Indentation creep of an Fe-based bulk metallic glass

Abstract

abstract In this study, the room temperature creep behavior of Fe 41 Co 7 Cr 15 Mo 14 C 15 B 6 Y 2 bulk metallic glass wasinvestigated using nanoindentation technique with the maximum applied load ranging from 1 mN to100 mN under different loading rates (0.01–2.5 mNs 1 ). The creep stress exponent was derived from therecorded displacement–holding time curve. It was found that the stress exponent increases rapidly from2.87 to 6.37 with increasing indentation size, i.e. exhibiting a positive indentation size dependence.Furthermore, as the indentation loading rate increases from 0.01 mNs 1 to 2.5 mNs 1 , the stressexponent decreases gradually from 4.93 to 0.94. The deformation mechanism causing the nano-indentation creep is discussed in the light of the ‘‘shear transformation zone’’ (STZ) which providesqualitative explanation for the observed plasticity in amorphous alloy. 2008 Elsevier Ltd. All rights reserved. 1. IntroductionThe last twodecades havewitnessed rapid development of bulkmetallic glasses (BMGs), which exhibit some unique mechanicalproperties and high potentiality for application as advancedstructural and functional materials [1–3]. While extensive plasticstrain can be reached in few localized shear bands, BMGs usuallyexhibit little or no macroscopic plasticity at room temperature[2–4]. Recently, several researchers have studied the plastic defor-mation mechanisms of BMGs using nanoindentation (see forexample Refs. [5,6]). Study of time-dependent deformation (i.e.creep) of metallic glasses using nanoindentation has been rare[7–9]. Apart from its scientific interest [10], creep under sub-micrometercontact is an issue of practical engineering significancefor the applications of MEMS and NEMS. From the indentationcreep studies of crystalline materials, a few indentation creepmethods have been proposed and used [11]. The constant loadholding method is the most widely used one, where the indentertip is maintained at a prescribed load, for an extended time period,while the displacement of the indenter into the surface iscontinuouslyrecorded,akintotheconstantloadcompressioncreeptests [11–13].The purpose of the present work is to investigate the creepbehaviors of an Fe-based bulk metallic glass. The stress exponentvalues were derived fromthe displacement–holding time curves. Itwas found that the stress exponent values obtained are dependenton the applied peak loads and indentation loading rates. Thedeformation mechanisms of metallic glasses are discussed.2. Experimental procedureAlloy ingots with nominal composition of Fe