Plastic deformation of NiTi shape memory .Plastic deformation of NiTi shape memory alloys ......

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    Acta Materialia xxx (2012) xxxxxx

    Plastic deformation of NiTi shape memory alloys

    Tawhid Ezaz a, J. Wang a, Huseyin Sehitoglu a,, H.J. Maier b

    a Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL 61801, USAb University of Paderborn, Lehrstuhl fur Werkstoffkunde, D-33095 Paderborn, Germany

    Received 13 December 2011; received in revised form 6 September 2012; accepted 9 September 2012

    Abstract

    Dislocation slip in B2 NiTi is studied with atomistic simulations in conjunction with transmission electron microscopy (TEM). Theatomistic simulations examine the generalized stacking fault energy (GSFE) curves for the {011}, f211g and {001} planes. The slipdirections considered are h100i, h111i and h011i. The results show the smallest energy barriers for the (011)[100] case, which is con-sistent with the experimental observations of dislocation slip reported in this study. To our knowledge, slip on the (011)111 system isillustrated for the first time in our TEM findings, and atomistic simulations confirm that this system has the second lowest energy barrier.Specimens that underwent thermal cycling and pseudoelasticity show dislocation slip primarily in the austenite domains while the bulk ofmartensite domains does not display dislocations. The results are discussed via calculation of the ideal slip nucleation stress levels for thefive potential slip systems in austenite. 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

    Keywords: Shape memory; Slip; NiTi; Dislocations; Density functional theory

    1. Introduction

    The shape memory alloy NiTi has considerable techno-logical relevance and has also been scientifically perplexing.For some time, the plastic deformation of austenite via dis-location slip has not been fully understood, although it isvery important as it limits the shape memory performance[16]. The role of slip in austenite has drawn significantattention recently [710].

    The understanding of NiTi has been empowered withrecent atomistic simulations. The simulations provide theenergy levels for the different phases [1113], the latticeparameters [14], the elastic constants [15] and the energybarriers for martensite twinning [16,17]. Beyond theseadvances, a detailed consideration of the dislocation slipbehavior via simulations is urgently needed to comparewith the experimental findings of active slip systems. Uponestablishing the GSFE (generalized stacking fault energy)

    1359-6454/$36.00 2012 Acta Materialia Inc. Published by Elsevier Ltd. Allhttp://dx.doi.org/10.1016/j.actamat.2012.09.023

    Corresponding author. Tel.: +1 217 333 4112; fax: +1 217 244 6534.E-mail address: huseyin@illinois.edu (H. Sehitoglu).

    Please cite this article in press as: Ezaz T et al. Plastic deformation of10.1016/j.actamat.2012.09.023

    curves in the austenitic (B2) phase, we study the propensityof five potential slip systems, and note the formation ofanti-phase boundaries (APBs) in certain cases. Conse-quently, we assess the magnitude of ideal stresses neededto activate slip in these different systems. We find the(011)[100] system to be the most likely one consistent withexperiments. The occurrence of (011)111 slip is reportedin our experimental findings in Section 2.1, which has notbeen reported earlier to our knowledge. This is the secondmost likely slip system after [10 0] slip.

    To gain a better appreciation of the role of slip on shapememory behavior we show two results in Fig. 1 for NiTi. Inthe first case, the temperature is cycled at a constant stress(Fig. 1a). The range of temperature is 100 to 100 C,which is typical for application of NiTi. It is evident thatas the stress magnitudes exceed 150 MPa, the strain uponheating is not fully recoverable. A small plastic strainremains and this is primarily due to residual dislocationsin the B2 matrix. In Fig. 1b the experiment is conductedat a constant temperature. The sample is deformed andaustenite-to-martensite transformation occurs, with

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    NiTi shape memory alloys. Acta Mater (2012), http://dx.doi.org/

    http://dx.doi.org/10.1016/j.actamat.2012.09.023mailto:huseyin@illinois.eduhttp://dx.doi.org/10.1016/j.actamat.2012.09.023http://dx.doi.org/10.1016/j.actamat.2012.09.023http://dx.doi.org/10.1016/j.actamat.2012.09.023

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    Fig. 1. Shape memory and pseudoelasticity experiments on solutionized 50.1% NiTi: (a) The development of macroscopic plastic (residual) strain upontemperature cycling (100 C to 100 C) under constant stress [18], (b) the plastic (residual) strain under pseudoelasticity at constant temperature(T = 28 C).

