Ageing of a Pd-35 at % Ag-25 at % Cu solid solution alloy

A palladium-35 at % silver-25 at % copper alloy has been prepared to simulate a succesful, commercial alloy. The commercial alloy is widely used because of its good corrosion resistance and formability and the fact that it develops good wear resistance and low electrical resistivity after ageing. However, little is known about the transformations which result from heat treatment of this alloy. Thus, the laboratory prepared material was used for determination of ageing transformations by means of resistance measurements, metallography, transmission microscopy, and X-ray diffraction. The following sequence of reaction was found to occur: $$\begin{gathered} Supersaturated \to Spinodal \to Ordering and discontinuous \hfill \\ solid solution decomposition precipitation \hfill \\ \end{gathered} $$ The transformations are quite similar to those occurring in several other solid solution systems whose initial spinodal decompositions are followed by discontinuous transformations with one or more ordered phases.     

Precipitation of vanadium carbide in an Ni-38 at.% Fe 3 at.%V-2.4 at.% C alloy

An electron microscopy investigation of the precipitation of vanadium carbide in a nickel-iron base alloy is reported. Three types of precipitate occur, matrix-dot, stacking fault and secondary precipitate depending upon ageing time and temperature. The morphology identity, and orientation relationships of the secondary precipitate have been determined. The features of matrix-dot precipitate free and enhanced zones have been studied, and a mechanism is proposed to explain the latter.     

Elevated temperature deformation behaviour of multi-phase Ni-20 at % Al-30 at % Fe and its constituent phases

The mechanical behaviour of the multi-phase ( + /) alloy Ni-20 at % Al-30 at % Fe and alloys similar to its constituent and / phases, Ni-30 at % Al-20 at % Fe and Ni-12 at % Al-40 at % Fe, respectively, were investigated. When tested in tension at 300 K, the alloys exhibited 20%, 2% and 28% elongation, respectively. At elevated test temperatures (700, 900 and 1100 K), the multi-phase alloy exhibited increased ductility, reaching an elongation in excess of 70% at 1100 K without necking or fracture. Similarly, the alloy demonstrated increased ductility with increasing test temperatures. In contrast, the / alloy showed greatly reduced ductility with increasing temperature and was quite brittle both at 900 and 1100 K. Thus, whilst at room temperature the / phase improved the ductility of the + / aggregate, at elevated temperatures the phase alleviated the brittleness of the / phase, thereby preventing any embrittlement of the multi-phase alloy over the temperature range 300–1100 K. Also, whilst the phase improved the room-temperature strength of the multi-phase alloy, at elevated temperatures where the phase is known to be weak, the / phase improved the strength of the multi-phase alloy up to 900 K, beyond which the strength deteriorated due to disordering and lack of anomalous strengthening in the / component.     

Clustering phenomena in Al-4.4 at. % Zn alloy with 0.03 and 0.08 at. % Ag additions

The influence of 0.03 and 0.08 at. % Ag additions on the clustering of Zn atoms in an Al-4.4 at. % Zn alloy has been studied by resistometry. The effect of quenching and ageing temperatures shows that the ageing-ratio method of calculating the vacancy-solute atom binding energy is not applicable to these alloys. Zone-formation in Al-Zn is unaffected by Ag additions, but the zone-reversion process seems to be influenced. Apparent vacancy-formation energies in the binary and ternary alloys have been used to evaluate the v-Ag atom binding energy as 0.21 eV. It is proposed that, Ag and Zn being similar in size, the relative vacancy binding results from valency effects, and that in Al-Zn-Ag alloys clusters of Zn and Ag may form simultaneously, unaffected by the presence of each other.     

Creep of austenitic 20% Cr/35% Ni stainless steels at low stresses

The present work comprises measurements of the secondary creep-rate at different stress levels with rates between about 2×10^–5 %/h and 10%/h and the grain-boundary sliding at 700° C in two austenitic 20 wt % Cr/35 wt % Ni stainless steels. One alloy was a pure 20 wt % Cr/35 wt % Ni steel, whereas the other contained about 0.5 wt % Ti and 0.5 wt % Al so that it precipitated during creep at 700° C. Special care was taken to assure equivalent microstructure in the specimens and precise creep conditions so as to obtain accurate and reproducible creep-rates. Both materials exhibited decreasing stress-dependence of the creep-rate at low stresses. Neither the stress-dependence of the creep-rate, nor the absolute creep-rate was consistent with diffusion-creep. The amount of grain-boundary sliding was measured separately by means of scribed grid lines on the creep specimens for the pure material at stresses above the creep yield. The values for the component of the creep-rate due to grain-boundary sliding coincide very well with the extrapolated line of the low-stress branch of the creep-rate/stress curve. All these results taken together suggest that the most likely explanation of the creep yield in 20 wt % Cr/35 wt % Ni steels is the one based upon grain-boundary sliding.     

Tensile behaviour of two DO3-ordered alloys: Fe3Si and Fe-20 at % Al-5 at % Si

The tensile behaviour, including fracture modes and deformation substructures, of two powder-produced DO₃-ordered alloys having compositions Fe-25 at % Si (Fe₃Si) and Fe-20 at % Al-5 at % Si, has been investigated from room temperature to 800° C. The brittle-to-ductile transition temperature for the Fe₃Si alloy occurred at a temperature between 500 and 550° C, while that of the Fe-20 at % Al-5 at % Si alloy was approximately room temperature. In both alloys fracture occurred by transgranular cleavage at room temperature, with the occurrence of an increasing proportion of intergranular cavitation with increasing temperature. At low strains plastic deformation occurred chiefly by movement of perfect superlattice dislocations which, with increasing strain, dissociated to produce next-nearest-neighbour antiphase boundary trails.     

Crystallization kinetics of Zr-33 at % Ni amorphous alloy

The amorphous to crystalline transition in Zr-33 at% Ni amorphous alloy has been found to occur by polymorphic crystallization. The product of the cyrstallization process has been identified as the equilibrium Zr₂Ni intermetallic phase. The kinetics of crystallization have been studied independently using differential scanning calorimetry (DSC) and transmission electron microscopy. The activation energy of crystallization has been evaluated by isothermal and continuous heating in DSC. The isothermal anneals have revealed that the crystallization follows the Johnson-Mehl-Avrami kinetics with an Avrami exponent close to 4. The microstructural changes accompanying crystallization have been studied for an interpretation of the Avrami exponent. The nucleation and growth rates of crystals have been estimated at different temperatures in order to determine the activation energies of the two processes. It has been found that nucleation is thermally activated and growth is interface controlled.     

Structural and Phase Transformations in Alloys during Spark Plasma Sintering of Ti + 23.5 at % Al + 21 at % Nb Powder Mixtures

We have studied the effect of sintering temperature on the structural and phase transformations of alloys produced by the spark plasma sintering of Ti + 23.5 at % Al + 21 at % Nb powder mixtures at temperatures in the range 1100–1550°C. The sintered alloys have been characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy (elemental X-ray mapping). The alloys sintered at temperatures of 1100 and 1200°C have been shown to have a nonuniform microstructure. According to electron microscopy results, the alloys consist of grains of the α₂ and Nb₂Al phases and small precipitates of the O-phase (intermetallic compound Ti₂AlNb). In addition, there are particles of unreacted niobium and titanium. The alloys sintered at a temperature of 1300°C have a uniform lamellar structure.     

Superconductivity of heat-treated Nb-65 at.% Ti alloy

Brief reviews are given of the effect of heat-treatment on the microstructure of cold-worked bcc metals and the superconducting properties of niobium alloys. Particular attention is paid to the influence of interstitial impurities in these processes. The annealing effects in microstructure and superconducting properties of a cold-worked Nb-65 at.% Ti alloy, containing oxygen as a major impurity, have been studied. The precipitation process takes the form TiNb + O₂ various Ti oxides Ti + TiO. Differences in precipitation sequence are described for vacuum-annealing and annealing in impure argon. Flux-pinning is related to the microstructural observations. At least three pinning mechanisms appear to operate; dislocation pinning and two types of precipitate pinning. These observations are in accord with previously proposed pinning models.     

Creep behaviour of Cu-30% Zn at intermediate temaperatures

The creep properties of single-phase Cu-30% Zn alpha brass were investigated in the intermediate temperature range 573–823 K (0.48–0.70T _m, whereT _m is the absolute melting point). Inverse, linear, and sigmoidal primary creep transients were usually observed above 573 K under stresses resulting in minimum creep rates between 10^–7 and 2×10^–4s^–1, while normal primary creep occurred under all other conditions. The creep stress exponent decreased from about 5.4 at 573 K to about 4.1 above 623 K, and the activation energy for creep varied between 170 and 180 kJ mol^–1. A detailed analysis of the data, as well as a review of the literature, suggests that no clearly defined class M to class A to class M transition exists in this alloy, although the characteristics of both class A and class M behaviour are observed under nominally similar stresses and temperatures. It is concluded that Cu-30% Zn does not conform to the normally accepted characterization of class A or class M solid solution alloys.     

