The deformation characteristics of metastable β -phase in a Ti-15 wt % Mo alloy

An investigation has been made of the deformation characteristics of metastable-phase in a commercial Ti-15 wt% Mo alloy within the temperature range –196 to + 146° C. Deformation occurred by slip, and by mechanical twinning on the {332} 113 system. Twinning occurred preferentially with decrease in deformation temperature. Enhanced ductility in tension, observed in the range – 94 to + 20° C, was attributed to the inhibition of necking resulting from strengthening by twin formation. A thinninginduced transformation occurred during foil preparation for electron microscopy.Formerly Imperial College.     

TEM study of metastable β-phase decomposition in rapidly solidified Ti-6Al-4V alloy

A new rationale is presented for various decomposition products obtained from the metastable -phase found in Ti-6A1-4V alloy produced by hot isostatic pressing comminuted melt-spun fibres and cooled to room temperature by furnace cooling. This alloy has an -matrix with about 8 vol% retained -phase, which is supersaturated with -stabilizers to such an extent that the martensitic transformation has been suppressed. The metastable -phase decomposes by different modes during continuous cooling, depending on the actual composition of individual -grains. Less enrichment of vanadium and iron favours the direct formation of the equilibrium -phase from the -matrix, while greater enrichment of vanadium and iron leads to a spinodal decomposition of the metastable -phase, resulting in a + two-phase structure. During further continuous cooling, the -phase which is lean in -stabilizers will transform into isothermal -phase. In addition, an unknown phase has also been observed in the -phase, which is typified by the appearance of 1/2{112} reflections in the SAD patterns.     

Ageing behaviour of t′-phase in a hot-pressed ZrO2(4 mol% Y2O3) ceramic

The ageing behaviour of unequilibrium tetragonal (t) phase and its resultant effect on the mechanical properties of hot-pressed ZrO₂(4 mol% Y₂O₃) ceramic have been investigated by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Experimental results show that t-phase which is the product of diffusionless transformation from cubic (c) phase during rapid cooling after sintering is unstable when aged in a temperature range of 1400–1600 °C for up to 80 h in that it decomposes diffusionally into equilibrium tetragonal(t) phase and c-phase. Yttria contents of phases formed during decomposition are basically in agreement with those indicated by phase diagram. The stability of t-phase characterized by the existence of anti-phase domain microstructure under the microscopic dark field image is significantly associated with the tetragonality(c/a) measured by XRD and the larger the tetragonality, the more unstable the t-phase. Metastable precipitates of t-phase are triggered by applied stress to transform to monoclinic (m) phase during which the fracture toughness is enhanced and transformability of t-phase is critically dependent upon the solute content as well as size. It is found that when t- and m-phase coexist with adequate fractions of c- and t-phase, the fracture toughness of the aged specimen demonstrates a peak value that moves to shorter ageing times with increasing temperature while the Vickers hardness decreases monotonically with ageing time regardless of ageing temperature due to grain growth.     

Oxygen release behavior of metastable tetragonal t′meta-(Ce0.5Zr0.5)2 phases prepared by reduction and successive oxidation of t′ phase

Tetragonal CaF₂-related-type t′-/Ce_0.5Zr_0.5)O₂ phase was reduced at 773 K ≤ T_red. ≤ 1073 K to prepare precursors with various oxygen compositions: Ce₂Zr₂O_712d /0.111 < δ < 0.379). Metastable tetragonal t^′_meta-(Ce_0.5Zr_0.5)O₂ phases were prepared by oxidizing in O₂ at 873 K the precursors, and subjected to evolved oxygen gas analysis by heating the samples at a constant rate. When the precursor was prepared at T_red: ? 773 K; i.e. δ ? 0:379; the oxygen release behavior of the tetragonal phase agreed approximately with that of the t^′. For δ < 0.29, distinct features of the t^′_meta-(Ce_0.5Zr_0.5)O₂ appeared in the XRD results and Raman spectra. The temperature exhibiting the maximum release rate of oxygen from the t^′_meta became lower with decreasing the d value. It could be concluded that the thermodynamic behavior of the t^′_meta is related closely to the precursor composition and becomes more unstable with decrease in the δ value. The t^′ and t^′_meta phases with disordered arrangement of Ce and Zr ions may belong to the same space group; nevertheless, difference in the random arrays of the Ce and Zr ions was suggested on the basis of the present experimental results.     

