Elevated Temperature Stability Research Articles

The thermal stability of the fibrous copper-chromium eutectic

The elevated temperature stability of the fibrous copper-chromium eutectic was studied and found to depend strongly upon the extent of structural defects in the as-grown eutectic. Both highly branched and nearly “ideal,” or regular, fibrous structures were obtained by controlling the crystal growing conditions. The branched structure coarsens at a much faster rate than the regular structure. In the regular structure, the initial stage of coarsening is described by a simple two-dimensional Ostwald ripening mechanism modified to take into account the effects of fiber geometry and volume fraction. The coarsening rate is limited by volume diffusion of the chromium solute in the copper matrix. The activation energy for coarsening is approximately 298 kJ/mol. After long time annealing, the chromium-rich fibers begin to pinch off and three dimensional coarsening is initiated which leads to a rapid increase in the coarsening rate. Application of theoretical analyses to the observed coarsening rate yields the productDΣ as 1.3 × 10−13 J/s at 1000°C, whereD is the diffusivity of Cr in solid Cu and Σ the Cu-Cr interfacial energy. Using reported values ofD, Σ is bracketed between 0.27 and 2.5 J/m2. The latter is quite high for a solid-solid interfacial energy, but is consistent with the rapid coarsening observed in this system.

Elevated temperature stability of carbon-fibre, nickel-matrix composites: morphological and mechanical property degradation

Carbon-fibre, nickel-matrix composites exhibit excellent microstructural stability when the composites are isothermally annealed at temperatures to 1200° C for several hundred hours; morphological degradation of the reinforcing phase takes place at temperatures in excess of 1250° C. The degradation temperature is less than 1200° C when the composites are subjected to thermal cycling or temperature gradients.

Stability of reactive and refractory metal bor1des in ternary chromium-base alloys

Dispersion hardening by the Group IVb and Vb metal carbides has been demonstrated to improve the high temperature strength of chromium significantly 1, and has created interest in the similar use of the borides of these metals. The purpose of this work was to characterize the elevated temperature formation and stability of these borides in chromium-base alloys, about which little is currently known.

Elevated temperature stability of the Al-Al 3Ni eutectic composite : Smartt, H. B., Tu, L. K. and Courtney, T. H. Metallurgical Transactions, Vol 2, No 9, pp 2717–2727 (September 1971)

Elevated temperature stability of the Al-Al 3Ni eutectic composite : Smartt, H. B., Tu, L. K. and Courtney, T. H. Metallurgical Transactions, Vol 2, No 9, pp 2717–2727 (September 1971)

Elevated temperature stability of the Al−Al3Ni eutectic composite

The elevated temperature stability of the Al−Al3Ni rod eutectic is reexamined at 600°C. The original coarsening process is found to be two-dimensional Ostwald ripening. The morphology produced by this process is intrinsically stable but is subject to subsequent breakdown which is most likely due to the presence of longitudinal growth defects. Coarsening resumes after breakdown of the stable structure and is characterized by two-dimensional anisotropic Ostwald ripening. Extensive fiber shortening also takes place in this latter stage of coarsening.

Some properties of a fibre-reinforced nickel-base alloy

Abstract Some improvement in tensile strength and creep-rupture strength has been demonstrated on reinforcing Nimocast 713C with 20 v/o of 0·050 in. diam. tungsten wire. A more significant enhancement in both strength and specific strength was achieved using tungsten-5% rhenium as the reinforcement. Theoretically, higher specific strength at elevated temperatures could be obtained by reinforcement with molybdenum or niobium alloys provided that suitable barrier coatings can be developed to prevent interaction between matrix and reinforcement. The fatigue properties of Nimocast 713C in tension-tension loading are not measurably influenced by reinforcement with 0·050 (or 0·040) in. diam. tungsten wire, in contrast to previous reports on composites of a nickel-base alloy plus 0·010 in. diam. wires. This behaviour is interpreted in terms of the relative effectiveness of the fibre arrays of given volume fraction in arresting the propagating fatigue cracks. A limited number of tests have shown that the composites are stable in thermal cycling conditions. Chemical interaction between matrix and reinforcement occurs to a degree which is considered acceptable on fibres of greater than 0·030 in. diam. The present data indicate that the choice of refractory metal wire diameter must be made on the basis of a compromise of fatigue strength, creep-rupture strength, and long term elevated temperature stability and the ability to retain alignment during infiltration. The feasibility of using extrusion as a fabrication technique has been demonstrated.

