Processing, Microstructure and Creep Behavior of Mo-Si-B Based Intermetallic Alloys for Very High Temperature Structural Applications. Annual Technical Report, July 1, 2003-June 30, 2004.
This research project is concerned with developing a fundamental understanding of the effects of processing and microstructure on the creep behavior of refractory intermetallic alloys based on the Mo-Si-B system. These materials are representative of the next generation of refractory alloys being considered for very high temperature structural applications. During this period of performance, we have studied the microstructure and mechanical properties of a three-phase Mo-3 wt.% Si-1 wt.% B (Mo-8.9Si-7.71B in at.%) alloy that was supplied by UES/AFRL. A powder metallurgy route was used by Plansee (Austria) to process this material into bulk form. Microstructural observations of samples in various conditions were conducted using back-scattered electron imaging in a scanning electron microscope. The Mo-3Si-1B alloy was observed to be three-phase, consisting of a-Mo, Mo3Si and T2-Mo5SiB2 phases, with the combined volume fraction of the latter two being approx. 30%. Samples for mechanical testing were prepared by electro discharge machining. Bend tests revealed that the elastic limit strength of the alloy remained quite high until 1200DGC with a value of 800MPa, but dropped rapidly thereafter to a value of 220 MPa at 1400DGC. Compression creep tests were conducted under inert atmosphere at 1200DGC at stress levels of 250, 300, 400 and 500 MPa in an Applied Test Systems creep frame equipped with computerized data acquisition. The results showed that the creep rates at 1200DGC were quite high and varied nearly linearly from a value of 3.58 x 10-5 min-1 at a stress of 250 MPa to 8.04 x 10-5 min-1 at 500 MPa. The stress exponent was ascertained to be approx. 1.18 from this data, which is near the value of 1 expected for diffusional creep mechanisms. This value of the stress exponent differs from the values of 2-7 reported for similar alloys by other investigators. Microstructural observations of post-crept samples indicated shape change and deformation of the a-Mo grains, together with the presence of many voids in these grains. A few cracks in the intermetallic particles and along their interfaces with the a-Mo matrix were also observed. These observations suggest that the bulk of the deformation and strain during creep is carried by the a -Mo.
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