Formation of β-phase microcrystals from the melt of PVF2-PMMA blends induced by quenching

High-rate quenching at lower temperatures will result in the formation of -phase microcrystals in PVF₂-PMMA blends with low PMMA contents directly from their melts. Annealing at higher temperature (120°C) promotes the growth of phase. Almost pure -phase crystallization in thin films of the blends could be obtained by this procedure. The morphology of the blends quenched and then annealed is similar to that of pure PVF₂.     

Internal oxidation of Fe-Si alloys in γ-phase region

Internal oxidation measurements of Fe-0.070, 0.219, 0.483, and 0.920 wt % Si alloys were made in the γ-phase region in order to discuss kinetics of internal oxidation, to evaluate the diffusion coefficient of oxygen in the internal oxidation layer, and to determine the diffusion coefficient of oxygen in γ-iron. Internal oxidation of these alloys was conducted at temperatures between 1223 and 1323 K using a powder mixture of iron and Fe₂O₃. The internal oxidation front in Fe-Si alloys with between 0.070 and 0.483 wt % Si advances in parallel to the specimen surface. The internal oxidation in these alloys obeys a parabolic rate law, which indicates that the internal oxidation is controlled by an oxygen diffusion process in the alloy. The diffusion coefficient of oxygen, D _O ^IO , in the internal oxidation layer where SiO₂ particles disperse was determined by using the thermodynamic data for the solution of oxygen in γ-iron. D _O ^IO increases with the increase of the volume fraction of the oxide, f ^IO, in the oxidation layer at a given temperature. The diffusion coefficient of oxygen, D _O, in γ-iron was evaluated by extrapolating D _O ^IO to f ^IO=0. D _O may be given by the following equation: $$D_O = \left( {6.42\begin{array}{*{20}c} { + 4.37} \\ { - 2.60} \\ \end{array} } \right) \times 10^{ - 5} exp \left[ { - \frac{{159 \pm 5(kj mol^{ - 1} )}}{{RT}}} \right]m^2 \sec ^{ - 1} $$ .     

Formation and transformation of metastable phases during electrodeposition and annealing of cobalt–iron alloy films

During the electrodeposition of Co–Fe alloy films from a CoSO₄·7H₂O-FeSO₄·7H₂O bath, the formation of metastable phases, such as a complex cubic Co–Fe phase isostructural to α-Mn and the HCP ε-Co/Fe and Ω-Co/Fe phases, appears to be related to the incorporation of metal hydroxide/oxide precipitates into the plated alloy films. In the absence of the incorporated precipitates, the plated films are the equilibrium α-Fe solid solution BCC phase. Thus, the addition of stabilizing reagents (such as ammonium citrate), and/or a lowering of solution pH, prevents the formation of the precipitates and promotes the formation of the BCC phase. On the other hand, increasing temperature causes the formation of metastable phases, possibly through the weakening of the stabilizing effect of the ammonium citrate, or the promotion of the formation of metal hydroxides/oxides precipitates. The BCC phase has higher saturation magnetic flux densities and lower coercivities than the metastable phases. Annealing of the films transforms the metastable phases, if present, into the BCC phase, leading to a decrease in the coercivity. An increase in the magnetic flux density after annealing is, however, not observed, possibly due to the cracking or delamination of the films as a result of annealing. Cracking and delamination make the determination of the film volume, which is required for magnetic flux density calculation, questionable.     

Formation of pure $$tau$$ τ -phase in Mn–Al–C by fast annealing using spark plasma sintering

Journal of Materials Science - Mn–Al–C is intended to be one of the “gap magnets” with magnetic performance in-between ferrites and Nd-Fe-B. These magnets are based on the...     

The Behaviour of Mg in γ-phase in a Ni-base Superalloy

1.IntroductionSince the biginning of 1960s,the effectof Mg upon superalloy has been studiedand some satisfactory developments havebeen achieved.The role of Mg on the alloycan be summarized as follows:(1)Themechanical plasticity of Ni-base superalloycan be improved by adding Mg asdesulphurizer,because of forming insolubleMgS instead of other low melting eutectic     

Identification of the metastable phase α′m in a Zn/Al alloy containing Cu and Mg

Samples of the commercial pressure-diecast alloy ZA8 containing 8.1% Al, 1.1% Cu and 0.024% Mg have been examined by transmission electron microscopy (TEM). During decomposition of the high temperature f c c phase, on cooling after casting, a transitional phase was formed within the phase in both dendrites and eutectic. This phase was identified as the metastable phase _m containing 11% Al or 23% Al, with a f c c crystal structure and lattice parameter (at 11%) of about 0.394 nm. It had adopted a symmetrical cube/cube orientation relationship with the surrounding f c c phase. The stability of this metastable phase at ambient temperatures was greatly increased by the presence of copper.     

