Martensitic transformations in Ti-Mo alloys

A detailed investigation has been made of the structure of alloys of the Ti-Mo system containing up to 10wt% Mo, water-quenched from the-phase region. With increase in molybdenum content, the martensite structure changes from hexagonal () to orthorhombic () at 4 wt% Mo, and at 10 wt% Mo, the structure is completely retained . For alloy compositions <4 wt% Mo, there is a diffusional component in the transformation of the -phase at the quench rates employed. There is a transition, with increase in molybdenum content, in morphology (from massive to acicular) and in substructure (from dislocations to twins). However, the transitions in crystallography, morphology and sub-structure are not directly related to one another except for an abrupt loss of dislocation substructure at the/ transition. The to crystallographic transition has the characteristics of a second order transformation, and evidence has been obtained of the existence of a spinodal within the metastable orthorhombic system. The orthorhombic martensites of Ti-6 and 8 wt% Mo decompose during quenching producing a fine modulated structure within the martensite plates, consistent with a proposed spinodal mode of decomposition.     

Mössbauer studies of α″-Fe16N2 and α′-Fe8N films

Conversion-electron Mössbauer spectra of epitaxial -Fe₁₆N₂ and -Fe₈N films have been studied and their differences are discussed in detail. The Mössbauer spectrum of -Fe₁₆N₂ can be decomposed into three subspectra, which correspond to the 4d, 8h and 4c sites. The Mössbauer spectrum of -Fe₈N can be fitted using four spectra based on a nitrogen-atom-random-distribution model. The average hyperfine field is larger (3%) for -Fe₁₆N₂ than for -Fe₈N, which is approximately consistent with a 4.1% enhancement of the magnetic moments for -Fe₁₆N₂. The iron moments tend to locate in the film plane for -Fe₁₆N₂ and to arrange perpendicularly to the film plane for -Fe₈N.     

Transformation-induced plasticity in two-phase (α+γ) Fe-Cr-Ni alloys

The relationship between the fracture elongation and the value of martensite per unit tensile strain has been studied for the two-phase ( + ) Fe-Cr-Ni alloys containing 10%, 35% and 52% 7. The elongation has a peak in the elongation-test temperature curve. The peak elongation is dependent upon both the delay of necking and a suitable value of martensite per unit tensile strain.     

The age hardening effect in Ti-6 Al-4 V due to ω and α precipitation in the β grains

The structure at room temperature of a quenched TA6V titanium alloy has been investigated. This structure is + or + according to the treatment temperature; it is always metastable. During ageing the grains decomposed by the reaction + + +; this decomposition was accompanied by a large increase of the 0.2% yield stress. No structural modification was observed in. The and phase of TA6V were separately investigated in the form of single-phase alloys. The hardness of was insensitive to ageing, while was considerably hardened by and; we deduced that the strengthening of the minor phase during ageing is mainly responsible for the hardening of TA6V.     

Phase transformations in permanent-mould-cast aluminium bronze

The phases obtained in aluminium bronze (Cu-10Al-4Fe) cast into a permanent mould were investigated. The parameters examined were the pre-heating temperature of the mould and the graphite coating thickness. The phases and ₂ were detected as well as the metastable phases and . The intermetallics of the system Fe-Al were obtained in various stoichiometric compositions. The different cooling rates of the casting resulted in two mechanisms of transformation to grains out of the unstable phase, one being nucleation and growth producing needle-shaped grains, the other exhibiting a massive transformation to spherical grains. These two mechanisms determine the changes in the size of the a grains as result of changes in the cooling rate in its various ranges.     

Dielectric Dispersion in Yttrium and Erbium Formate Dihydrate Crystals

The dielectric properties of Y(HCOO)₃ · 2H₂O and Er(HCOO)₃ · 2H₂O are studied. The frequency dependences (0.01 Hz to 20 kHz) of the real () and imaginary () parts of dielectric permittivity ( = – i) are shown to follow a fractal scaling law for the dielectric response of solids. In the tan versus temperature curves, a number of maxima are revealed in a narrow temperature range. The experimental data are used to evaluate the activation energies of relaxation processes. The observed anomalies are assumed to be associated with changes in the dynamics of protons in hydrogen bonds.     

