Study of 11bar2 1 Twinning in αTi b

Authors: Leyun Wang Rozaliya Barabash Thomas Bieler Wenjun Liu Philip Eisenlohr

Publish Date: 2013/04/04

Volume: 44, Issue: 8, Pages: 3664-3674

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Abstract

Activity of the 11bar2 1 langle bar1 bar1 26 rangle extension twinning T2 mode was analyzed in a commercial purity Ti sample after 2 pct tensile strain imposed by fourpoint bending The sample had a moderate caxis fiber texture parallel to the tensile axis Compared with the many 10bar1 2 langle bar1 011 rangle extension T1 twins that formed in 6 pct of the grains T2 twins were identified in 025 pct of the grains by scanning electron microscopy SEM and electron backscattered diffraction EBSD maps Most of the T2 twins exhibited irregular twin boundaries TBs on one side of the twin Highresolution EBSD revealed both intermediate orientations at some matrix/twin interfaces and substantial lattice rotation within some T2 twins Interactions between matrix 〈c + a〉 dislocations frac13 langle 1bar2 13 rangle and a 11bar2 1 T2 twin were investigated by combining SEM/EBSD slip trace characterization and Laue microdiffraction peak streak analysis 〈c + a〉 dislocations that originally glided on a pyramidal plane in the matrix were found on other planes in both the matrix and the twin which was attributed to extensive crossslip of the screw component whose Burgers vector was parallel to the twinning plane On the other hand thickening of the twin could engulf some pileup edge components in front of the TB During this process these 〈c + a〉 dislocations transmuted from a pyramidal plane 0bar1 11 in the matrix to a prismatic plane bar1 010 textT in the twin lattice Finally possible mechanisms for the nucleation and growth of T2 twins will be discussedMechanical twinning has a large effect on the plasticity of hexagonal metals such as Mg Ti and Zr On one hand nucleation and propagation of twins in a grain can effectively accommodate deformation along the caxis thus enhancing the ductility of the material12 On the other hand dislocation–twin interaction can make a significant contribution to the work hardening of the material3 4 5 6 The latent hardening model has typically been used to describe dislocation–twin interaction in crystal plasticity frameworks that generally give good prediction of the measured stress–strain curves78 However the latent hardening model may be too simplified to incorporate detailed accommodation mechanisms around twins that were experimentally observed which include 1 glide dislocations’ dissociation at the twin boundary TB4910 2 transmutation of existing matrix dislocations into the twin lattice11 12 13 14 15 3 formation of kink bands or secondary twins at the twin–twin intersection4101617 etc Furthermore cracks can sometimes nucleate at twin–twin or twingrain boundary intersections because of local shear incompatibility18 19 20 21 There is not a single computational model that can comprehensively describe twinning in hexagonal metals that include their nucleation growth interaction with matrix dislocations other twins and grain boundaries To achieve this aim more experimental characterization is needed to identify active mechanismsTitanium is one of the most widely used hexagonal metals In Ti four twinning modes and their twinning shears γ have been reported namely 10bar1 2 langle bar1 011 rangle T1 γ = 017 11bar2 1 langle bar1 bar1 26 rangle T2 γ = 063 11bar2 2 langle 11bar2 bar3 rangle C1 γ = 010 and 10bar1 1 langle 10bar1 bar2 rangle C2 γ = 02222 T1 and T2 are referred to as tensile or extension twinning modes because generally these two twinning modes result in extension of the crystal in the cdirection and can be activated only in such grains that have their caxis strained in tension Correspondingly C1 and C2 are referred to as compressive or contraction twinning modes At room temperature T1 twinning is the most commonly observed twinning mode in Ti and other hexagonal metals with c/a ratio less than sqrt 8/3 such as Mg and Zr23 24 25 26 27 28 29 Several atomistic and grainscale models have been proposed to account for the nucleation and growth of T1 twins30 31 32 33 34 35 36 In contrast other twinning modes have received much less attentionThe current article focuses on the T2 twinning mode The large shear of T2 twins makes them potential damage nucleation sites in the material Though T2 twins are less frequent than T1 twins they have also been observed in Co17 Zr273738 Ti2629 and an Mgbased alloy39 ReedHill et al observed kink bands both inside the twin and outside the twin adjacent to the tip of some T2 twins in Zr37 The kink bands were attributed to a set of basal slip dislocation walls with frac13 langle 11bar2 0 rangle Burgers vector Vaidya and Mahajan17 examined T2 twins in Co by analyzing dislocations near TBs using transmission electron microscopy TEM Near the matrix/twin interface 〈c + a〉 dislocations Burgers vector of which was parallel to the twinning plane were identified17 Other than these two early articles little research focusing on T2 twinning has been published partly because of the lack of effective characterization tools and—at least in the case of Ti—because of the limited number of T2 twin observations compared with T1 twinsIn the last two decades advances in electron backscattered diffraction EBSD and Laue microdiffraction have led to new opportunities for the study of deformation microstructures Among many applications EBSD is convenient for statistically analyzing twinning282938 and characterizing local lattice rotation4041 Laue microdiffraction uses a highenergy polychromatic Xray microbeam to probe the material The ultrabrilliant Xray beam is able to penetrate metals for a distance greater than one hundred microns Using the differential aperture Xray microscopy DAXM technique42 43 44 45 Laue diffraction patterns from individual volume elements voxels along the beam path can be extracted Indexing of these Laue patterns reveals the crystal orientation of each voxel with an accuracy as high as 001 deg In a deformed specimen Laue patterns often display streaked peaks because of the existence of geometrically necessary dislocations GNDs45 By analyzing peak streak direction in a Laue pattern it is possible to identify the content of local GNDs which provides new means to gain crucial knowledge about local deformation history4647In the current study T2 twins were identified in five different grains in a commercial purity Ti specimen after small tensile strain These T2 twins and their parent grains were intensively characterized using EBSD and DAXM Some unusual features of T2 twins such as irregular TBs and large orientation gradients inside the twin lattice were observed The origin of these features will be discussed

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