On fatigue crack growth in a Ti-4.5Al-5Mo-1.5Cr alloy with metastable β-phase

Fatigue crack growth behavior has been examined in a Ti-4.5Al-5Mo-1.5Cr alloy, for two different levels of β-phase metastability. The resistance to fatigue crack growth appears to be marginally enhanced with the presence of metastable β-phase in a microstructure also containing some primary α-phase (~30 pct) of high aspect ratio. This enhancement appears slightly greater for β-phase water quenched from 899°C, than as more slowly cooled in helium from this same solution treatment temperature, at approximately an air-cooling rate. In the case of the former, nearly full retention of solute in the β-phase is apparent, while in the latter, significant precipitation of secondary α-phase is evident in thin-foil transmission electron micrographs.     

Ion-nitriding behaviour of Fe-Ti alloys in theα-phase region

The ion-nitriding behaviour of four iron alloys containing between 0.11 and 1.48 wt% titanium was investigated in the-phase region to discuss kinetics of the growth of the nitriding layer. The ion-nitriding experiments have been made at 823 K. Two nitriding layers were observed: a thin surface layer which mainly consists of Fe₄N; an internal nitriding layer beneath the surface layer, where the nitride formed was found to be TiN. The growth of the internal nitriding layer is controlled by a diffusion process of nitrogen in the matrix metal. The apparent diffusion coefficient of nitrogen in the nitriding layer, evaluated using the rate equation proposed for internal oxidation, increases linearly with the volume fraction of titanium nitride. Furthermore, by excluding the effect of the titanium nitride from the apparent diffusion coefficient, the diffusion coefficient of nitrogen in-iron was calculated, being in good agreement with that reported so far. In addition, the increase in hardness in the internal nitriding layer has been discussed.     

Formation of γ-alumina nanorods in presence of alanine

Boehmite and alumina nanostructures were prepared using a simple green sol-gel process in the presence of alanine in water medium at room temperature. The uncalcined (dried at 200 °C) and the calcined materials (at 500, 600 and 700 °C for 4 h) were characterized using XRD, TEM, SEM, N₂ physisorption and TGA. Nanorod aluminas with a possible hexagonal symmetry, high surface area and relatively narrow pore size distribution were obtained. The surface area was enhanced and crystallization was retarded as the alanine content increased. The morphologies of the nanoparticles and nanorods were revealed by a transmission electron microscope (TEM).     

γ′ Formation and morphology evolution in the undercooled structure of DD3 Single crystal superalloy

An investigation of the formation and its morphology evolution in the bulk undercooled DD3 single crystal superalloy is performed. The application of a molten salt denucleating technique combined with thermal cycle enables such investigation over a wide range of undercooling up to 210 K. The microstructure formation has been respectively discussed with respect to undercooling, nucleation and solute segregation during solidification and post-solidification, by employing the classical nucleation theory. Furthermore, TEM and SEM technique are adopted to reveal the microscopy and to illustrate other factors correlated to the precipitation. It is found that the size, fraction, and distribution of precipitated in as-solidified DD3 single crystal superalloy are all influenced by melt undercooling.     

Thermodynamics of Bainitic Formation in Carbon-depleted Region in Fe-C Alloys

Thermodynamic analysis for the formation of bainitic ferrite in carbon -depleted region has been conducted. The results show that the driving force of bainitic formation increases with depleting of carbon in parent austenite and with decreasing the transformation temperature. The critical driving force (absolute value) at the Bs temperature is 600-1 200 J / mol, which increases as the mean carbon content of Fe-C alloys increases. The freshly-formed bainitic ferrite has partial supersaturation of carbon, which increases smoothly with decreasing the transformation temperature Therefore, the displacive formation of bainitic ferrite in carbon-depleted region is thermodynamically feasible in the whole temperature range of bainitic reaction.     

Formation of elongated particles in β-SiC compacts

The mechanism of bonding of β-SiC particles in a compact in a sintering process at relatively low temperatures has been investigated by TEM. The initial sign of bonding is observed at around 1800°C. At these temperatures, SiC particles are found to bond together only when their mutual orientations are the same. Similarly oriented grains at contact reorient themselves by rotation to the same orientation and then bond, thus forming elongated particles. As the temperature increases, the tendency to form elongated particles increases. The formation of boundaries between differently oriented particles is observed only at around 2100°C and above in similar compacts. The mechanism of bonding of particles in the same orientation at low temperatures is ascribed to high self-energy of dislocations common to covalent-bonded materials and, hence, the high energy of formation of grain boundaries.