High-frequency dielectric properties of Mg-Al-Si,Ca-Al-Si and Y-Al-Si oxynitride glasses

Recently developed coaxial line techniques [1] have been used to determine, at room temperature, the values of the real () and imaginary (') parts of the dielectric constants for some Mg-Al-Si, Ca-Al-Si and Y-Al-Si oxynitride glasses over the frequency range 500 MHz to 5 GHz. The frequency dependencies of and ' are consistent with the universal law of dielectric response in that (-_t8)^(n–1) and '^(n–1) for all glass compositions; the high experimental value of the exponent (n=1.0±0.1) suggests the limiting form of lattice loss [2] situation. In this frequency range, as previously reported [3] at longer wavelengths, the addition of nitrogen increases the dielectric constant, (); in both the oxide and oxynitride glasses is also influenced by the cation, being increased with cation type in the order magnesium, yttrium, calcium as at lower frequencies.     

Complex microstructural changes in as-cast eutectoid Zn-Al alloy

Phase transformations and microstructural changes of an as-cast eutectoid Zn-Al alloy (ZnAl₂₂Cu₂) were investigated during isothermal holding. The typical dendritic structures consisted of _s phase as a core with the edge of decomposed _s phase and decomposed _s in the interdendritic regions. A series of complex phase transformations was observed. Both decompositions of _s and _s were determined at an early stage of ageing and a four-phase transformation, _f+T+, was observed at the boundaries of _f phase and the phase, instead of clearly observed at the boundaries of phase, in a solution-treated Zn-Al alloy during prolonged ageing.     

Dislocations and faults in some alloys containing copper and aluminum

By using the method of variance of the respective X-ray diffraction line profile, the effective particle size (p) and r.m.s.. strain (S ^21/2) of four alloys containing copper and aluminium (Al-0.08, 4 and 10 wt% Cu and Cu-5.87 wt% Al) at different annealing stages (starting from the cold filed stages) have been determined. From the peak shift of these samples, their average stacking fault probability = – where is the intrinsic stacking fault probability and the corresponding extrinsic stacking fault probability, respectively, were determined. Similarly, from the angular distance between the peak and the centroid of the diffraction profiles, the magnitude of + 4.5, where is the twinning fault probability, was measured. From these, the true particle sizeT, the stacking and twinning fault probabilities , and and the minimum stacking fault widthD _min have been determined. The hardness of alloys at different annealing stages was found to be dependent on the dislocation density and the stored energy in the alloys. The relationT S ^21/2 = constant was found to be valid for the alloys and from this a mechanism of grain growth with annealing has been suggested. With annealing the dislocation density and stored energy per unit volume were found to decrease until finally, in the fully annealed stages, they disappeared. The same was found to be valid for stacking and twinning fault probabilities as well as the minimum stacking fault width. A mechanism for the creation and annihilation of the intrinsic and extrinsic stacking faults has been suggested.     

Torsional Oscillations of a Rotating Column of 3He-B

No Heading We have analysed the axisymmetric and non-axisymmetric modes of a continuum of vortices in a rotating superfluid. We have investigated how changing the temperature affects the growth rate of the disturbances. We find that, in the long axial wavelength limit the condition q = /(1 – ) = 1, where and are temperature-dependent mutual friction parameters, is the crossover between damped and propagating Kelvin waves. Thus at temperatures for which q > 1, perturbations on the vortices are unlikely to cause vortex reconnections and turbulence. These results are in agreement with the recent discovery of Finne et al¹ of an intrinsic condition for the onset of quantum turbulence in ³He-B.PACS numbers: 67.40. Vs, 67.57.–z     

The influence of ageing on the stability of β-γ/γ′ derived microstructures in Ni-Al-Cr-(Co) alloys

Multiphase Ni-Al-(Fe)-(Cr)-(Co)-based intermetallics with (B2)- (A1)/(L1₂), - or - microstructures can exhibit significant room-temperature tensile ductility. In the case of Ni-Al-Cr-based alloys, microstructural development is complicated by the precipitation of -Cr, which can supplant the -phase during ageing of three-phase -/ microstructures. An investigation of the stability, during ageing, of cast Ni-Al-Cr-(Co) alloys with microstructures derived from -/ is reported. In the as-cast condition, the materials investigated consisted of a dendritic matrix containing L1₀ type martensite and interdendritic /. Extensive intra- and interdendritic -Cr precipitation was also observed. The stability during ageing of the interdendritic / microstructure is also considered and transformation of the L1₀ martensite is examined.     

