Received: from mx03.ecn.purdue.edu (mx03.ecn.purdue.edu [128.46.105.218]) by mailbag.ecn.purdue.edu (8.14.4/8.14.4) with ESMTP id q3CDXNVL003321 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NOT); Thu, 12 Apr 2012 09:33:23 -0400 (EDT) Received: from mailhub128.itcs.purdue.edu (mailhub128.itcs.purdue.edu [128.210.5.128]) by mx03.ecn.purdue.edu (8.14.4/8.14.4) with ESMTP id q3CDWTUw013309 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NOT); Thu, 12 Apr 2012 09:32:29 -0400 Received: from WPPEXHUB03H.purdue.lcl (wppexhub03h.itap.purdue.edu [172.21.6.92]) by mailhub128.itcs.purdue.edu (8.14.4/8.14.4/mta-nopmx.smtp.purdue.edu) with ESMTP id q3CDWTRQ000670; Thu, 12 Apr 2012 09:32:29 -0400 Received: from VPEXCH02.purdue.lcl ([169.254.1.4]) by WPPEXHUB03H.purdue.lcl ([172.21.6.92]) with mapi; Thu, 12 Apr 2012 09:32:28 -0400 From: "Stacey, Lisa A" To: "'msefaculty-list@ecn.purdue.edu'" , "'msepostdoc-list@ecn.purdue.edu'" , "'msegradstudent-list@ecn.purdue.edu'" Date: Thu, 12 Apr 2012 09:32:28 -0400 Subject: SEMINAR NOTICE: Ph.D. Final Exam-K. Vishnu 4/19/12 Thread-Topic: SEMINAR NOTICE: Ph.D. Final Exam-K. Vishnu 4/19/12 Thread-Index: Ac0YsLPRhHemtB7KT32yv/i0pNboNQ== Message-ID: <92F872D971E7AA498CF64DD847E5594D83C438C1EE@VPEXCH02.purdue.lcl> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: yes X-MS-TNEF-Correlator: acceptlanguage: en-US Content-Type: multipart/related; boundary="_004_92F872D971E7AA498CF64DD847E5594D83C438C1EEVPEXCH02purdu_"; type="multipart/alternative" MIME-Version: 1.0 X-PMX-Version: 5.5.9.388399 X-PerlMx-Virus-Scanned: Yes X-ECN-MailServer-VirusScanned: by amavisd-new X-ECN-MailServer-Origination: mailhub128.itcs.purdue.edu [128.210.5.128] X-ECN-MailServer-SpamScanAdvice: DoNotScan --_004_92F872D971E7AA498CF64DD847E5594D83C438C1EEVPEXCH02purdu_ Content-Type: multipart/alternative; boundary="_000_92F872D971E7AA498CF64DD847E5594D83C438C1EEVPEXCH02purdu_" --_000_92F872D971E7AA498CF64DD847E5594D83C438C1EEVPEXCH02purdu_ Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MATERIALS SCIENCE AND ENGINEERING SEMINAR Shape Memory in Nanostructured Metallic Alloys by Karthik Guda Vishnu Purdue Ph.D. Final Exam Advisor Professor A. Strachan ABSTRACT Materials with nano-scale dimensions show mechanical and structural propert= ies different to those at the macro scale and engineering their nano-struct= ure opens up potential avenues for designing materials tailored for a speci= fic application. This work is focused on shape memory materials, an importa= nt class of active materials with wide variety of applications in medical i= ndustry, aerospace and automobile industries, due to their two important pr= operties of super-elasticity and shape memory. These unique properties orig= inate from a solid-solid transformation called martensite transformation an= d the main objectives of this research are to i) study the atomic mechanism= s of the martensite transformation, ii) study the effect of nano-structure = on shape memory behavior and iii) computationally explore avenues through w= hich their performance is optimized. A combination of density functional th= eory (DFT) and molecular dynamics (MD) simulations is used to achieve this.= This approach gives an atomic level description and the effects of size, s= urfaces and interfaces are explicitly described. Detailed analysis of the a= tomic mechanisms of the martensite transformation in NiTi using DFT reveale= d a new phase transformation (B19'') that sheds light on why the theoretica= lly predicted ground state (BCO) is not observed experimentally and that th= e experimentally observed martensite phase (B19') can be stabilized by inte= rnal stresses. This finding is very important as the theoretically predicte= d ground state does not allow for shape memory in nano-scale NiTi samples. The size effects caused by the presence of free surfaces and the role of na= no-structure in martensite transformation have been investigated in thin Ni= Ti slabs. Surface energies of B2 phase (austenite), B19 (orthorhombic), B19= ' (martensite) and the body centered orthorhombic phase (BCO) are calculate= d using DFT. (110)B2 surfaces with in-plane atomic displacements stabilize = the austenite phase with respect to B19' and BCO, thus slabs with such orie= ntations are predicted to exhibit a decrease in martensite transition tempe= rature with decreasing thickness. It has been predicted that a thickness of= 2 nm is critical for the transition to occur. The opposite trend is observ= ed in slabs with atomic displacements along the surface normal; the phase t= ransformation temperature increases with decreasing size. Finally, it has also been demonstrated via large scale molecular dynamics s= imulations that the austenite - martensite energy landscapes of NiAl based = thermo-elastic shape memory superlattice structures can be engineered via e= pitaxial integration. This allows for tuning their thermal hysteresis and t= ransition temperatures. This resulted in materials with very low thermal hy= steresis and with minimal degradation in phase transition strain thereby ma= king them ideal for actuating applications. Date: Thursday, April 19, 2012 Time: 3:30 P.M. Place: ARMS Room 1028 Lisa Stacey Secretary/Development Assistant Purdue University School of Materials Engineering 765/494-4100 --_000_92F872D971E7AA498CF64DD847E5594D83C438C1EEVPEXCH02purdu_ Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable = <= meta name=3DGenerator content=3D"Microsoft Word 12 (filtered medium)">

