Formation of a DO19 phase in zirconiumaluminium martensites

Journal of Materials Science -     

A new ordered phase in the AlPd system

Using electron diffraction and electron microscopy a new phase has been observed in a rapidly solidified Al-6%Pd alloy. Depending on the cooling rate this phase is present in the as quenched cellular-dentritic structure along the cell boundaries or in finer precipitates mostly along the domain boundaries of the fcc solid solution. In both cases the precipitates agglomerate and grow to bigger size after heating in the electron microscope. The new phase having a complicated structure and a large unit cell is closely related to the Al₄Mn phase which is hexagonal.     

Mechanical alloying of the Ti-Al system

Ti-AI alloys have been prepared by mechanical alloying (MA) of pure titanium and aluminium powders for four compositions: Ti₈₀AI₂₀, Ti₇₅AI₂₅, Ti₅₀AI₅₀, and Ti₄₀AI₆₀. The 50 < Ti(at%) < 80 compositions could be fully amorphized with a milling time strongly dependent on the starting chemical composition. For the Ti₄₀AI₆₀ composition only partial amorphization was observed. The investigation on the early stage of MA shows that the different systems amorphized through two different paths. On the titanium rich side (Ti=75,80 at%) the MA first leads to the formation of an h.c.p. Ti(AI) solid solution with an aluminium content of 14–16% which subsequently collapses into the amorphous state. For the aluminium rich side (Al=50,60 at%), MA sooner promotes the nucleations of disordered forms of Ti₃AI and TiAl intermetallic compounds respectively. The inhibition of the ordering transition of the observed intermetallic phases is ascribed to the low temperature at which the SSR takes place. The complete (Ti₅₀AI₅₀) or partial (Ti₄₀AI₆₀) amorphization of the powder is then attained through a destabilization of the disordered Ti₃AI and TiAl phases. The present results confirm the existence of a metastable f.c.c. phase as final alloying stage for the Ti₇₅AI₂₅ and Ti₄₀AI₆₀ compositions.     

An investigation of phase stability in the Y2O3-Al2O3 system

The stability of the three known phases in the Y₂O₃-Al₂O₃ pseudo-binary system has been investigated. YAlO₃ (YAP) and Y₄Al₂O₉ (YAM) decompose at elevated temperatures, the products of the reaction being the third compound Y₃Al₅O₁₂ (YAG) and an unknown phase (designated X). The decomposition is most evident in powders but can also be initiated on the surface of bulk single crystals. X-ray diffraction studies have been performed in an attempt to identify the structure and composition of the unknown phase. The thermal decomposition has been found to be surface controlled and an optical and scanning electron microscope study of the associated morphological changes in YAlO₃ indicates that the reaction involves localized surface melting, probably with the loss of oxygen which effectively moves the composition off the binary join.     

An experimental investigation into the formation of an equiaxed zone in ingot casting: Pb-Sb alloy system

An experimental investigation was undertaken to obtain a better understanding of the phenomenon of equiaxed zone formation in ingot castings with particular attention to the thermal conditions during dendritic growth and the mechanism of columnar-to-equiaxed transition. Thermal conditions during dendritic growth were measured in the laboratory-scale Pb-Sb castings. Solidification was also directly observed in castings of the cyclohexanol-phenol red system. A close correlation was noted between the general behaviour of the thermal history curves, especially when taken on the centreline of the ingots, and the resulting macrostructure. The dendritic solidification front was found to grow with considerable solute undercooling and also with a slight, but positive, temperature gradient ahead of the front in the bulk liquid. Moreover, the dendritic front temperature was also noted to increase with distance from the mould wall surface, the nominal rate of advance of the front being sensitive to the solute content and the cooling rate. Heterogeneous nucleation ahead of the columnar front was believed to have played a major role in equiaxed zone formation. However, columnar-to-equiaxed transition was observed not to take place immediately when the equiaxed crystals formed, but some time later, equiaxed dendrites growing at a somewhat higher temperature than the columnar dendritic front. Finally, combining the information gained from thermal analysis data with the observations made upon non-metallic castings, a novel picture of how columnar-to-equiaxed transition takes place has been proposed.Associate Professor of Physical Metallurgy.     

