High Temperature Deformation Behavior of a Mechanically Alloyed Mo Silicide Alloy
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0980-II06-06
High Temperature Deformation Behavior of a Mechanically Alloyed Mo Silicide Alloy Martin Heilmaier1, Holger Saage1, Pascal Jéhanno2, Mike Böning2, and Jens Freudenberger3 1 Institute for Materials & Joining Technology, University Magdeburg, Grosse Steinernetischstrasse 6, Magdeburg, D-39016 Magdeburg, Germany 2 Technology Centre, Plansee SE, Reutte/Tyrolia, A-6600, Austria 3 Leibniz-Institut fuer Festkoerper- und Werkstoffforschung Dresden, Helmholtzstr. 20, Dresden, D-01069, Germany ABSTRACT 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, the temperature varied between 1200 and 1400°C. With a stress exponent of about 2 and an activation energy close to that of Mo-self diffusion the material exhibits superplasticity at temperatures as low as 1300°C with strain to failures up to 400%, thus, making wrought processing on industrial-scale facilities typical for refractory metals and alloys feasible. To enhance creep resistance the alloys were annealed at 1700°C for 10h for coarsening the microstructure. While, still, the average sizes of all phases were below 10 microns, a reduction in minimum creep rate was noted. This finding also demonstrates the extraordinary high thermal stability of this 3-phase Mo-silicide alloy. INTRODUCTION Mo-Si-B alloys consisting of Mo solid solution (Moss) and the intermetallic phases Mo3Si and Mo5SiB2 (T2) hold promise as structural materials for ultra-high temperature applications in excess of 1100°C in air, taking advantage of (i) the beneficial oxidation resistance of the silicides and (ii) the intrinsically good mechanical properties of molybdenum. An alloy composition with balanced properties was given as Mo–8.9Si–7.7B (all compositions are given subsequently in at.%) [1, 2]. However, Berczik [1] reported that wrought processing of such alloys with conventional grain sizes for making semi-finished products needed temperatures as high as 1760 °C which is extremely difficult to manufacture reproducibly on industrial equipment [3]. Furthermore, from the three phases under consideration here the Mo(ss) possesses the highest melting point by far [4, 5]. Therefore, (usual) metallurgical approaches which come from the liquid phase will always lead to primary solidification of the Mo(ss) phase before eventually the brittle intermetallic compounds will solidify as a part of the eutectic network surrounding the toughening Mo(ss) particles. To overcome this undesired microstructure which is brittle in nature up to temperatures above 1000°C [4], the patents of Berczik [1, 6] suggest the appl
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