Gencang Yang

Effect of Ca and Y additions on oxidation behavior of AZ91 alloy at elevated temperatures

In order to develop the ignition-proof magnesium alloy, the effect of alloying elements, Ca and Y, on the oxidation behavior of AZ91 magnesium alloy at elevated temperatures was investigated. The ignition-proof performance, oxide products and oxidation kinetics of Ca- and Y-containing AZ91 alloys were studied. The results indicate that the proper addition of Ca can increase the ignition point of AZ91 alloy greatly. However, the oxide film of Ca-bearing AZ91 alloy formed at elevated temperature is thick and brittle, which is prone to crack in melting and cooling process. In addition, the oxide film of AZ91-xCa alloy is incompact and cannot inhibit the diffusion of reaction particles. The oxide film of AZ91-xCa alloy turns to thin and plastic one after Y is added, and the density of the oxide film increases greatly due to the formation of composite oxide film composed of MgO, CaO and Y2O3.

Mode of dendrite growth in undercooled alloy melts

Abstract The mode of dendrite growth in the undercooled Ni–50at%Cu alloy was investigated. At lower undercoolings, the dendrite growth is mainly controlled by solute diffusion, and the formed dendritic morphologies are similar to those of the conventional as-cast equiaxed crystals, except that here the branches are much denser. At higher undercoolings, however, the severe solutal trapping that results from high dendrite growth velocity weakens the effect of solute diffusion on the dendrite growth. In this case, the dendrites branch in the bunching form. The dendrite spacings were measured, and the results were interpreted with the current dendrite growth theories.

Microstructure evolution and metastable phase formation in undercooled Fe-30 at.% Co melt

Abstract The microstructure evolution and phase selections of Fe–30 at.% Co alloy at various undercoolings were investigated in this paper. The metastable body-centered cubic (b.c.c.) phase was detected in a highly undercooled Fe–30 at.% Co alloy. Applying the regular solution model, the equilibrium Fe–Co alloy phase diagram was evaluated and its metastable extension was taken into account to explain the formation of the metastable b.c.c. phase. Based on the classical nucleation theory, the activation energy ΔG* and the steady-state nucleation rate Ihs have been calculated in terms of stable face-centered cubic (f.c.c.) (γ) and metastable b.c.c. (δ) phases. The crystal growth velocities of b.c.c. and f.c.c. phases as a function of undercoolings ΔT and tip radius R were also given according to Bottinger, Coriell and Trivedi (BCT model). The critical undercooling for the formation of metastable b.c.c. (δ) phase in Fe–30 at.% Co alloy was 204 K. The transmission electron microscopy results indicated that the microstructure of the primarily nucleated b.c.c. phase exhibited fine dendritic structure. The composition analysis showed that the b.c.c. dendrite structure was enriched in cobalt.

The dependence of damping capacity of PMMCs on strain amplitude

Abstract For the particulate-reinforced metal–matrix composites (PMMCs), when the interface is considered as ideal, the damping mechanisms mainly come from two aspects. One is the intrinsic damping of component phase and the other is the energy dissipation caused by local micro-plastic deformation during external loading. According to this principle, the dependence of the damping capacity of SiC P /Al composite, at room temperature, on strain amplitude, has been simulated by employing cell method in this paper. The results show that the damping capacity of SiC P /Al composite is independent of the strain amplitude e 0 when e 0 is comparatively low. But, when the strain amplitude e 0 reaches a critical value e crit , the damping capacity increases rapidly with the strain amplitude e 0 . The results also show good coincidence to the model of G–L dislocation damping theory. It can be concluded that the dependence of the damping capacity on the strain amplitude e 0 is due to the energy dissipation caused by local micro-plastic deformation near the interface of Al/SiC for the difference of the elastic modulus.

Experimental investigation on thermal wear of high speed steel rolls in hot strip rolling

Abstract The thermal wear ratio of a high speed steel roll was investigated experimentally in hot strip rolling with a DTW- 166 thermal wear testing machine developed by the authors. It is clear that the wear ratio increased with number of cycles. Some of the increase in wear was because of the black oxide layer generated on the roll surface at the beginning. The wear ratio also increased as slippage ratio and loads increased. Loads played a more important role than slippage ratio for thermal wear. The appearance of the roll surface was observed by SEM under different conditions. The mechanism of thermal wear was composed of adhesive, microploughing, microcutting, oxidation, and plastic slippage wear.

