The observation of surface-softening in Fe-based metallic glass
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The observation of surface-softening in Fe-based metallic glass Ansar Masood1, V. Ström1, L. Belova1, J.J. Oak2, J. Ågren1, K. V. Rao1 1
Materials Science and Engineering Department, The Royal Institute of Technology, Brinnelvägen 23, 10044 , Stockholm, Sweden 2
Institute of Materials Research, Tohoku University, Sendai 980-8577, Japan
ABSTRACT Depth sensing nano-indentation investigations have been performed to determine the radial dependence of the hardness through the cross section of a Fe-based bulk glassy rod. We have found the hardness of the material decreases along radius from the centre to the outermost surface. This phenomenon is attributed to the ‘cooling rate induced surface softening’. Furthermore, a significant change (~15 %) in elastic modulus is noticed along the radius as well. INTRODUCTION The study of mechanical properties of bulk metallic glasses (BMG) is an intriguing field of research from fundamental as well as application point of view [1-17]. One of the major areas of active pursuit being carried out is how to improve the ductility of these materials, an essential factor for structural applications [2-5, 7]. A recent breakthrough in this direction is the discovery of induced surface softening in BMG. It is emphasized that the surface softening by residual stresses induced by shot-peening may improve the ductility in amorphous glassy materials [7]. Such surface softening can also be obtained by thermal tempering [7,8]. However the occurrence of such a phenomenon is extremely rare in the case of BMG because of their high thermal conductivity. In the present letter, we report the manifestation of surface softening in Fe-based BMG. Among the different classes of BMG, the Fe-based materials are particularly interesting because they exhibit high strength and excellent soft magnetic properties along with very high saturation magnetization [1, 6-10]. The lack of ductility hinders them from different structural applications. Although there are many studies regarding the magnetic properties of these materials, the mechanical properties of these materials are not well investigated. In the present work, we focus on the mechanical properties of Fe71B23Nb6 bulk metallic glass. Our study indicates that, the hardness and the elastic modulus of the material considerably decreases at the outermost surface in comparison to the central part indicating the occurrence of ‘surface-softening’. This kind of surface softening may originate due to the different cooling rates at central and outer part of the rod during casting. Earlier experimental studies indicate that FeBNb system has some unique network like structure which enables it to have high mechanical strength along with appreciable compressive plasticity [15]. Therefore it is considered as an excellent material for structural applications. The current work is to gain further insight into the mechanical properties of the FeBNb system.
EXPERIMENTAL DETAILS The glassy amorphous rod of well known BMG composition of Fe71B23Nb6 of diameter ~ 0.5 mm was fabric
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