Nanoquasicrystalline phase formation in binary Zr–Pd and Zr–Pt alloys

This paper reports nanoquasicrystalline phase formation in Zr_100−xPd_x (x=30 and 35) and Zr₈₀Pt₂₀ binary alloys and the kinetics of the nanoquasicrystallization process. While the icosahedral phase (i-phase) forms as a metastable phase in the transient stage during the crystallization of Zr–Pd amorphous alloy, it forms directly from the liquid during melt-spinning of Zr–Pt alloy. The isothermal kinetics studies show that i-phase forms from the Zr₇₀Pd₃₀ amorphous alloy by the primary crystallization process with the Avrami exponent in the range of 1.5–2.5. Three-dimensional atom probe analysis results suggest that the i-phase is slightly enriched with Zr with respect to the matrix and its composition is close to Zr₇₅Pd₂₅. The tendency of quasicrystallization of Zr-based alloys appears to have correlation with the enthalpy of mixing of the system.     

Effects of Sr on microstructure and corrosion resistance in simulated body fluid of as cast Mg–Nd–Zr magnesium alloys

Mg–2·2Nd–xSr–0·3Zr alloys (wt-%, x?=?0, 0·4, 0·7 and 2·0) were prepared by gravity casting to study the effects of Sr addition on the microstructure and corrosion resistance of Mg–Nd–Zr alloys in simulated body fluid (SBF). Phases were identified by X-ray diffraction, and microstructure was observed with optical microscopy and scanning electron microscopy. Corrosion resistance of the alloys was determined by evaluating mass loss and hydrogen evolution during immersion in SBF. Mg₁₇Sr₂ phase was formed, and the grain size decreased with additional Sr addition. For the grain refinement and more continuous second phase, which could improve the corrosion resistance, the alloy with 0·7 wt-%Sr showed the slowest corrosion rate, whereas the alloy with 2·0 wt-% showed the fastest corrosion rate due to the increased volume fraction of Mg₁₇Sr₂, which led to severe local microgalvanic corrosion.     

Dependence of phase selection on cooling rates in as-cast A1-Si-Mg alloys

The effect of cooling rate on the solidification process of Al-2.06%Si-1.58%Mg was numerically and experimentally investigated. The solidification paths and the phase precipitation sequence were predicted based on the solute transportation analysis in the solidification process by coupling the thermodynamic calculation. Due to the different solute diffusion speeds, the solidification paths can be largely influenced by the cooling rates. Different phase precipitation sequences can be obtained through calculation under different cooling rates. And the later experiments have also proved this phenomenon. In the researched Al-2.06%Si-1.58%Mg alloy, the solidification sequences are α（Al）-α（Al）＋Si-α（Al）＋Mg2Si＋Si under low cooling rate and α（Al）- α（Al）＋Mg2Si-α（Al）＋Mg2Si＋Si under high cooling rate, respectively. The experimental results confirm the calculation predications.     

The role of strontium in modifying aluminium–silicon alloys

Small amounts of strontium can transform the morphology of the eutectic silicon phase present in Al–Si casting alloys from coarse plate-like to fine fibrous networks. In order to understand this industrially important but hitherto insufficiently understood effect, the strontium distribution was studied in atomic resolution by atom probe tomography and in nanometre resolution by transmission electron microscopy. The combined investigations indicate that Sr co-segregates with Al and Si within the eutectic Si phase. Two types of segregations were found: (i) nanometre-thin rod-like co-segregations of type I are responsible for the formation of multiple twins in a Si crystal and enable its growth in different crystallographic directions; (ii) type II segregations come as more extended structures, restrict growth of a Si crystal and control its branching. We show how Sr enables both kinds of mechanisms previously postulated in the literature, namely “impurity-induced twinning” (via type I) and growth restriction of eutectic Si phase (via type II).     

