Article

Creep of platinum alloys at extremely high temperatures

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Abstract

Extremely high temperatures, mechanical stresses and severe corrosion conditions act in combination in equipment for the production of optical glasses. Common solid solution platinum base alloys such as Pt-10%Rh or Pt-5%Au often cannot be used because they can cause a detrimental discoloration of the glass. The materials selection is therefore limited to pure platinum or a dispersion hardened pure platinum matrix. However, compared to conventional alloys, dispersion hardened alloys are usually less ductile. In the present investigation creep tests have been carried out on pure Pt (99.95 weight%) and the oxide dispersion hardened alloy Pt DPH in the temperature range between 1473 K and 1873 K. The specimens were heated directly by an alternating electric current in specially designed creep test facilities. Strain was measured with a video extensometer. The measurements confirm much better creep rupture strength and lower stationary creep rates at high temperatures for the oxide dispersion hardened alloy Pt DPH compared to pure Pt. Activation energies and stress exponents are given. The hardening effect in Pt DPH is achieved by finely dispersed zirconium and yttrium oxides, which are formed during an internal oxidation heat treatment of the alloy in a compact state. Oxide particles can be found both inside grains and at grain boundaries. SEM and TEM investigations of the microstructure were performed in order to discuss the mechanical properties of the alloy at high temperatures.

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Article
Specially designed facilities for tensile testing of ultra-high temperature alloys are presented. Ohmic heating is chosen for easy access to the sample, fast attainable heating and cooling rates, simplicity in design and operation. Strain is measured with a video extensometer by means of the software SuperCreep. The algorithm for the strain measurements is described. Stability and accuracy of the test system were determined by testing an oxide dispersion strengthened Pt alloy. Performance of the video extensometer was checked by thermal expansion tests on pure Pt. Tensile tests of the oxide dispersion strengthened alloy Pt-10 wt% Rh DPH at 1600°C have proven the reliability of the equipment.
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Article
Creep behavior has been investigated with high purity platinum (99.999 mass%) at high temperatures from 1373K up to 1773K. Stress and temperature dependence of the creep rate has been discussed in detail by using microscopic observations and X-Ray diffraction measurements. Platinum shows the power-law creep with the stress exponent of the steady state rate of about 6. Recrystallization to bamboo-type grains from the initial polycrystalline micro-grains has been observed. In these specimens a strain burst (a sudden increase in the strain) has been detected. The occurrence of this burst depends on the concentration and kinds of minute impurities, test-temperature, applied stress, and the detailed process of the specimen preparation. The fracture mode changes from the void coalescence to the necking or shearing-off by the recrystallization. The activation energy for the steady state creep is (3.1±0.3)×105 J/mol.
Article
The paper reports from an ongoing project which is intended to help the design engineers at SCHOTT GLAS to choose the right platinum base alloy for any particular application in the production of special glasses and high-quality glass fibers. In order to investigate the creep and fracture properties at very high temperatures special test facilities were designed and built. The specimens are heated directly by alternating current. The temperature is measured by an infrared thermometer. Strain is measured with the aid of a high resolution camera and the image processing system SuperCreep. Creep tests have been carried out on the solid solution alloys Pt 10 wt.% Rh and Pt 20 wt.% Rh and four oxide dispersion hardened alloys (ODS alloys) with nominal compositions of 90 wt.% Pt and 10 wt.% Rh. The hardening effect in Pt-10 wt.% Rh ODS alloys is due to finely dispersed zirconium and yttrium oxides. The measurements confirm much better creep rupture strength and lower stationary creep rates at high temperatures for oxide dispersion hardened Pt-10 wt.% Rh alloys compared to the conventional alloys Pt-10 wt.% Rh and Pt-20 wt.% Rh.
Article
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Article
A new fundamental approach to the design of high strength, high thermal conductivity dispersion-strengthened copper alloys for applications in actively cooled structures is developed. This concept is based on a consideration of the basic principles of thermodynamics, kinetics and mechanical properties. The design requirements for these materials include a uniform distribution of fine particles for creep and fatigue resistance, a high thermal conductivity, thermodynamic and chemical stability at temperatures up to 1300 K, a small difference in the coefficients of thermal expansion between the particle and matrix, and low particle coarsening rates at the processing and service temperatures. The theory for creep of dispersion-strengthened metals developed by Rösler and Arzt is used to predict the optimum particle size for a given service temperature and to illustrate the need for a high interfacial energy. Resistance to coarsening leads to a requirement for low diffusivity and solubility of particle constituent elements in the matrix. Based on the needs for a low difference in the coefficients of thermal expansion to minimize thermal-mechanical fatigue damage and low diffusivity and solubility of the constituent elements, several candidate ceramic phases are compared using a weighted property index scheme. The results of this quantitative comparison suggest that CeO2, MgO, CaO and possibly Y2O3 may be good candidates for the dispersed phase in a copper matrix.
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