Platinum Metal Plating—A Process and Applicational Survey

SummaryProcesses for the electrodeposition of the platinum group metals are reviewed, from the viewpoint of general electrolyte and deposit characteristics rather than that of detailed plating procedures. Applications are briefly discussed, with special reference to the electronics field. On grounds of limited availability, the platinum metals can offer no serious challenge to the pre-eminence of gold but they offer advantages in respect of wear resistance, elevated temperature stability, and erosion resistance.

The Influence of Small Additions of Silver on the Structure and Properties of Aged Aluminum Alloys

1. Small additions of silver can exert marked effects on the structure and properties of several aluminum alloys. It is believed that so-called trace elements effects such as this will assume increasing importance in the future because, in many alloy systems, the combinations of major alloying elements have been investigated. 2. The most significant effects of silver additions are observed in alloys based on the Al-Zn-Mg system. On the basis of endurance lives in a range of environments, the stress-corrosion resistance of Al-Zn-Mg-Cu alloys is increased while still maintaining a high level of strength. The addition of silver has been shown to increase the threshold stress for stress-corrosion cracking in an industrial atmosphere but this has not been confirmed for alternate immersion tests in a NaCl solution. Overall, the effects of silver are most marked if the addition of silver is made in conjunction with minor modifications to certain other alloying elements, and a higher ageing temperature is used (e.g. 165°C). 3. There is evidence that the stress-corrosion resistance of welded Al-Zn-Mg alloys is increased if silver is added to the parent metal or solely to the filler rod. 4. Silver additions may modify the structure and properties of aged Al-Mg and Al-Cu-Mg alloys (providing the Mg: Cu ratio is sufficiently high). One possible practical implication of these results is that silver may increase the elevated temperature stability of certain Al-Cu-Mg alloys and preliminary results have been obtained for one experimental composition. 5. From the fundamental viewpoint, it is considered that the effects of silver on the precipitation processes in certain aluminum alloys arise because of an interaction between silver atoms, magnesium atoms, and vacant lattice sites. The interaction is believed to modify the nucleation processes of the particular precipitates although the mechanisms differ depending on the alloy concerned. In alloys based on the Al-Zn-Mg system, silver stimulates nucleation of the existing precipitate whereas silver promotes the formation of a different precipitate in a wide range of Al-Cu-Mg alloys.

OBSERVATIONS ON THE EFFECT OF VACUUM HEAT-TREATMENT (UP TO 4000°F) ON DISPERSIONS OF VARIOUS OXIDES, NITRIDES, AND BORIDES IN MOLYBDENUM

The compatibility between molybdenum and several refractory compounds present as dispersed phases in the 3000-4000°F (1650–2210°C) temperature range has been evaluated. Compounds included in the study were BN, ZrN, HfN, LaB6, ZrB2, TaB2, Y2O3, MgO, ZrO2, and ThO2. Hot-pressed alloys, consisting of nominally 2.5 vol.%, of the refractory phase in a molybdenum matrix, were vacuum-annealed to determine elevated-temperature stability. The evaluation was based on changes in grain size, dispersed-phase morphology, hardness, and gross macroscopic deterioration. The materials HfN, ZrO2, and ThO2 showed promise for dispersion-strengthening and were recommended for further development.

A Mathematical Interpretation of the Kinetics of Elevated Temperature Stability Studies as Applied to Solid Formulations

When using kinetic principles and the Arrhenius equation to predict shelf life, it is convenient to use exaggerated storage temperatures. However, it is sometimes difficult to attain temperature equilibrium rapidly in these cases, and because accurate heating time estimates are essential for precise shelf life predictions, the time lags during sample heating and cooling may seriously affect the prediction accuracy. In this paper an equation is presented which relates the sample heating rate, storage time, temperature, and the heat of activation of the breakdown reaction. The solution of the equation is expressed as “Equilibrium Temperature Time Equivalent” or ETTE, and represents the effective time of storage at the oven temperature. The ETTE takes into account the finite time lag in attaining temperature equilibrium during the heating phase and during cooling after removal from the oven. A sample calculation is presented to indicate the magnitude of the error involved in using sample storage time which does not correct for the time necessary to attain temperature equilibrium. The ETTE permits the formulator to correct either storage times or his sampling times, thus making the error negligible.