Correlation analysis on partition of rare earth in ion-exchangeable phase from weathered crust ores

1Introduction Weathered crust ores which are widely deposited in Jiangxi,Fujian,Guangdong,Hunan,Yunnan and Guangxi provinces in the south of China[1?3]are the main resources of mid-heavy RE.Their development and utilization have solved the shortage proble…     

Substitution of Cu and/or Al in η phase (MgZn2) and the implications for precipitation in Al–Zn–Mg–(Cu) alloys

The effects of Cu and Al substituting for Zn within bulk samples of η phase (nominally MgZn₂) have been studied by laboratory X-ray powder diffraction and nuclear magnetic resonance. Increasing Al concentration causes both of the η phase lattice parameters to increase linearly, while increasing Cu concentration causes both parameters to decrease linearly. These effects also appear to combine in a linear fashion if both Al and Cu are substituted into the MgZn₂ structure, particularly in the case of the a lattice parameter. Al was found to substitute evenly onto both Zn sites, while Cu substitutes preferentially onto the 6(h) site at low Cu concentrations, before causing significant disruptions to the structure at concentrations above 1.1 at.%, leading to the transition to long period stacking phases at the expense of η. High-resolution synchrotron powder diffraction from a commercial Al–Zn–Mg–(Cu) alloy revealed that the η phase precipitates with lattice parameters that are substantially smaller than for pure MgZn₂, indicating Cu concentrations of at least 8.9 at.% and probably higher. It is likely that the Al matrix provides a mechanical constraint on the formation of any long period stacking phases and allows the η phase to exist in these alloys with such high Cu concentrations.     

On the effect of strain on peritectic reactions and transformations in Fe–Ni and Fe–Cu binary alloys

Abstract

Differential thermal analysis (DTA) experiments conducted on Fe–Ni and Fe–Cu alloys showed undercooling below the equilibrium peritectic temperatures, T_P . The intervals between the observed liquidus and peritectic temperatures were on average 11°C and 8°C larger than the intervals obtained from equilibrium phase diagrams of Fe–Ni and Fe–Cu respectively. The transformation from δ-Fe to γ-Fe during the peritectic reaction is associated with density change and strain build up at the δ-Fe/γ-Fe interface. Thermodynamic calculations showed that by introducing the strain energy at the δ-Fe/γ-Fe interface, T_P dropped 9 K below its equilibrium value and the increase in the liquidus-to-peritectic temperature interval was in reasonable agreement with the experimental observations. The growth rate of γ-Fe during a peritectic transformation was calculated based on the strain-induced undercooling in T_P and the results showed partial agreement with observations obtained from CSLM directional solidification experiments conducted earlier on Fe–Ni alloys.     

Atmospheric corrosion of Mg–rare earth alloy in typical inland and marine environments

The atmospheric corrosion behaviour of peak aged Mg–5Y–7Gd–1Nd–0·5Zr alloys in typical land (Beijing) and marine (Qingdao) environments were investigated by optical microscopy, SEM and X-ray diffraction. The weight loss rates and residual mechanical properties were analysed. The atmospheric corrosion rates of specimens in a typical marine environment were higher than those in land environment. The results demonstrated that temperature, relative humidity and inorganic salts at field exposed sites played an important role in atmospheric corrosion behaviours. The results of our investigation may find wider applications for Mg–rare earth alloys in atmospheric environments.     

Ternary rare earth (R) alloys occurring in some ∼RPd2−xGex sections

Ternary rare earth (R) alloys with palladium and germanium, on a stoichiometric ratio close to RPd_2−xGe_x, were synthesised and structurally determined. New compounds were identified for R=La, Pr, Nd, Gd, Tb, Dy and Y and their crystal structures were found to be related to the hexagonal hP3–AlB₂ type. A homogeneous alloy was obtained for Lu too, on the stoichiometric ratio 1:1:1. This compound was found to crystallise in the orthorhombic oI12–KHg₂ (=CeCu₂) type structure, like the other equiatomic RPdGe alloys. In some systems the stability composition range of AlB₂ structure type was also analysed.     