MATERIALS SCIENCE AND ENGINEERIN= G

<= span style=3D'font-size:14.0pt'>SEMINAR

&nbs= p;

Shape Memory in Nanostructured Metallic Alloys

&n= bsp;

by

 

Karthik Guda Vishnu

Purdue Ph.D. Final Exam<= /o:p>

 

Advisor

Professor A. Strachan

 

ABSTRACT

 

Materials with nano-scale dimensions show mechanica= l and structural properties different to those at the macro scale and engin= eering their nano-structure opens up potential avenues for designing materi= als tailored for a specific application. This work is focused on shape memo= ry materials, an important class of active materials with wide variety of a= pplications in medical industry, aerospace and automobile industries, due t= o their two important properties of super-elasticity and shape memory. Thes= e unique properties originate from a solid-solid transformation called mart= ensite transformation and the main objectives of this research are to i) st= udy the atomic mechanisms of the martensite transformation, ii) study the e= ffect of nano-structure on shape memory behavior and iii) computationally e= xplore avenues through which their performance is optimized. A combination = of density functional theory (DFT) and molecular dynamics (MD) simulations = is used to achieve this. This approach gives an atomic level description an= d the effects of size, surfaces and interfaces are explicitly described. De= tailed analysis of the atomic mechanisms of the martensite transformation i= n NiTi using DFT revealed a new phase transformation (B19’’) th= at sheds light on why the theoretically predicted ground state (BCO) is not= observed experimentally and that the experimentally observed martensite ph= ase (B19’) can be stabilized by internal stresses. This finding is ve= ry important as the theoretically predicted ground state does not allow for= shape memory in nano-scale NiTi samples.

The size effects caused by the presence of = free surfaces and the role of nano-structure in martensite transformation h= ave been investigated in thin NiTi slabs. Surface energies of B2 phase (aus= tenite), B19 (orthorhombic), B19’ (martensite) and the body centered = orthorhombic phase (BCO) are calculated using DFT. (110)B2 surfa= ces with in-plane atomic displacements stabilize the austenite phase with r= espect to B19’ and BCO, thus slabs with such orientations are predict= ed to exhibit a decrease in martensite transition temperature with decreasi= ng thickness. It has been predicted that a thickness of 2 nm is critical fo= r the transition to occur. The opposite trend is observed in slabs with ato= mic displacements along the surface normal; the phase transformation temper= ature increases with decreasing size.

F= inally, it has also been demonstrated via large scale molecular dynamics si= mulations that the austenite – martensite energy landscapes of NiAl b= ased thermo-elastic shape memory superlattice structures can be engineered = via epitaxial integration. This allows for tuning their thermal hysteresis = and transition temperatures. This resulted in materials with very low therm= al hysteresis and with minimal degradation in phase transition strain there= by making them ideal for actuating applications.

 

Date:   Thursday, April 19,= 2012

Time:   3:30 P.M.

Place:   ARMS Room 1028

 

 

 

L= isa Stacey

Secretary/De= velopment Assistant

P= urdue University

School= of Materials Engineering

765/494-4100

 =

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