Effect of crystalline phase, orientation and temperature on the dielectric properties of poly (vinylidene fluoride) (PVDF)

The effect of crystalline phase, uniaxial drawing and temperature on the real () and imaginary () parts of the relative complex permittivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency range between 10² and 10⁶ Hz. Samples containing predominantly and phases, or a mixture of these, were obtained by crystallization from a DMF solution at different temperatures. phase samples were also obtained from melt crystallization and from commercial films supplied by Bemberg Folien. Different molecular orientations were obtained by uniaxial drawing of and phase samples. The results showed that the crystalline phase exerts strong influence on the values of and , indicating that the _a relaxation process, associated with the glass transition of PVDF, is not exclsively related to the amorphous region of the polymer. An interphase region, which maintains the conformational characteristics of the crystalline regions, should influence the process decisively. The molecular orientation increased the values of for both PVDF phases and modified its dependence with temperature over the whole frequency range studied. The influence of the crystallization and molecular orientation conditions on the dc electric conductivity (_dc) were also verified. The value of _dc was slightly higher for samples crystallized from solution at the lowest temperature and decreased with draw ratio.     

Intensity of critical opalescence of helium-4

We present measurements of the critical opalescence of helium-4. The results are analyzed by the Einstein and Ornstein-Zernike theory and the power laws. We obtain ==1.17±0.02, ==0.62±0.1,/=4.5±0.3,P _c =1706.008 mm Hg, andT _c =5,189.863 mK (T ₅₈ ). The critical behavior of helium-4 is almost the same as that of classical fluids and the influence of the quantum nature of helium-4 is not as evident as has been claimed.     

Distribution of α-AlFeSi and β-AlFeSi particles in surface layer of AA6063 alloy billets after heat treatment

We developed the EPMA mapping method of small -AlFeSi(Al_8.3Fe₂Si) and -AlFeSi(Al_8.9Fe₂Si₂) particles in the billets of Al-Mg-Si alloys such as AA6063 alloys. To discriminate between -AlFeSi and -AlFeSi particles we used the relative X-ray intensities of Fe/Si ratio, the I _Fe/I _Si ratio, instead of the Fe/Si mass ratio. To obtain the I _Fe/I _Si ratio, we used a Monte Carlo method. In this study, using this method the mapping of -AlFeSi and -AlFeSi particles in the surface layer of AA6063 billets after the heat treatment (for 2 h at 580°C) was done. Namely, the distribution of -AlFeSi and -AlFeSi particles of zones from the billet surface to a depth of 800 m was measured. Results showed the zone from the surface to a depth of 200 m was occupied mainly by -AlFeSi particles and the zone from a depth of 200 m toward the center was occupied mainly by -AlFeSi particles. From these results, it was found that if we remove zones from the surface to a depth of 200 m, we can remove the majority of the -AlFeSi particles, and thus improve the quality of anodizing performance of Al-Mg-Si alloys extrusions.     

The mechanism of continuous dissolution of the lamellar structure in aged Al 22 at% Zn alloy

The mechanism of continuous dissolution was studied using transmission electron microscopy, X-ray diffraction and hardness measurements. The metastable phase was identified in the first stage of dissolution nucleating at the - boundaries. Interface dislocations at the - boundaries forming a perpendicular net in the 110 directions were identified as of screw character and of Burgers vector b=1/2a 110. During dissolution a change of shape of precipitates occurs leading to their fragmentation and to a change of the direction of boundaries into 110 directions.     

The effect of Ag additions on the microstructure of aluminium–lithium alloys

Minor quantities of Ag have been added to Al–Li–Cu–Mg–Zr alloys. Their microstructure has been studied by means of optical metallography, transmission electron microscopy and X-ray diffraction. In the high Li, low Cu : Mg ratio alloys the main phases found were , , S and T₁, while fewer T₂ and Al₇Cu₂Fe precipitates were also observed. The addition of up to 0.5 wt% Ag diminishes the and T₁ precipitates size. This is attributed to a small increase of Li solubility in the matrix. In the low Li, high Cu : Mg ratio alloy the addition of 0.2 wt % Ag resulted in the precipitation of phase simultaneously with , , S and T₁ phases. Due to the low Li concentration an unusual growth of the / precipitates at the expense of the precipitates was also observed.     