Influence of initial textures on dynamic recrystallization and textures in AZ31 magnesium alloys

1　INTRODUCTIONMagnesiumisthelightestmetallicstructurema terialwithhighspecificstrengthandthereforeiswidelyusedinautomotive ,electronicsandaerospaceindustries[1,2 ] .However ,magnesiumoftenshowsinsufficientplasticityatroomtemperatureduetoitsHCPstructurewithlessindependentsystemsofbasalslip .Toenhanceformabilityofmagnesium ,ahigherdeformingtemperatureisusuallyusedwithtwopur poses .Thefirstistoactivatenewslipsystemsbesidesbasalslip ,sothatmorethanfiveindependentslipsystemscanbeprovided ,be…     

Effects of Zn content on as-cast microstructure and mechanical properties of Mg-xZn-4A 1 alloys containing TiC and rare earth elements

The effects of Zn content on the as-cast microstructure and mechanical properties of Mg-xZn-4Al alloys containing TiC and rare earth elements were investigated by optical microscopy（OM）, scanning electron microscopy（SEM） analysis, X-ray diffraction （XRD） analysis and tensile test. The results show that Zn content which increased from 8% to 12% does not obviously influence on the alloy phase type of the Mg-xZn-4Al experimental alloys containing 0.25%RE and 1%TiC, but with Zn content increasing from 8% to 12%, the amount of Mg32（Al,Zn）49 phase in the as-cast microstructure of the experimental alloys increases and its distribution becomes more continuous. In addition, the Mg-10Zn-4Al alloy containing 0.25%RE and 1TiC has the highest ultimate tensile strength at room temperature and 150 ℃ and highest yield strength and elongation at 150 ℃ Furthermore, with Zn content increasing from 8% to 12%, the yield strength and elongation of Mg-xZn-4Al experimental alloys containing 0.25%RE and 1%TiC increase and decrease at room temperature, respectively.     

The crystal structure of the equilibrium Φ phase in Mg–Zn–Al casting alloys

The crystal structure of the equilibrium intermetallic Φ phase formed in a Mg–Zn–Al casting alloy has been characterised using transmission electron microscopy. Electron diffraction patterns recorded from particles of the Φ phase in the casting alloy can be well indexed according to a primitive orthorhombic unit cell, with lattice parameters a=0.90 nm, b=1.70 nm, and c=1.97 nm. Examination of the whole pattern symmetry of principal zone axis diffraction patterns indicates a space group of Pbcm. A model for the decoration of the unit cell of the Φ phase is proposed, in which the Mg₅(Zn,Al)₁₂ Friauf polyhedron is the key structural unit. The Zn and Al atoms are all in icosahedral coordination, but their icosahedral shells are distorted due to the presence of Mg atoms. A total of 84 Mg atoms and 68 Zn/Al atoms can be accommodated in the orthorhombic unit cell, resulting in a formula of Mg₂₁(Zn,Al)₁₇ that is consistent with the composition obtained experimentally. Computer simulations of electron diffraction patterns provide very good agreement with experimental observations.     

The effect of Sn on autoclave corrosion performance and corrosion mechanisms in Zr–Sn–Nb alloys

The desire to improve the corrosion resistance of Zr cladding material for high burn-up has resulted in a general trend among fuel manufacturers to develop alloys with reduced levels of Sn. While commonly accepted, the reason for the improved corrosion performance observed for low-tin zirconium alloys in high-temperature aqueous environments remains unclear. High-energy synchrotron X-ray diffraction was used to characterize the oxides formed by autoclave exposure on Zr–Sn–Nb alloys with tin concentration ranging from 0.01 to 0.92 wt.%. The alloys studied included the commercial alloy ZIRLO® (ZIRLO® is a registered trademark of Westinghouse Electric Company LLC in the USA and may be registered in other countries throughout the world. All rights reserved. Unauthorized use is strictly prohibited.) and two variants of ZIRLO with significantly lower tin levels, referred to here as A-0.6Sn and A-0.0Sn. The nature of the oxide grown on tube samples from each alloy was investigated via cross-sectional scanning electron microscopy. Atom probe analysis of ZIRLO demonstrated that the tin present in the alloy passes into the oxide as it forms, with no significant difference in the Sn/Zr ratio between the two. Synchrotron X-ray diffraction measurements on the oxides formed on each alloy revealed that the monoclinic and tetragonal oxide phases display highly compressive in-plane residual stresses with the magnitudes dependent on the phase and alloy. The amount of tetragonal phase present and, more importantly, the level of tetragonal-to-monoclinic phase transformation both decrease with decreasing tin levels, suggesting that tin is a tetragonal oxide phase stabilizing element. It is proposed that in Zr–Nb–Sn alloys with low Sn, the tetragonal phase is mainly stabilized by very small grain size and therefore remains stable throughout the corrosion process. In contrast, alloys with higher tin levels can in addition grow larger, stress stabilized, tetragonal grains that become unstable as the corrosion front continues to grow further inwards and stresses in the existing oxide relax.     

