Influence of zirconium content on microstructure and creep properties of Mo–9Si–8B alloys

Highlights

• Creep of three phase Mo–9Si–8B alloys with zirconium addition is measured.

• Formation of Si-oxide on the grain boundaries can be prevented with zirconium addition thus enhancing creep strength.

• Samples with 1 at.% zirconium exhibit higher ductility than samples with 2 at.%.

• Creep is dislocation controlled accompanied by grain boundary sliding.

Abstract

Mo–Si–B alloys with a molybdenum solid solution accompanied by two intermetallic phases and Mo₅SiB₂ are a prominent example for a potential new high temperature structural material. In this study the influence of 1, 2 and 4 at.% zirconium on microstructure and creep properties of Mo–9Si–8B (at.%) alloys produced by spark plasma sintering is investigated. Creep experiments have been carried out at temperatures of 1100 °C up to 1250 °C in vacuum. The samples exhibit sub-micron grain sizes as small as 450 nm due to the chosen production route. With addition of 1 at.% zirconium, formation of SiO₂ on the grain boundaries can be prevented, thereby enhancing grain boundary strength and creep properties significantly. Moreover ZrO₂ particles also enhance creep resistance of the molybdenum solid solution. Creep deformation is a combination of dislocation creep in the grains including dislocation-particle interaction and grain boundary sliding leading to intergranular fracture surfaces. It is promising to use grain size adjustments in order to balance the creep and oxidation resistance of the investigated material.

Introduction

Three-phase Mo–Si–B alloys pioneered by Berczik [1], [2] are potential candidates for high temperature structural applications. They usually have a molybdenum solid solution providing adequate toughness and two intermetallic phases Mo₃Si and Mo₅SiB₂ (T2 phase) forming a protective boro-silicate glass layer for oxidation resistance [3]. In order to reduce the ductile to brittle transition temperature and provide room temperature toughness a continuous molybdenum matrix with embedded intermetallic phases is preferred [4].

Recently, several production techniques with the aim of establishing a Mo-Mo₃Si–Mo₅SiB₂ microstructure with a continuous Mo matrix have been developed. Examples range from arc melting with additional annealing [5] to a powder metallurgical approach followed by hot pressing in a graphite die and further annealing [6]. Krüger et al. [4] reported a method of manufacturing the three-phase material with a continuous Mo matrix using powder metallurgy combined with mechanical alloying in a planetary ball mill followed by cold isostatic pressing at 200 MPa, sintering in hydrogen at 1600 °C and hot isostatic pressing at 1500 °C and 200 MPa. Hereby it turned out that mechanical alloying yielding in powders with a nanoscale grain size is an important step resulting in shorter diffusion paths during the following processing steps leading to a homogeneous three-phase morphology.

In order to evaluate the deformation mechanisms at high temperatures (above 1100 °C) of these three-phase alloys, compressive creep tests have been carried out [7], [8]. However only little is known about the tensile creep behaviour of the materials.

Jéhanno et al. [9] produced a three-phase Mo–2.7Nb–8.9Si–7.7B (at.%) alloy with mechanical alloying as the essential processing step resulting in sub-micron grain sizes. Tensile testing of these alloys lead to superplasticity with strain to failures up to 400% at 1400 °C. Since the grains did not deform substantially during creep deformation, it is indicated that grain boundary sliding has a significant role in tensile deformation at elevated temperatures. Superplasticity was also observed by Li et al. using a Mo–9Si–8B-3Hf (at.%) alloy at temperatures ranging from 1400 °C to 1560 °C [10].

Jain and Kumar [11] carried out tensile creep tests of different Mo–Si–B alloys produced by the plasma rotating electrode process with additional hot isostatic pressing. They investigated the tensile strength of a single phase molybdenum solid solution alloy (Mo–3Si–1.3B (at.%)), a two phase alloy consisting of Mo solid solution and Mo₅SiB₂ (Mo–6Si–8B (at.%)) and a three-phase alloy with Mo solid solution, T₂ phase and Mo₃Si (Mo–8.6Si–8.7B (at.%)). The high dissolved Si level in the solid solution significantly strengthened the creep resistance of these alloys. While the three-phase alloy had only marginally better creep strength than the two-phase alloy, both were far superior than the solid solution alloy. The three-phase alloy showed strain to ruptures of 10–20% and the results suggest that at 1200 °C dislocation climb is the rate controlling mechanism at a grain size smaller than 3 μm.

A major issue connected with the powder metallurgical process is oxygen uptake leading to SiO₂ particles forming on grain boundaries of the alloy [6]. In order to getter oxygen during the production process Zr can be added in small amounts to form ZrO₂ particles [12]. As a beneficial side effect, zirconium additions reduces the grain size, and thus, limited plastic deformation is noted for room temperature bend tests [13]. The zirconium primarily segregates at grain and phase boundaries [14]. In addition Mo₂Zr particles as small as 10 nm could be observed in the molybdenum solid solution [12].

This study focuses on the influence of Zr addition on microstructural changes and creep in a Mo–9Si–8B (at.%) base alloy. The oxidation resistance of these alloys has already been studied in a previous work [15], [16]. Microstructures and mechanical properties during creep of four alloys Mo–9Si–8B–xZr (x = 0,1,2,4) (at.%) were determined. The microstructure before and after creep is compared to get an insight into the deformation behaviour of a three-phase alloy with a continuous solid solution matrix under varying zirconium levels.