The possible role of partial dislocations in facilitating transformations of the Nishiyama–Wassermann type

Crystalline 9-cyanoanthracene undergoes photodimerization to give the trans dimer which is unexpected on the basis of the topochemical preformation theory. The possibility that the reaction occurs at defects is investigated; and the nature of the structural imperfections are described, as are also the types of product nuclei and their modes of growth. Interference-contrast and fluorescence microscopy have been employed for the examination of cleaved and partially dimerized faces of the monomer. It is shown that there is an active slip plane (221), and consideration of feasible dislocation reactions, particularly those involving unit strength dislocations which have a component of the Burgers vector in [100], reveals that, within stacking-fault regions (bounded by partial dislocations), the monomer molecules are in trans registry. It is suggested that molecules in such stacking faults act as traps for the excitation energy, and that reaction occurs at these sites.

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Nucleation of γ' plates from G.P. zones in an Al-Ag alloy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100144231 ◽

1986 ◽

Vol 44 ◽

pp. 552-553

Author(s):

J.M. Howe ◽

R. Gronsky

Keyword(s):

Alternative View ◽

Ion Milling ◽

Frank Loop ◽

Shockley Partial ◽

Partial Dislocations ◽

Thin Foils ◽

Nucleation Sites ◽

Equilibrium Phases ◽

Experimental And Theoretical Studies

Experimental and theoretical studies on the role of G.P. zones in the nucleation of either transition or equilibrium phases have been an active area of research for many years. In Al-Ag alloys, these zones are not usually thought to serve as nucleation sites for metastable γ' plates, although the alternative view was expressed during early research on this system. Instead, γ' plates are nucleated either by the splitting of perfect a/2<110> dislocations into pairs of a/6<112> Shockley partial dislocations, or by the propagation of a Shockley partial dislocation across an a/3<111> Frank loop. The stacking faults produced by these reactions thus create an initial two-atom layer of hep γ' precipitate. However, in this study it was found that γ' plates can nucleate from G.P. zones during ion milling of thin foils prior to TEM examination.

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The Yield Stress Anomaly in Fe-Al Alloys: The Role of the Local Climb Locking Mechanism

MRS Proceedings ◽

10.1557/proc-364-79 ◽

1994 ◽

Vol 364 ◽

Cited By ~ 3

Author(s):

David G. Morris ◽

David Peguiron

Keyword(s):

Yield Stress ◽

Detailed Examination ◽

Al Alloys ◽

Al Alloy ◽

Al Content ◽

Partial Dislocations ◽

Locking Mechanism ◽

Crystallographic Axes ◽

Ordered State

AbstractA detailed examination of the variation of yield stress with temperature in an Fe3Al alloy shows a maximum at a temperature of about 500°C, slightly below the critical temperature for loss of DO3 order. At this temperature the dislocations present in the material change from being <111= superdislocations separated by APB to being single dislocations with Burgers vector <100=. At slightly lower temperatures the superdislocations become pinned by a local climb process involving point defect transfer between the partial dislocations.Analysis of the forces between the dislocations which induce the local climb locking process allows an estimation of the role which will be played by variations in composition of the Fe-Al alloy considered, changes in deformation rate and orientation of the applied stress.Examination of data available in the literature shows that each of the three aspects discussed, namely the influence of variations in ordered state, in Al content over the range 25% to 50%, or additions of alloying elements such as Si, straining at very fast or at slow rates, and stressing along different crystallographic axes, is completely consistent with the model proposed.

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Phase boundary effects on the mechanical deformation of core/shell Cu/Ag nanoparticles

Journal of Materials Research ◽

10.1557/jmr.2009.0263 ◽

2009 ◽

Vol 24 (7) ◽

pp. 2210-2214 ◽

Cited By ~ 7

Author(s):

Bin Zheng ◽

Yi Nong Wang ◽

Min Qi ◽

Elissa H. Williams

Keyword(s):

Phase Boundary ◽

Ag Nanoparticles ◽

Mechanical Deformation ◽

Atomic Level ◽

Single Type ◽

Core Shell ◽

Cu Nanoparticles ◽

Partial Dislocations ◽

Slip Planes

The uniaxial compressive deformation of core/shell-type Cu/Ag nanoparticles and naked Cu nanoparticles were simulated by molecular dynamics, revealing the role of nanophase boundaries in the mechanical deformation. The simulations show that single type of partial dislocations glide across the entire slip planes of the Cu cores, resulting in elongated Cu cores compared with circular Cu cores of naked Cu nanoparticles. The phase boundary is the nucleation source of dislocations, and the ultrahigh atomic level stress of part atoms in the phase boundary can ensure the movement of the single type of dislocations under compressed.

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The bacterial flagellum as an imperfect cylindrical crystal: Flagellar geometry, movement and polymorphism, and the role of partial dislocations

Journal of Theoretical Biology ◽

10.1016/0022-5193(74)90199-4 ◽

1974 ◽

Vol 47 (2) ◽

pp. 295-308 ◽

Cited By ~ 4

Author(s):

William Frith Harris

Keyword(s):

Bacterial Flagellum ◽

Partial Dislocations ◽

Cylindrical Crystal

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Nanoscale Grain Boundary Dissociation: Role of Shockley Partial Dislocations

Microscopy and Microanalysis ◽

10.1017/s1431927602108002 ◽

2002 ◽

Vol 8 (S02) ◽

pp. 1166-1167

Author(s):

D.L. Medlin ◽

D. Cohen ◽

G. Lucadamo ◽

S.M. Foiles

Keyword(s):

Grain Boundary ◽

Shockley Partial ◽

Partial Dislocations

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Grain Scale Assessment of Variant Selection in a Thermomechanically Processed TRIP Steel

Materials Science Forum ◽

10.4028/www.scientific.net/msf.539-543.4762 ◽

2007 ◽

Vol 539-543 ◽

pp. 4762-4767

Author(s):

You Liang He ◽

Stéphane Godet ◽

John J. Jonas

Keyword(s):

Electron Backscatter Diffraction ◽

Variant Selection ◽

Orientation Relationships ◽

Partial Dislocations ◽

Commercial Steel ◽

Austenite Grains ◽

Multi Phase ◽

Backscatter Diffraction ◽

Good Agreement

Orientations of both the α and γ phases in a multi-phase commercial steel were measured by means of electron backscatter diffraction (EBSD) techniques. Using the average orientation of each austenite grain as the reference frame, the orientation relationships between the two lattices were compared with the common orientation relationships (i.e. the Kurdjumov-Sachs and Nishiyama-Wassermann) in Rodrigues-Frank space. The occurrence of variant selection in individual austenite grains was examined using a recent dislocation-based model. This model considers the role of the slip systems that were active during prior deformation, as well as those of in-plane reactions, cross-slip and the partial dislocations that are linked to specific variants. It also unites the competing K-S and N-W relationships through the dissociation of perfect dislocations. Reasonably good agreement was observed between the predictions and the observations. Possible explanations for some of the discrepancies are also presented.

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