1:13 phase formation mechanism and first-order magnetic transition strengthening characteristics in (La,Ce)Fe_(13–x)Si_x alloys

The effects of the introduction of Ce to La_(1-x)Ce_xFe_(11.5)Si_(1.5) alloys on 1:13 phase formation mechanism,the first-order magnetic phase transition strengthening characteristics,and magnetocaloric property were studied,respectively.The results show that the formation mechanisms of 1:13 and La Fe Si phases in La_(1-x)Ce_xFe_(11.5)Si_(1.5) alloys are the same as those of Ce_2Fe_(17) and CeFe_2 phases in Ce–Fe binary system,respectively.The substitution of Ce in 1:13 phase which is limited can make the first-order magnetic phase transition characteristics strengthen,which can make thermal and magnetic hysteresis increase,the temperature interval of temperatureinduced phase transition decrease,and the critical magnetic field of field-induced magnetic phase transition(HC)increase,respectively.Owing to the lattice shrink of 1:13phase with the increase in Ce content,the Curie temperatures(TC) show a linear decrease.The maximum change in magnetic entropy gradually increases due to the decrease in temperature interval of temperature-induced phase transition,but the relative cooling capacities are all about80 Jákg-1at magnetic field of 2 T.     

The role of trace element segregation in the eutectic modification of hypoeutectic Al–Si alloys

Modification of the eutectic silicon in hypoeutectic Al–Si alloys causes a structural transformation of the silicon phase from a needle-like to a fine fibrous morphology and is carried out extensively in industry to improve mechanical properties. It has been documented that the fibrous silicon phase in chemically modified alloys is heavily twinned. It has been proposed that this increased density of twinning results from the adsorption of impurity-elements at the solid–liquid interface perpetuating further growth according to a twin plane re-entrant edge (TPRE) model. In this paper, we discuss this mechanism of eutectic modification in light of experimental data obtained by synchrotron radiation and predictions based on theoretical calculations. This research utilizes a μ-XRF (X-ray fluorescence) technique at the SPring-8 synchrotron radiation facility X-ray source and reveals that different elements which theoretically satisfy the geometric requirements of the impurity-induced TPRE model do not have the same effect on the growth of silicon during solidification. Europium for instance is distributed relatively homogeneously in the silicon phase while ytterbium was not found in the silicon phase at all. This has important implications for the fundamental mechanisms of eutectic modification in hypoeutectic aluminium–silicon alloys.     

A model for the thermodynamics of and strengthening due to co-clusters in Al–Mg–Si-based alloys

An expanded model for the thermodynamics of co-clusters and their strengthening is presented and applied to predict co-cluster formation and strengthening in Al–Mg–Si alloys. The models were tested against data on a wide range of Al–Mg–Si alloys aged at room temperature. The strengthening due to co-clusters is well predicted. The formation of co-clusters was studied in an Al–0.5 at.% Mg–1 at.% Si alloy using three-dimensional atom probe analysis. The results correspond well with the model. It is shown that in general (short-range) order strengthening due to co-clusters will be the main strengthening mechanism in these alloys. Apart from the co-clusters, Si clusters also form, but due to their low enthalpy of formation they contribute little to the strength.