M. Böning

Molybdenum alloys for high temperature applications in air

Abstract Molybdenum base silicide alloys exhibit promising oxidation resistance in addition to the inherent high temperature strength of refractory metals. However, alloys with sufficient oxidation resistance are effectively brittle up to temperatures above 816°C (1500°F). Recent progress in alloy and process development, utilising a PM manufacturing route with mechanical alloying as a crucial step, has allowed significant improvement of both oxidation resistance and mechanical properties via micro-alloying additions including nano-dispersed second phase oxide particles.

Investigation of current breaking capacity of vacuum interrupters with focus on contact material properties with the help of a reference model vacuum circuit breaker

This paper presents the performance of current breaking tests with a model vacuum circuit breaker and copper chromium disc-shaped contacts. The specifications of the model circuit breaker and the characteristics of the test circuit define a reference for the current breaking capability that serves as basis for further optimization. This reference breaking capability is defined with a standard powder-metallurgical contact material with 75 wt% copper and 25 wt% chromium. Information about the statistical reproducibility of the current interruption capability, the permissible number of high current switching events without contact wear, the recovery of breaking capability after current over-stress and the increase of breaking capability of virgin contacts due to high current conditioning could be gathered.

Parameters influencing the electrical conductivity of CuCr alloys

Copper-Chromium alloys are commonly used as contact materials for energy distribution. One of the major requirements is a good electrical conductivity for non-dissipative conduction of the nominal current. Since Cu and Cr are mutually nearly insoluble, CuCr alloys are two-phase composites, usually consisting of chromium particles embedded in a copper matrix. A decrease in electrical conductivity following e.g. a change in process parameters may be caused by (a) the geometry of the Cr phase, (b) phase boundaries between Cr particles and Cu matrix, or (c) diffusion of impurities into the Cu matrix. In the present work the influences of these different factors on the electrical conductivity of powder metallurgically produced CuCr25 material will be investigated. Regarding microstructure, several theoretical (analytical) models are available in literature, which do, however, not explicitly consider measurable features of the microstructure. We tackle this problem by combining quantitative microstructural analyses with an empirical relationship generated from FEM simulations based on simplified microstructures to obtain a prediction of the electrical conductivity of the composite. The electrical conductivities determined show considerable deviation from experiment, which is ascribed to the above mentioned factors (b) and (c) having not been incorporated into the model yet. However, the comparison of longitudinal and transverse conductivity measurements enables the separation and quantification of the effects arising by phase boundaries and impurities.

Investigation of the heat affected volume of CuCr contact material for vacuum interrupters

This work is focused on the microstructural changes during switching of simple geometry, butt-type contacts in a demountable vacuum chamber. The heat affected volume in copper-chromium (CuCr) with different chromium (Cr) content and the particle size in the re-solidified area were investigated. It was found that there is only a negligible difference between copper (Cu) with 25 (CC75) and 43 (CC57) wt% Cr. For CC57 a sudden change in the investigated parameters is observed for transferred charges above 35 As and a peak current (Ip) higher than 6 kA. It is assumed that a transition into the arc mode occurs. The results were assessed against published work, in which the switching process was observed using a camera and compared with the development of arc voltage.

Influence of processing on the microstructure and mechanical behaviour of Mo-Si-B alloys

Mo-Si-B materials consisting of a Mo(Si) solid solution and the intermetallic phases Mo3Si and Mo5SiB2 (T2) were prepared by mechanical alloying (MA) as the crucial step of a powdermetallurgical process. After consolidation via an industrial processing route (cold isostatic pressing, sintering, hot isostatic pressing) the resulting microstructures of Mo-Si-B alloys up to 45% of intermetallic phases reveal a continuous α-Mo matrix with embedded, homogeneously distributed intermetallic particles. Clearly, increasing the amount of Mo solid solution reduces the BDTT (demonstrated by three point bending tests between room temperature and 1200°C), however, values below 900°C could not be obtained due to grain boundary embrittlement caused by Si segregation. Alloying with Zr was proven by Auger analysis in Mo-Si solid solutions to reduce this segregation. Therefore, in a second trial Zr as a (micro-) alloying element was added. The influence of microalloying on ductility and strength is comparatively discussed with reference compositions Mo-6Si-5B and Mo-9Si-8B.

High Temperature Deformation Behavior of a Mechanically Alloyed Mo Silicide Alloy

A 3-phase Mo-Si-B alloy consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 (T2) was manufactured employing mechanical alloying (MA) as the crucial processing step. After consolidation via cold compaction, sintering in hydrogen atmosphere and final hot isostatic pressing (HIP) at 1500°C, one obtains an ultra-fine microstructure with a nearly continuous Mo(ss) matrix and the sizes of all phases being in the 1 micron range. Tensile tests were carried out in vacuum at initial strain rates ranging from 10-4 to 10-2 s-1 and the temperature varied between n1200 an 1400 °C. With a stress exponent of about 2 and the activation energy being close to that of Mo-self diffusion the material exhibits superplasticity at temperatures as low as 1300°C and tensile strain to failures up to 400%, thus, making sound wrought processing on industrial-scale facilities at temperatures typical for refractory metals and alloys feasible. To enhance creep resistance at high temperatures the alloys were annealed at 1700°C for 10h for a coarsening of the microstructure. While, still, the average sizes of all phases were below 10 microns, a considerable reduction in minimum creep rate was noted. This finding also demonstrates the extraordinary high thermal stability of this 3-phase Mo-silicide alloy.

Interdependency of test environment and current breaking capacity of a model vacuum switch

In contact material development it is elementary to have a fundamental understanding of the effects of the test environment on the switching performance. However, the complexity of those dependencies make it difficult to identify a definite cause-effect relationship between contact material and switching properties, especially if the effect of small variations in the contact material, such as the particle distribution and the morphology, shall be investigated. This paper sensitizes to the strong interdependencies of test conditions and current breaking capacity. To identify these dependencies the test environment was systematically modified. The modifications could reproducibly be correlated with their effect on the test results. In particular, the topology of the high current circuit and the contact bolt design showed strong effects. The circuit topology was optimized for reduction of the magnetic flux density within the contact gap, and it was standardized to ensure long-term reproducibility of future results. The modifications of the test circuit could also be recognized in the characteristic arcing voltage.