Elevated Temperature Mechanical Properties of Devitrified Metallic Glass
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Elevated Temperature Mechanical Properties of Devitrified Metallic Glass Nathan A. Mara, Alla V. Sergueeva, and A.K. Mukherjee Materials Science Division, University of California, Davis One Shields Avenue Davis, CA 95616, USA ABSTRACT Elevated temperature tensile tests of different microstructures arising from different heat treatments of the Fe-based metallic glass Vitroperm (Fe73.5Cu1Nb3Si15.5B7) are presented. An anneal at 6000C for 1h yields a single phase α-Fe microstructure with equiaxed, randomly oriented 15 nm grains, which is an ideal candidate for study of material properties at diminishing length scale. This microstructure has good stability during tensile testing at 6000C, showing a strain rate exponent correlating to grain boundary sliding (m=0.5), but little ductility, and strengths to 1250 MPa. The brittle behavior could be attributed to the lack of dislocation activity at such length scales. At temperatures up to 7250C, grain growth occurs, leading to elongations as large as 65% at flow stresses of 250 MPa. Precipitation of a second Nb-rich phase accompanies the grain growth. This investigation is supported by NSF, Division of Materials Research, grant NSF-DMR-0240144. INTRODUCTION High-temperature mechanical properties of nanocrystalline materials can often be quite challenging to ascertain due to the inherent instablility of the microstructure at length scales below 100nm. At the test temperatures necessary for diffusion-driven mechanisms to play a significant role in deformation (in excess of 0.5Tm), grain growth that occurs during testing can cause the microstructure to coarsen to the point that the sample is no longer truly nanocrystalline. Based on findings in the literature, there is a large scatter in the available experimental data, with wide ranges given for characteristic strain rate exponent values and for the activation energy of deformation [1-5]. These quantities give important information as to the dominant deformation mechanism at hand, and the scatter in their values reflects an important point—that a nanocrystalline structure does not point clearly to a single dominant deformation mechanism. Although there are many methods of producing nanocrystalline materials, devitrification of amorphous materials can offer some attractive advantages, so long as heat treatments are carefully carried out. With the correct chemistry, this method allows for many microstructures of uniform overall chemistry to be evolved by simply altering the annealing parameters, effectively avoiding some of the shortfalls associated with other production methods. In recent years, alloys have been developed that after crystallization yield a fully crystalline matrix with grain sizes as low as 20 nm [6]. Fe-based alloys with composition close to that of FINEMET show some promise in this area [7]. Commercially available alloys such as Vitroperm yield extremely fine one-phase microstructures, or bimodal grain size distributions consisting of several phases depending on the annealing parameters chosen. In thi
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