FEM‐Simulation of Real and Artificial Microstructures of Mo‐Si‐B Alloys for Elastic Properties and Comparison with Analytical Methods

Abstract

In view of their outstanding intrinsic properties, notably the high melting point (approaching 2000°C) and, thus, the very good mechanical properties and creep strength at elevated temperatures, refractory metal (RM) silicide alloys are considered first choice replacements for Ni-base superalloys with the aim of increasing the (thermodynamic) efficiency of gas turbine engines. [1] Hence, the main objective of work on RM silicide alloys is to manufacture a composite material that takes advantage of (i) the beneficial oxidation resistance of the silicides and (ii) the outstanding mechanical properties of molybdenum. Nowotny et al, [2] first pointed out that alloys in the ternary system Mo-Si-B might be able to fulfil the above requirements. Later on, Berczik [3,4] followed up on this work and patented alloy compositions in the Mo-rich corner which were proved to possess balanced properties regarding high temperature creep strength, room temperature toughness and oxidation resistance. However, in his approach two manufacturing steps have appeared to be at least problematic to obtain sound material in sufficient quantities at reasonable costs: (1) Berczik employed a rapid solidification (RS) step via Helium gas atomization to obtain a matrix of Mo solid solution for adequate fracture toughness and ductility at temperatures below 600°C with embedded intermetallic compounds Mo 3 Si and Mo 5 SiB 2 (the T2 phase) for oxidation resistance due to the formation of a dense borosilicate glass layer on the metal surface, and (2) he also reported that for sound wrought processing temperatures above 1700°C were needed which makes industrial up-scaling hardly feasible.In order to overcome the above limitation (2) we suggested a powder-metallurgical manufacturing route with mechanical alloying (MA) as the technique replacing the (costly) RS gas atomization step to economically obtain large quantities of three phase Mo-Si-B material with a nearly continuous Mo solid solution matrix. [5] This material beneficially proved superplastic tensile deformation at temperatures as low as 1300°C, exhibiting an ultrafine microstructure with extraordinary thermal stability.Besides the above mentioned requirements applications as high temperature structural materials, for example as guide vanes in a turbine environment, also require the knowledge about the temperature dependent elastic properties and their dependence on microstructure. It is, therefore, the object of the present paper to combine an experimental and numerical approach for predicting the elastic properties of Mo-Si-B composite materials with varying microstructures. Experimental and Numerical Approach Microstructural characterizationThe microstructures of the samples were characterized by scanning electron microscopy (SEM) combined with energy dispersive X-ray microanalysis (JEOL 6400 SEM and FEI XL30 FEG equipped with EDX and EBSD). Three different alloy compositions and manufacturing routes were considered for subsequent modelling: while two cast alloys, Mo12Si8.5B and Mo12Si10...

On the fracture toughness of fine-grained Mo-3Si-1B (wt.%) alloys at ambient to elevated (1300 °C) temperatures

“…Such K JC values represent the plane stress fracture toughnesses that would have been obtained if a sample large enough to maintain small-scale yielding could have been used. The elastic modulus at 1300 C for these materials was estimated as 260 GPa, based on the experimental and finite element simulations performed by Biragoni and Heilmaier [36]. It should be noted that Kruzic, et al [21] did not test an alloy with w50 vol.% a-Mo at 1300 C, but the results for their 46 vol.% a-Mo material should be reasonably comparable to the present alloys.…”

Mo‐Si‐B Alloys for Ultrahigh‐Temperature Structural Applications

“…Hot pressing of blended powders provides a relatively simple means of creating a two‐ or three‐phase microstructure without the pitfalls of direct solidification; for this reason it is now widely used in processing of Mo‐Si‐B alloys. The technique has been used to successfully compact and sinter mixtures of molybdenum silicides, Mo and B,81, 113 precast Mo‐Si‐B powders,82–87 elemental Mo, Si and B114 as well as rapidly‐solidified,18, 19, 32, 95–101 surface‐modified,115–118 reaction synthesized15, 119–123 or mechanically alloyed4, 10, 11, 14, 123–137 Mo‐Si‐B powders. Heat treatment is required after hot pressing to further homogenize the material.…”

“…Mechanical alloying has received much attention recently4, 10, 11, 14, 124–137 as a method for producing ultra‐fine grained in situ Mo‐Si‐B composites. High energy collisions during mechanical alloying lead to repeated cold welding and fracturing of powder particles 138.…”

“…Only a few studies of the basic tension, compression or bending constitutive behavior of Mo‐Si‐B alloys at elevated temperatures have been published 9, 10, 14, 15, 94, 114, 134–137. However, the elastic constants of the Mo‐4Si‐1B alloy are known, determined by finite element modeling (FEM) simulations and confirmed by the resonant beam technique; these are listed, specifically Young's modulus, E , the shear modulus μ and the bulk modulus B , in Table 2 134…”

Mechanically activated combustion synthesis of molybdenum borosilicides for ultrahigh-temperature structural applications

“…The obtained Mo 5 SiB 2 /TiB 2 material has better elastic modulus and hardness: 135 GPa and 2.40 GPa, respectively. Yet, for both materials these properties are worse than for Mo-Si-B materials reported in the literature [28][29][30]. This is apparently associated with the insufficient density and significant porosity of the combustion synthesis products.…”