    2 T. Ezaz et al. / Acta Materialia xxx (2012) xxxxxx

    austenite domains primarily undergoing dislocation slip toaccommodate the transformation strains (see Section 2.2for further details). Upon unloading, a small but finiteamount of plastic strain remains. The plastic strainsbecome noticeable at the macroscale at stress levels exceed-ing 600 MPa for the solutionized 50.1% Ni NiTi. The smallplastic strains can accumulate over many cycles and deteri-orate the shape memory effect. Apart from the residualstrain that is produced, the presence of plasticity reducesthe maximum transformation strain and increases thestress hysteresis, two other measures of shape memory per-formance. This will be discussed further in Section 2.3.

    Under fatigue loading, the NiTi alloys exhibit gradualdegradation of pseudoelasticity with cycles, and this deteri-oration has been attributed to slip deformation [3,4]. Asstated above, the domains that undergo slip curtail thereversibility of transformation. Therefore, a higher slipresistance is desirable to achieve pseudoelasticity overmany cycles in NiTi. Based on this background, it is extre-mely worthwhile to develop a quantitative understandingof dislocation slip; specifically, it is crucial to determinethe energy barriers (GSFE curves) for the most importantslip systems. In this study, the simulation results (in Sec-tions 3 and 4) are aimed towards building a frameworkfor a better comprehension of shape memory alloys.

    The glide planes and directions of possible slip systemsin austenitic NiTi are shown in Fig. 2. We note that thereare multiple planes within the same family of slip systems,and only one of the planes is shown in Fig. 2 for clarity.The potential slip planes are {011}, f211g and {001}.In the studies of Chumlyakov et al. [7] and Tyumentsevet al. [18], NiTi is deformed at high temperatures(>473 K) where slip dominates. The slip systems were iden-tified as {110}h0 10i and {10 0}h010i. The {110}h010isystem was proposed by Moberly et al. [19]. More recently,Simon et al. [8], and Norfleet et al. [9] provided details oftransformation-induced plasticity and indexed the{10 1}h010i slip system. The transformation-inducedplasticity refers to the nucleation and build-up of slip inaustenite to accommodate the rather high transformation

    Please cite this article in press as: Ezaz T et al. Plastic deformation of10.1016/j.actamat.2012.09.023

    strains [2022] upon traversing martensite interfaces [22].Recently, Delville et al. [23] argued the source of the irre-versibility in shape memory alloys as primarily slip defor-mation; we note that residual martensite can also prevail,and contribute to the irreversibility.

    NiTi alloys exhibit considerable ductility. This high duc-tility behavior is viewed as unusual since B2 intermetallicalloys are expected to exhibit limited ductility [24]. As dis-cussed above, the {011}h1 00i permits glide only in threeindependent slip systems. The presence of only three inde-pendent systems for the {011}h10 0i case was discussed inthe textbook by Kelly et al. [25] and more recently in apaper on NiTi by Pelton et al. [10]. If loading was appliedat any point along the cube axis the cube is not able todeform because the resolved shear stress is zero for all pos-sible h100i directions, hence certain orientations produceno glide if there are less than five independent slip systems.Given that at least five independent slip systems arerequired for dislocations to accommodate arbitrary defor-mations [26], additional slip systems must be present inB2 NiTi, contributing to the enhanced plasticity. In B2alloys, the potentially operative {110}h11 1i slip providesan additional nine deformation modes that can contributesignificantly to the superior ductility. The slip system{110}h1 11i, though observed in a few ordered intermetal-lic alloys of B2 type such as b-CuZn [27] and FeCo [28],has, however, not been reported for NiTi to our knowl-edge. Rachinger and Cottrell [29] classified the B2 typeintermetallic compounds to two categories: (i) those ofionic binding with slip direction in h100i and (ii) thosedominated by metallic binding with slip direction h111i.In this paper, we report experimental evidence with trans-mission electron micrographs of {110}h1 11i slip in B2NiTi in Section 2, and provide an energetic rationale incomparison to the other possible B2 slip modes such as(01 1) 011, (001)[010] and 2 11[11 1] in Section 4.

    The underlying basis of dislocation motion in a certainglide plane and direction is described by the generalizedstacking fault energy curve (GSFE) [30]. Simulations incor-porating electronic structure are capable of predicting

    NiTi shape memory alloys. Acta Mater (20