sn is more powerful to improve the ductility of the doped α2 phase than Hf, but Zr can increase the brittleness. The densities of states (DOS and PDOS) and the difference charge density are obtained to reveal the underlying mechanism of the effect of alloying elements.

Phase-field simulation of tip splitting in dendritic growth of Fe-C alloy

Two types of dendrite tip splitting including dendrite orientation transition and twinned-like dendrites in Fe-C alloys were investigated by phase-field method. In equiaxed growth, the possible dendrite growth directions and the effect of supersaturation on tip splitting were discussed; the dendrite orientation transition was observed, and it was found that the orientation regions of anisotropy parameters were reduced from three to two with increasing the supersaturation, which was due to the effect of interfacial anisotropy controlled by the solute in front of S/L interface changing with the increase of supersaturation. In directional solidification, it was found that the twinned-like dendrites were formed with the fixed anisotropy couples and no seaweed dendrites were observed; these were concluded from the results of competition between process anisotropy and inherent anisotropy. The formation process of twinned-like dendrite was investigated by tip splitting phenomenon, which was related to the changes of dendrite tips growth velocity. Then, the critical speed of tips splitting and solute concentration of twinned-like dendrites were investigated, and a new type of microsegregation in Fe-C alloys was proposed to supplement the dendrite growth theories.

First-Principles Investigation of Mechanical and Thermodynamic Properties of Nickel Silicides at Finite Temperature

First-principles calculations are performed to investigate lattice parameters, elastic constants and 3D directional Young’s modulus E of nickel silicides (i.e., β-Ni3Si, δ-Ni2Si, θ-Ni2Si, ε-NiSi, and θ-Ni2Si), and thermodynamic properties, such as the Debye temperature, heat capacity, volumetric thermal expansion coefficient, at finite temperature are also explored in combination with the quasi-harmonic Debye model. The calculated results are in a good agreement with available experimental and theoretical values. The five compounds demonstrate elastic anisotropy. The dependence on the direction of stiffness is the greatest for δ-Ni2Si and θ-Ni2Si, when the stress is applied, while that for β-Ni3Si is minimal. The bulk modulus B reduces with increasing temperature, implying that the resistance to volume deformation will weaken with temperature, and the capacity gradually descend for the compound sequence of β-Ni3Si > δ-Ni2Si > θ-Ni2Si > ε-NiSi > θ-Ni2Si. The temperature dependence of the Debye temperature ΘD is related to the change of lattice parameters, and ΘD gradually decreases for the compound sequence of ε-NiSi > β-Ni3Si > δ-Ni2Si > θ-Ni2Si > θ-Ni2Si. The volumetric thermal expansion coefficient αV, isochoric heat capacity and isobaric heat capacity Cp of nickel silicides are proportional to T3 at low temperature, subsequently, αV and Cp show modest linear change at high temperature, whereas Cv obeys the Dulong-Petit limit. In addition, β-Ni3Si has the largest capability to store or release heat at high temperature. From the perspective of solid state physics, the thermodynamic properties at finite temperature can be used to guide further experimental works and design of novel nickel–silicon alloys.

Preparation of bulk crystallite alloys by rapid quenching of bulk undercooled melts

ABSTRACT Rapid quenching of deeply bulk undercooled alloy melts (200–250 K) before recalescence was used to produce three-dimensional bulk crystallite alloys. It was found that the micro-grain sizes of the prepared bulk crystallite alloys decreased with increasing undercooling. Dense crystal defects such as dislocation networks and annealing twins could be observed in the microstructures of the crystallite alloys. Substantial recrystallisation could be observed when annealing these crystallite alloys. These new findings could expand and enrich not only the traditional methods of preparing three-dimensional bulk crystallite alloys but also the traditional technologies of recrystallisation.

The Effect of Alloying Elements on the Structural Stability, Mechanical Properties, and Debye Temperature of Al3Li: A First-Principles Study

The structural stability, mechanical properties, and Debye temperature of alloying elements X (X = Sc, Ti, Co, Cu, Zn, Zr, Nb, and Mo) doped Al3Li were systematically investigated by first-principles methods. A negative enthalpy of formation ΔHf is predicted for all Al3Li doped species which has consequences for its structural stability. The Sc, Ti, Zr, Nb, and Mo are preferentially occupying the Li sites in Al3Li while the Co, Cu, and Zn prefer to occupy the Al sites. The Al–Li–X systems are mechanically stable at 0 K as elastic constants Cij has satisfied the stability criteria. The values of bulk modulus B for Al–Li–X (X = Sc, Ti, Co, Cu, Zr, Nb, and Mo) alloys (excluding Al–Li–Zn) increase with the increase of doping concentration and are larger than that for pure Al3Li. The Al6LiSc has the highest shear modulus G and Young’s modulus E which indicates that it has stronger shear deformation resistance and stiffness. The predicted universal anisotropy index AU for pure and doped Al3Li is higher than 0, implying the anisotropy of Al–Li–X alloy. The Debye temperature ΘD of Al12Li3Ti is highest among the Al–Li–X system which predicts the existence of strong covalent bonds and thermal conductivity compared to that of other systems.