Title page for etd-0521118-091514

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URN etd-0521118-091514
Author Wen-Shuo Chuang
Author's Email Address No Public.
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Department Materials and Optoelectronic Science
Year 2017
Semester 2
Degree Ph.D.
Type of Document
Language English
Title Deformation mechanisms and sample size effects of Mg-Zn-Y 18R LPSO
Date of Defense 2018-06-10
Page Count 197
  • Mg-Zn-Y alloys
  • 18R LPSO
  • micro-compression
  • sample size effect
  • deformation mechanism
  • nanoindentation
  • Abstract With the long-period stacking ordered (LPSO) second phase, a novel Mg-Zn-Y based alloys with excellent mechanical properties have been developed. Because the LPSO phase plays an important role in strengthening, it is urgent to clarify the deformation mechanisms within the LPSO phase. Until now, there have been many research works done on the LPSO structure examination and the deformation mechanism along its growth direction, which is [11"2" ̅0], but all in the mini-meter scales. However, because the grain size of the LPSO is typically in 30 to 150 m, it is essential to shrink the testing sample sizes in order to achieve a clear examination on LPSO single crystals. Moreover, the mechanical properties and the deformation mechanisms on different crystal orientations have not been clarified. 

    In this study, the yield strength as well as the deformation mechanisms of the Mg-Zn-Y 18R LPSO structure with two different orientations, namely, (11"2" ̅0) and (0001), will be systematically examined and analyzed in the micro-meter scale by micropillar compression, and the basic mechanical properties by nanoindentation. As for compressing along [11"2" ̅0], different deformation behavior have been observed between different sample sizes. The prism slip has been observed in smaller micro-pillars while the deformation kink has been observed in larger micro-pillars. The origin of deformation kink has been found out to be the prismatic dislocation slip. According to the Frank’s rule, the interaction between prismatic dislocations would cause the stair-rod dislocations, which would form the kink boundaries and nucleate the basal dislocations to induce the deformation kinks. It indicates that the yield mechanisms in Mg-Zn-Y 18R LPSO single crystal during compressing along [11"2" ̅0] are the same which is proved by using both TEM analysis and ln-lnd curves fitting. The different deformation behaviors are caused by the sample size effect. With larger sample sizes, the probability of forming stair rod dislocations becomes higher, resulting in higher probability of forming deformation kinks. Moreover, these results will be compared with those in the literature on the mini-meter scale.
    As for compressing along [0001], since there are few studies with deformation along [0001], we attempt to figure out the whole deformation mechanism. It has been found that the basal dislocations activate first causing the bending of the micro-pillars. The bending situation results in the formation of prism screw dislocations forming the slip band with 45 degrees with respect to the [0001] loading direction. It is considered that the bending can cause the shear stress along [0001] and would induce the nucleation of prism dislocations along 45 degrees with respect to the [0001] loading direction to form the slip band. This slip band causes the first pop-in shown in the stress strain curves. From our results, the deformation behavior are the same with different sample sizes. Moreover, the sample size effect of the first pop-in stress has also been examined using the ln-lnd curves fitting. Interestingly, the first pop-in stress in the 3.8 m micro-pillar rises to the level of 2.7 m micro-pillar. The reason is because the original dislocations influence the slip of basal dislocations, resulting in the delay of slip band forming.
    Both experimental results show the trend of higher flow stresses with smaller sample sizes because of the sample size effect. In comparison with the slope of the ln-lnd curves (or in transferring into the Weibull modulus) with the data in literatures, it can be extracted that both the yielding under compression along [11"2" ̅0] and the first pop-in under compression along [0001] are caused by the prismatic slip, consistent with our transmission electron microscopy analysis.
    Advisory Committee
  • Jason Shian-Ching Jang - chair
  • H.K. Lin - co-chair
  • C.T. Pan - co-chair
  • S.R. Jian - co-chair
  • J. C. Huang - advisor
  • Files
  • etd-0521118-091514.pdf
  • Indicate in-campus at 2 year and off-campus access at 2 year.
    Date of Submission 2018-06-21

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