In-situ observation of Nb/Nb 5 Si 3 two-phase alloys during bending at various temperatures
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In-situ observation of Nb/Nb5Si3 two-phase alloys during bending at various temperatures Seiji Miura1, Yukiyoshi Tsutsumi*2, Tetsuo Mohri1 1
Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University,
Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Hokkaido, Japan. 2 Now with KOBELCO Construction Machinery Co., Ltd. *
Graduate Student, Graduate School of Engineering, Hokkaido University
ABSTRACT In order to understand the deformation and fracture behavior of Nb-Si alloys, in-situ observation was conducted during bending of small specimens at room and high temperatures. Nb-Si alloy ingots containing 18.1 at.%Si, 1.5 at.%Zr and 100 ppmMg were prepared by arc melting, followed by uni-axial solidification in an optical floating zone apparatus and a heat-treatment to obtain Nb/Nb5Si3 two-phase microstructure. Chevron-notched specimens with a dimension of 1x2x10mm were used for in-situ observation of bending tests under a confocal laser scanning microscope (CLSM) at room temperature and at 1140 oC. At room temperature the Nb-Si alloy shows a fracture toughness of 8 MPa m1/2 and the crack propagation velocity seems to be not uniform, presumably due to the ductile Nb. At 1140 oC the toughness of the alloy was about 20 MPa m1/2 and slower plastic deformation prior to the cracking was observed. The SEM observation of crack surfaces revealed that plastic deformation of Nb enhances the toughness of the alloy. INTRODUCTION For several decades Nb alloys have been investigated as a candidate for high temperature materials. Comparing to other refractory metals such as W, Ta and Mo, Nb has much lower density (as light as Ni), while the melting point is 2469 oC (about 1000 oC higher than that of Ni) [1]. Recently developed Nb-based alloys strengthened by intermetallic phases such as Nb5Si3 have suitable properties to be used as turbine foils [2-4]. The present authors have proposed a unique microstructure control method for Nb-based alloys with Nb5Si3 through two invariant reactions (eutectic and eutectoid reactions) to improve its room temperature toughness [5-9]. The microstructure of a Nb-Si eutectic alloy containing 1.5 %Zr and 100 ppm Mg after eutectic solidification is composed of rod-like Nb and a Nb3Si
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matrix. It is confirmed by EBSD analysis that all the fine Nb rods have the orientation
same crystallographic within each of the
eutectic
cells. The eutectoid decomposition of Nb3Si starts at the Nb/Nb3Si interface and all of the Nb plates formed by the decomposition have the same crystallographic orientation with Nb rods. Therefore, all the Nb phases in a eutectic cell have the same crystallographic orientation and connect each other without any Nb/Nb grain boundaries. On the other hand, the Nb5Si3 silicide phase, which is the counterpart of the Nb phase in the eutectoid decomposition, is formed in the plate-shape during the Figure 1. A schematic illustration of the microstructure reaction but tends to be spherical evolution from (a) liquid phase to (b) eutectic cells as the shaped af
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