Contact mass transfer and phase formation in the Au-Pb system

Contact mass transfer and phase formation processes in the Au-Pb system have been studied by x-ray diffraction, scanning electron microscopy, tunneling electron microscopy, and x-ray microanalysis. The results indicate that dynamic and thermal influences give rise to gold dispersion followed by the formation of intermetallics and eutectic systems.     

Calculation of the shear energy profile in alloys with DO19 superstructure

The shear energy surfaces in Mg₃Cd and Ti₃Al alloys possessing a DO₁₉ superstructure were studied by computer simulation with an allowance of the crystal lattice anisotropy. Stable planar defects in the (0001) basal plane are revealed.     

An investigation into the amorphous phase separation characteristics of an ionomer glass series and a sodium-boro-silicate glass system

Amorphous phase separation of ionomer glasses, also known as fluoro-phospho-alumino-silicate glasses, of generic composition SiO₂-Al₂O₃-P₂O₅-CaO-CaF₂ were investigated. A sodium-boro-silicate glass system, which is known to undergo amorphous phase separation was also investigated. High Temperature Dynamic-Mechanical Thermal Analysis (DMTA), combined Differential Thermal Analysis/Thermo Gravimetric Analysis (DTA/TGA) and Scanning Electron Microscopy (SEM) were the principal analytical techniques used in this study. High temperature DMTA was used to measure the glass transition temperatures (T _g) of the original starting glass compositions, as well as being able to follow amorphous phase separation (APS) within the glass. High temperature DMTA traces of both the ionomer glasses and the sodium-boro-silicate glasses exhibited two maxima in tan , corresponding to two glass transition temperatures and demonstrating that amorphous phase separation of the parent glass into two glass phases had occurred. DTA/TGA of the ionomer glasses detected a glass transition and two crystallisation peaks for apatite and mullite, accompanied by a gradual weight loss of 1–3% on passing through the crystallisation region. The sodium-boro-silicate base glass showed no evidence of a glass transition, but a prominent glass transition was detected for a second sample which had undergone a heat-treatment of 240 min at 580°C. SEM analysis of the ionomer glass compositions revealed smooth spherical droplets of 2–15 nm while the background morphology appeared rough and speckled. A classic interconnected structure was observed for the sodium-boro-silicate glass.     

Electron microscopy study of a new ordered phase in the platinum-vanadium (8 : 1) system

Electron diffraction and high resolution electron microscopy revealed a new ordered phase in the platinum-vanadium system. It appears in the vicinity of the composition Pt₈V and has a body centered tetragonal unit cell with point group 4/m 2/m 2/m; the new phase is isostructural with the ordered Pt₈Ti structure.     

An investigation of segregation and mixing in dense phase pneumatic conveying

Pneumatic conveying of bulk materials has become an important technology in many industries: from pharmaceuticals to petro-chemicals and power generation. Particulate segregation has been investigated in many solids handling processes. However, little work has been published on the segregation and mixing in pneumatic conveying pipelines, particularly in dense phase pneumatic conveying. Due to the character of dense phase flow, it is difficult to investigate the segregation in a flowing plug. A sampling device was designed and built to take samples from the pneumatic conveying pipeline after “catching a plug”. Several experiments were conducted over a range of gas–solids flow conditions with 3 mm nylon pellets and 3 mm ballotini as a segregating mixture. Experimental data combined with video footage were analysed to describe the segregation and mixing of solids plugs in pipes. This investigation provides initial research on establishing a segregation index in a flowing plug. A gas–solids two-dimensional mathematical model was developed for plug flow of a nylon-glass particulate mixture in a horizontal pipeline in dense phase pneumatic conveying. The model was developed based on the discrete element method (DEM). The model was used to simulate the motion of particles both in a homogeneous flow and as binary mixtures taking into account the various interactions between gas, particles and pipe wall. For the gas phase, the Navier Stokes equations were integrated by the semi-implicit method for pressure-linked equations (SIMPLE) using the scheme of Patankar employing the staggered grid system. For the particle motion the Newtonian equations of motion of individual particles were integrated, where repulsive and damping forces for particle collision, the gravity force, and the drag force were taken into account. For particle contact, a model with a simple non-linear spring and dash pot model for both normal and tangential components was used. This model employed a mixture of 3 mm pellets and ballotini as virtual materials with properties of nylon and glass. The results from the model are discussed and compared with experimental work and show qualitative agreement. Further modelling and experimental work in key areas is proposed.     

