Mechanism of Deformation by Intervariant Boundary Motion in Shape-Memory Materials
- PDF / 202,430 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 66 Downloads / 200 Views
Mechanism of Deformation by Intervariant Boundary Motion in Shape-Memory Materials Robert C. Pond and Steven Celotto Materials Science and Engineering, Department of Engineering, University of Liverpool, Brownlow Hill, Liverpool, L69 3GH, UK. ABSTRACT The shape-memory effect relies on deformation in the martensitic state by motion of intervariant boundaries in response to an applied stress. A mechanism for this process in terms of interfacial defect generation, motion and interaction is proposed. This mechanism is conservative and is expected to operate with modest applied stresses and without thermal activation. Experimental observations of intervariant boundaries are discussed and atomistic simulations of the mechanism in such boundaries in hexagonal metals are presented. INTRODUCTION Shape-memory materials are widely used as sensors and actuators [1]. Their thermomechanical properties arise from distinctive microstructures that can be modified by changes of temperature or applied stress. For example, as illustrated in figure 1, shape memory is manifested by cooling the material to temperatures below Mf and subsequently deforming to the required shape. On heating to a temperature above Af (indicated as dotted lines in figure 1(b)), the above mentioned plastic deformation can be completely removed when the material transforms back to austenite state, and thus returning to its original shape. In the martensitic state the material can be worked easily and the deformation mechanism is the motion of boundaries which separate crystallographically equivalent variants of martensite, hereafter referred to as intervariant boundaries. In NiTi for example, the stress required to make these boundaries move is about 50 MPa and is not sensitive to temperature at temperatures below Mf [2]. The applied stress causes favourably oriented variants to grow and plastic strains of up to about 8% are recoverable upon reheating to above Af. It is evident that the motion of intervariant boundaries in response to modest applied stresses is a key process in materials with good shape-memory properties. The objective of the present work is to discuss the structure of intervariant boundaries and the stress-activated mechanism that couples boundary migration and shear deformation. To illustrate structural principles, experimental observations of intervariant boundaries in hexagonal metals will be described, along with atomistic simulations of coupled migration/deformation processes in such materials. Some aspects of the theory of interfacial defects will be reviewed, particularly those concerned with diffusive fluxes arising during defect mechanisms. This is appropriate since deformation in shape-memory materials occurs at rates consistent with diffusionless mechanisms. In addition, feasible defect mechanisms should be activated at low stresses and should not need thermal activation.
Y11.1.1
(a) 100%
(iv)
0%
fM
fA 0%
100% MF MS AS AF Temperature
m Te
(iii)
u at r pe
re
(ii)
(b)
σ
(c)
AF (i)
(iv)
AS
(iii)
σ
MS
ε
(ii)
MF
Data Loading...