Early-stage phase separation in liquid-quenched Al-7 at % Zn

The kinetics of GP zone formation in liquid-quenched Al-7 at % Zn was studied with small-angle X-ray scattering. It is found that liquid quenching yields a solid solution in the as-quenched condition which is significantly less clustered than for solid quenching. In addition, the ageing kinetics of the liquid-quenched specimens was observed to be significantly slower than that for solid-quenched specimens. The results indicate that slow ageing in liquid-quenched alloys results from the absence of a high concentration of quenched-in mobile vacancies. It is thus suggested that liquid quenching effectively enables studies of the earliest stages of phase separation in precipitation-hardenable alloys.     

Al-3%Cu-2%Li合金的热变形行为

在Gleeble-1500热模拟仪上进行热压缩实验,研究了变形温度为350~500℃,应变速率为0.001,0.01,0.1和1s-1时Al-3%Cu-2%Li合金的热变形行为。利用双曲正弦本构关系分析热变形中的流变应力,采用金相分析热变形中合金的显微组织变化。结果表明,该合金流变应力的大小受变形温度、应变速率的强烈影响,它随变形温度升高而降低,随应变速率提高而增大,该合金高温流变应力可采用Zener-Hollomon参数的函数来描述,其热变形激活能为325.48kJ/mol。     

On annealed Ni-34.6 at % Al at 1523 K and effects of cooling rates

The microstructures and room temperature mechanical properties of melt spun Ni-34.6 at % Al annealed at 1523 K for 2 h and cooled at different rates were studied. The annealed microstructures and bend ductility were sensitive to the cooling rate. The distribution and relative content of precipitation were important factors for enhancement of bend ductility of the annealed ribbon. The best bend ductility of the annealed ribbon reached 7.6%, which was considerably better than the as-melt spun ribbon. The emission and transmission of dislocations from ductile into to satisfy the need of deformation compatibility played a key role in the improvement of the ductility.     

3He in 99.5% Porous Aerogel at the Normal-Superfluid Transition

We have used a torsional oscillator to measure the superfluid density and dissipation near the superfluid transition of ³ He in aerogel of 99.5% porosity. We used a new cell (constructed at Penn State) for which the aerogel was grown in the pores of a 100 m silver sinter. The cell was tested with ⁴ He and showed no signs of the second-sound resonances that have interfered with previous torsional oscillator measurements. The measurements with ³ He, presented here, were taken at pressures of 1.34 and 4.13 bars. We observed values of _s / in the T 0 limit of 0.05 and 0.14 respectively. Our measurements show an increase in the dissipation on warming through T _c . This series of measurements is ongoing and temperature sweeps at various pressures are planned.     

Bayesian reconstruction of nanodosimetric cluster distributions at 100% detection efficiency

Ionisation spectra in nanometric volumes at a given distance from a charged particle track are obtained by using electron (or ion) gas detectors, having non-uniformly distributed detection efficiency. Therefore, such spectra should be properly processed in order to reconstruct the frequency distribution of clusters really produced in the detector gas. A Bayesian unfolding has been applied to ionisation distributions due to 5.4 MeV alpha particles in a 20-nm site obtained by Monte Carlo simulations, taking into account different detection efficiency conditions. It will be shown that Bayesian analysis provides a valid tool for reconstructing the true ionisation distributions, well beyond the maximum measured cluster size.     

Mechanical twinning in copper-29.9 at.% gold single crystals

Three copper-gold single crystals containing 29.9 at. % gold and having the disordered, Cu₃Au I and Cu₃Au II structure respectively, were deformed in tension at room temperature, at dry ice temperature (just into the plastic region) and at liquid nitrogen temperature (until fracture) successively.For Cu₃Au I and Cu₃Au II ₀₀ does not vary practically with composition, being four times that of stoichiometric Cu₃Au [6]. At 78° K twinning was observed in the elastic region for Cu₃Au II, whereas Cu₃Au I twinned only under the influence of complex stresses; the disordered alloy showed no twinning at all.The strain-hardening _II at 78° K is almost the same for Cu₃Au I and Cu₃Au II at the 20.9 at. % composition.