The many facets of deformation mechanism mapping and the application to nanostructured materials
- PDF / 254,385 Bytes
- 8 Pages / 584.957 x 782.986 pts Page_size
- 56 Downloads / 164 Views
Terence G. Langdon Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089-14533; and Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, United Kingdom (Received 3 January 2013; accepted 19 February 2013)
Deformation mechanism maps are well established in the field of high temperature creep for materials having conventional coarse grain sizes but they are almost unknown within the field of nanostructured materials. This paper summarizes the background to deformation mechanism mapping, presents simplified examples that may be used to easily construct appropriate maps for any selected condition, demonstrates the potential extension of this approach to other areas such as creep fracture, and then considers the potential limitations associated with using the same approach to predict the deformation mechanisms in true nanostructured materials. Two representative deformation mechanism maps are shown for an ultrafine-grained alloy processed either by equal-channel angular pressing or by high-pressure torsion.
I. INTRODUCTION
The flow or creep of polycrystalline materials at elevated temperatures is now understood reasonably well. When a material is subjected to a constant stress or load that is below the fracture stress, the material gradually deforms with time until it ultimately fails. Generally, there is an initial instantaneous strain on application of the load and this is followed by a primary stage of creep where the creep rate decreases with increasing time, a steadystate region where the creep rate remains essentially constant and then a tertiary stage where the creep rate accelerates up to the point of failure. Often much of the flow occurs under steady-state conditions and then an important requirement, for any selected testing conditions, is to determine the rate-controlling flow mechanism. Extensive reviews are now available describing the nature of the flow processes occurring under creep conditions.1,2 Creep is a diffusion-controlled process occurring at temperatures above ;0.5 Tm, where Tm is the absolute melting temperature. Extensive experiments have established that the steady-state creep rate, e_ , may be expressed by a simple relationship of the form3,4
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.55 J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013
http://journals.cambridge.org
Downloaded: 09 Jan 2015
e_ ¼
ADGb b p rn kT d G
;
ð1Þ
where D is the appropriate diffusion constant [5 D0 exp(Q/RT), where D0 is the frequency factor, Q is the activation energy for the flow process, R is the gas constant, and T is the absolute temperature], G is the shear modulus, b is the Burgers vector, k is Boltzmann’s constant, d is the grain size, r is the flow stress, n and p are the exponents of the stress and the inverse grain size, respectively, and A is a dimensionless constant. It follows from inspection of Eq. (
Data Loading...