Mechanical properties and fracture mechanisms of aluminum matrix composites reinforced by Al9(Co, Ni)2 intermetallics

Abstract The microstructures and mechanical properties of Al matrix composites reinforced by different volume fractions of Al-Ni-Co intermetallic particles were investigated. Three different volume fractions of Al-Ni-Co particles were added to pure Al matrix using a stir-casting method. Microstructural analysis shows that with the increasing of the reinforcement volume fraction, the matrix grain size decreases and the porosity increases. The mechanical properties of the composites are improved over the matrix materials, except for the decreasing of the ductility. Fracture surface examination indicates that there is a good interfacial bonding between the Al matrix and the Al-Ni-Co particles and the fracture initiation does not occur at the particle-matrix interface.

Phase selection in highly undercooled Fe-B eutectic alloy melts

Abstract The high undercooling technique by molten glass slag purification and cyclical superheating in Ar atmosphere was applied to bulk Fe-B alloy melts. A hypercooling was achieved which suppressed the formation of stable phase and consequently promoted the nucleation of metastable phase. Fe-17%B and Fe-20%B alloys were investigated, respectively. TEM and X-ray powder diffraction analyses verify the formation of metastable phase in the highly undercooled Fe-B alloy melts. Besides, the critical nucleation work of Fe 2 B and Fe 3 B phases was calculated to predict phase selection in the undercooled melts. The results show that the metastable phase formation is a function of the undercooling achieved prior to nucleation. And the amount of undercooling is an important factor in determining microstructural development by controlling phase selection in the undercooled melts.

Kinetic effect of crystal growth on the absolute stability of a planar interface in undercooled melts

The stability of a planar interface during the non-equilibrium solidification of undercooled melts is reexamined by the linear stability theory of Mullins and Sekerka. The incorporation of the kinetic coefficient, which is dependent on the local interfacial temperature, into the analysis leads to a striking change of the absolute stability criterion proposed by Trivedi and Kurz. It is also shown that whether the absolute stability of a planar interface exists relies on a dimensionless parameter q which is a function of the physical properties of the melts. Only the melts whose q is smaller than 1 can solidify into a stable planar interface when the crystal growth velocity exceeds the critical value.

MICROSTRUCTURE FORMATION AND MECHANICAL PROPERTY INVOLVING ICOSAHEDRAL QUASICRYSTAL PHASE OF Y RICH Mg-Zn-Y QUASICRYSTAL ALLOY

The microstructure formation and mechanical property involving icosahedral quasicrystal (I-phase) in the Y-rich Mg-Zn-Y alloy have been studied. The equilibrium formation of I-phase from the Y-rich Mg-Zn-Y melt is through a peritectic reaction between the Y-rich melt and the primary W-phase, which is discussed in detail. The independent nucleation and coupling growth mechanism between the W-phase and the I-phase, from the melt, are revealed, which is significant for understanding the peritectic reaction process involving icosahedral quasicrystal in the Mg-Zn-Y alloy. The mechanism of the quasicrystal phase strengthened magnesium alloys is also discussed here.

Liquid-phase Separation in Rapid Solidification of Undercooled Fe-Co-Cu Melts

The homogeneous liquid was separated into two phases, (Fe, Co)-rich L1 and Cu-rich L2, once the melt was undercooled below a liquid-phase separation temperature T sep . If the duration from T sep to T s1 (solidification temperature of L1 phase), termed the liquid-phase separation interval Δ t , exceeded a critical value, an eggtype structure was observed. By utilizing differential thermal analyses (DTA), the solidification process of the undercooled Fe-Co-Cu alloys was studied. Additionally, an immiscible boundary was obtained, which was a convex parabola with a symmetrical axis of x Cu =0.52. Depending on the relative amounts of L1 and L2, the minor phase was nucleated firstly to form liquid droplets and separated from the original liquids at the beginning of liquid-phase separation.