A high-resolution-electron-microscopy study of a γ/γ′-α directionally solidified eutectic alloy

The microstructure of a /- directionally solidified (DS) eutectic alloy with a nominal composition of Ni-30.26Mo-6.08Al-1.43V (wt%) was investigated by means of high-resolution electron microscopy (HREM) and analytical electron microscopy. The -fibres exhibited a typical morphology with a rectangular cross-section and they displayed the Bain orientation relationship (OR) with the / matrix; that is, [001][001] and (110)(010). Misfit dislocations and lattice strain fields existed at the / interface for different habit planes; that is, (110)(010) and (100)(110) were analysed. EDAX (Energy dispersive X-ray) analysis showed that the composition of the -phase was approximately Ni₄(Mo, Al, V); it contained 90° rotational domains of Ni₃(Mo, Al, V) with a DO₂₂ structure and Ni₂(Mo, Al, V) with a Pt₂Mo structure.     

Phases in as-spun and aged binary aluminium-lithium ribbons

As-spun aluminium-lithium ribbons, containing up to 10wt% Li, and an Al-1.79 wt % Li ribbon aged at different temperatures (room temperature, 66, 175 and 200 °C) were investigated by transmission electron microscopy (TEM) and microhardness testing. It was found that the metastable phase Al₃Li () did not form either in the as-cast ribbons containing less than 1.79 wt% Li or in the featureless zone of Li-rich ribbons (Li content >3 wt%) but it did form, as the stable phase AlLi (), in the segregated zone of the later ribbons. The study of the early stage of precipitation showed that the mechanism of formation of depended on the ageing temperature. At a low temperature (66°C), the phase occurred by homogeneous spinodal decomposition. Whereas, it formed by heterogeneous nucleation at higher temperatures (175 and 200°C). These results are consistent with the presence of a spinodal curve within the miscibility gap ( + ). All investigations performed both on the as-cast and aged ribbons did not reveal any evidence for the formation of G.P. zones. It was also observed in this study that contamination of aluminium-lithium ribbons by impurities such as O₂ and CO₂ could alter the precipitation process and lead to the formation of lithium carbonate.     

Superplastic deformation in Ti-4%Al-4% Mo-2%Sn-0.5%Si (IMI550)

The superplastic deformation properties, i.e. flow scenes and stain rate sensitivities, of the commercial alloy Ti-4% Al-4% Mo-2% Sn-0.5% Si (IMI55O) have been assessed in the temperature range 805 to 915° C and for different initial microstructures. Fine grained + microstructures showed superplastic properties at temperatures above 850° C and grain coarsening, which led to increases in flow stresses and e reduction in superplastic properties, was only pronounced at the higher temperatures. In transformed microstructures the method of strain rate cycling led to a breakdown of the acicular platelets end the development of an equiaxed + microstructure. At the same time the flow stresses at low strain rates decreased and the strain rate sensitivities increased. Light and electron microscopy showed that the-phase became the continuous or matrix phase even at volume fractions below 50% (i.e. at the lower deformation temperatures) and that molybdenum segregation at the longitudinal interphase boundaries (parallel to the tensile axis) occurred. The results suggest that there is a strain distribution between the-phase and the -phase, the-phase effectively behaving as a deforming mantle around the grains.     

An investigation of the ordered DO19 phase formation in the Ti-Al system

For the Ti-rich, Ti-AI system, the- and- ₂ phase boundaries have been obtained through differential thermal analysis for alloy compositions ranging from 10.2 to 25.2 at% Al (6 to 16 wt% Al). It has been shown that in the above mentioned composition range, a metastable disordered-phase can be quenched in. This metastable then transforms to the ordered ₂(DO)₁₉ phase upon heating and/or isothermal ageing. It has also been observed that the kinetics of this ordering phenomenon is composition dependent.