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However the increase in α martensitic phase along

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However, the increase inα″martensitic phase along with decrease in dislocationdensity is always accompanied by significant reducing the fraction ofωphaseprecipitation. As shown before, compared with the upper surface treated at a rotation
MANUSCRIPTACCEPTEDACCEPTED MANUSCRIPTPage16of32speed of 225 r/min (Fig. 4(d)), much more deformation-inducedα″martensite ispresented in the middle stir zone (Fig. 6(c)).ωphase precipitates are considered to bedissolved with increasing the deformation amount [58]. Currently, there was aspeculation that the structural change fromωtoα″is more difficult to achievecomparing with the transformation fromβtoα″, which is ascribed to the decrease inthe amount ofωphase [61]. However, little experimental evidence could prove thisspeculation. In this work, the dissolving ofωphase is discussed in detail for thespecimens with different rotation speeds as indicated in Fig. 8.It can be observed that most dislocations glide in slip bands for both specimensFSP-processed at different rotation speed as marked by yellow arrows in Fig. 8(a) andFig.8(b). Several single dislocations glide in the grain and slip planes aredistinguished from slip traces left by dislocations at the sample surfaces (indicated bywhite arrows). Due to multiple dislocation slips, some slip traces are wavy, whileother traces are straight. Same scenario was also clearly reported in Ref. [62].Compared with the FSP-processed specimen at a rotation speed of 225 r/min, manymore slip bands accompanying with dislocation tangles are present in the specimen atthe speed of 300 r/min, as indicated in Fig. 8(b), which is attributed to much largerdeformation extent at higher rotation speed. Thus, metastableωphase dissolvingfeature is evident in the region where dislocations gliding is present, especially in thespecimen processed at a higher rotation speed (Fig. 8 (b)). After considering that theobserved areas possess the same thickness for both specimens, we compare theHRTEM images for these specimens as shown in Fig.8(c) and Fig.8(d). In the current
MANUSCRIPTACCEPTEDACCEPTED MANUSCRIPTPage17of32work, the details for the domains ofωdissolving show clearly the transformationfromωphase toβphase. Many moreωprecipitates are firstly promotedaccompanying with a fewωdissolving regions and gradually disappear withincreasing the rotation speed from 225 r/min to 300 r/min. As seen from Fig.8(d),many more domains containingβphase appear and more feature ofωdissolving isobserved clearly, which is promoted by dislocation gliding. Fig. 8 (e) shows theindexed diagram corresponding toβphase andωphase.Fig. 9 shows the schematic view of microstructural characteristics in SZ, TZ,HAZ and BM, accompanyingωphase transformation. The detailedωphasetransformation mechanism is shown in Fig. 9 (b). During severe plastic deformation,especially for the upper surface at a rotation speed of 300 r/min, much more vacanciesare formed owing to the increased dislocation slipping, acceleratingωphasedissolving, as clearly indicated in Fig. 9. During this process, a homogeneous latticedeformation and lattice invariant shears are also observed. It was reported [63] thatthe vacancy mediated atom diffusion normally has a lower energy barrier whichpromotes the phase transformation fromωphase toαphase. In the current work,vacancy-ordered metastableωphase dissolves and finally develops to the

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Term
Spring
Professor
lebrini
Tags
Tensile strength, Fsp, Shape memory alloy, sz

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