P. Jéhanno

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.

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.

The Oxidation Behaviour of Pack-Treated Heavy Refractory Alloys

The heavy refractory metals and alloys Molybdenum (Mo), Molybdenum – Silicon – Boron (Mo–Si-B; “MoSiBor”), Tungsten (W), Tungsten – Copper (W-Cu), Tungsten – Nickel – Iron (W-Ni-Fe; “Densimet D 176 and 185”) and Tungsten – Nickel – Molybdenum - Iron (W-Ni- Mo-Fe; “Densimet D2M”) were pack-treated at 1100°C with Silicon - powder to form siliconized zones and/or intermetallic phases which are intended to be more oxidation resistant than the plain base materials. These materials (especially the W-based ones) are used at ambient conditions as counterweights, radiation shields etc. because of their high density as well as at high temperatures (600 – 900°C) as metal forming tools, electrodes etc. because of their refractory metal content. In both areas of conditions oxidation of the plain materials occurs and leads to lower functionality or destruction. A suitable oxidation test has been defined to check the presumably enhanced oxidation resistance of the pack-treated materials: an isothermal high temperature oxidation test at 700 and 900°C for one week. At these conditions all untreated materials would have been more or less strongly oxidized. Improved oxidation resistance could be found for the materials with pack-cementation treatment except for sintered Tungsten (92% dense), sharp etched D 185 and D 176 at 900°C and Tungsten – Copper at both temperatures. More stable and dense superficial oxides were formed which led to decreased oxidation rates and could help to increase functional stability and the lifetime of the components. Different pack-treatments e.g. with chromium or silicon plus chromium could improve the behaviour of the materials which failed within this work.