An in situ TEM study of phase formation in gold-aluminum couples

In the gold wire bonding of aluminum in microelectronic devices the presence of aluminum oxide on the metallization surface may be expected. Electron transparent couples containing an oxide layer at the interface were heated in a TEM to determine the effects of a passivation layer on intermetallic formation. Intermetallic phases were evidenced by changes in sample appearance and their structure was determined by electron diffraction. The presence of an oxide at the interface hindered second phase formation at temperatures at which they were usually expected to form. In aluminum rich couples, the formation of the AuAl₂ intermetallic was not observed to form until about 350°C with the oxide present. In a reverse configuration involving a gold rich couple, an amorphous oxide phase was observed between the Al and the advancing Au₂Al front. The movement of the reaction front appeared to be controlled by surface diffusion across this phase.     

Ti-Al层状复合材料相界面扩散反应层的电子结构

采用热压扩散焊接法制备Ti-Al层状复合材料,在不同的焊接温度条件下进行热压扩散焊接,利用SEM、EDS、XRD以及基于密度泛函理论(DFT)的第一性原理模拟计算等分析测试手段研究了Ti-Al扩散偶的扩散反应层的反应产物以及电子结构特征。结果表明: 当焊接温度≥560 ℃时,Ti-Al扩散偶相界面扩散反应层的反应产物主要是以金属间化合物Al₃Ti为主,反应产物Al₃Ti的总态密度(TDOS)的赝能隙为2.8 eV,相邻的Al原子和Ti原子的局域态密度(PDOS)的赝能隙也为2.8 eV,金属间化合物Al₃Ti中所含的共价键较少,而金属键较多。因此,Al₃Ti表现出更多的金属特性,具有较好的导电性,为Ti-Al层状复合材料作为一种新型电极材料奠定了基础。     

The ordered phase fields of the iron-nickel-platinum equilibrium diagram

The approximate limits of the ordered phases in the Fe-Ni-Pt system at 600° C are established; some results are also given for high nickel alloys at 475° C. Continuous solid solutions form between FePt and NiPt and between Fe₃ Pt and Ni₃ Pt at temperatures below the relevant critical temperatures, but not between FeNi₃ and Ni₃Pt. Ordering of the iron and nickel atoms was not detected in the ternary alloy Fe₂₅ Ni₂₅ Pt₅₀ but two tetragonal phases were found in the alloys containing about 60 at. % Pt. The lattice parameters of alloys heat-treated at these temperatures are given.     

The effect of pressure on metastable phase formation in the undercooled Bi-Sn system

A metastable phase was produced by the solidification of highly undercooled Bi-48.6 at% Sn alloy droplet samples. During heating the metastable phase was observed to melt at 116° C at ambient pressure. The onset of the metastable endotherm was found to increase with increasing pressure, while the liquidus and eutectic temperature for the structure stable at ambient pressure decreased with increasing pressure. Based on the pressure dependence of the melting trend, the metastable phase will be stable at the expense of the stable ambient pressure structure under high hydrostatic pressure conditions (above 1 GPa). Both microstructural observations and X-ray examinations at ambient pressure revealed that the metastable phase was present in droplet samples and that the X-ray diffraction pattern was close to that of the high-pressure stable phase previously reported as a rhombic cell. High-pressure thermal analysis has also allowed for identification of the effect of pressure in promoting favourable formation kinetics and the kinetic transition from the equilibrium phases to the metastable phase at high undercooling.     

An investigation of the unstable region for dense phase conveying in sliding bed flow

For fine powdered materials, capable of being conveyed in dense phase, a conveying region exists in which instability in flow and pipeline blockage can occur if the pressure gradient available is insufficient to maintain a minimum value of solids loading ratio. A reduction in material flow rate can also lead to pipeline blockage in this region. Conveying trials under-taken with cement are used to illustrate the nature of the problem.