Mo-9Si-8B alloys with additons of Zr – microstructure and creep properties

Three phase Mo-9Si-8B (at.%) alloys are a prominent example for a potential new high temperature structural material. Due to their high melting point and excellent creep resistance. In this study the effect of Zr addition (0…4 at.%) on the microstructure and creep properties of Mo-9Si-8B (at.%) alloys is investigated. Two powder metallurgical processes, hot isostatic pressing (HIP) and spark plasma sintering (SPS), are used to prepare specimens. The resulting microstructures are examined using SEM and TEM analysis. SPS alloys exhibit smaller grain sizes and fewer oxides compared to the HIP alloys, because of the oxygen availability during HIP. The more Zr is present in the alloys, the more and finer the observed particles are. With addition of Zr the formation of SiO2 on the grain boundaries can be prevented completely, due to the formation of ZrO2. High temperature tensile creep tests are carried out under vacuum to determine the influence of the microstructure on creep properties. The creep rates are one order of magnitude lower for the Zr containing alloys. However with a level of 4 at.% Zr the minimum creep rates increase again. Please click Additional Files below to see the full abstract

Microstructural analysis and high temperature creep of Mo-9Si-8B alloys with Al and Ge additions

Refractory metals and their alloys show potential for high temperature applications, due to the increased melting point and creep resistance. Spark plasma sintering technology as well as argon arc melting is used to prepare quaternary and quinternary Mo-9Si-8B-xAl-yGe (x is 0 or 2; y is 0 or 2) samples. Compositions are stated in at.%. All the compositions consist of a Mo solid solution (α-Mo) and two intermetallic phases: Mo3Si (A15) and Mo5SiB2 (T2). On the one hand, no zirconium is added to the alloys to avoid evaporation of MoO3 due to the phase transformation from a monoclinic to a tetragonal crystallographic structure of ZrO2 at 1150°C. On the other hand, fractions of Al and Ge are alloyed to reduce the melting point of the intermetallic phases. The specimens are homogenized and coarsened by a subsequent heat treatment in a vacuum radiation furnace at 1850°C for 24 h. Both the reduction of the melting point and the heat treatment at a temperature of 1850°C result in an increase in diffusion rate. This procedure is expected to generate an α-Mo interpenetrating network. The resulting microstructures are investigated using SEM, EDX and XRD analyses. A creep testing device for a very short specimen heated in a radiation furnance up to 1400°C usable in air or vacuum is presented. Creep tests are performed at elevated temperatures in vacuum to investigate the influence of different fabrication techniques. Please click Additional Files below to see the full abstract

Finite element modelling of strains and stresses in platinum alloy bushings for textile glass fibre production

Platinum components are now very widely used in the manufacture and processing of glass. At first sight it is, therefore, surprising that finite element modelling (FEM) is seldom used in their design. The reason for this omission is to be found in the specific type of mechanical loading at high temperatures and the lack of appropriate mechanical property data for the structural materials. Using the example of a glass fibre bushing from an indirect melt fiberizing proeess, the paper will demonstrate the use of FEM to determine the stresses resulting from the hydrostatie pressure of the glass melt, the intrinsic weight of the bushing, the stresses caused by the forces applied to withdraw the fibres from the bushing, and the stresses induced by the inhomogeneous temperature distribution in the bushing. On the basis of these model calculations it was possible to localize regions of stress concentrations, to select an appropriate structural material and to modify the component design to reduce the stresses

Entropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe

Quantification of Solid Solution Strengthening and Internal Stresses through Creep Testing of Ni-Containing Single Crystals at 980 °C

Various alloy compositions were cast as single crystals in a Bridgman vacuum induction furnace and creep tested at 980 °C: pure Ni, the equiatomic alloys CoCrNi and CrMnFeCoNi (Cantor alloy), single-phase fcc (Ni) solid solution alloys (with the composition of the matrix-phase of CMSX-3 and CMSX-4), and two-phase Ni-based superalloys CMSX-3 and CMSX-4. Due to the single-crystal state, grain size effects, grain boundary sliding, and grain boundary diffusion can be excluded. The results identify two major strengthening mechanisms: solid solution strengthening and other mechanisms summarized as precipitation hardening. Configurational entropy does not increase creep strength: The Cantor alloy, with the highest configurational entropy of all alloys tested, shows a weak and similar creep strength at 980 °C in comparison to pure Ni with zero configurational entropy. The element Re is a very effective strengthener, both in single-phase fcc (Ni) solid solution alloys as well as in two-phase superalloys. Quantitative estimations of different strengthening mechanisms: internal back stress, misfit stresses, Orowan bowing, and γ′-phase cutting (in the case of two-phase superalloys) are presented. Finite element simulations allow estimating the influence of solid solution strengthening of the matrix on the creep behavior of the two-phase superalloys

Entropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy