Small stretches equal big strength gains?

Materials World magazine
,
1 Dec 2015

A new paper claims that metals can be strengthened by stretching them at the nanoscale. Simon Frost reports. 

Can repeated stretching at the nanoscale prevent metal fatigue? According to a new paper published in Proceedings of the National Academy of Sciences by researchers from USA universities MIT and Carnegie Mellon, and Xi’an Jiaotong University, China, it can. 

While it is well known that repeated bending of metals can cause them to weaken and eventually break, the researchers found that repeated stretching of nanoscale metals can eliminate defects in their crystalline structure, and therefore strengthen
the material. 

In Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals, the team uses the term ‘cyclic healing’ to describe a new method for strengthening metals, through reductions in dislocation density. The paper states, ‘Although monotonic loading can reduce or even eliminate dislocations in submicroscale single crystals, such “mechanical healing” causes severe plastic deformation and significant shape changes.’ 

The researchers examined small, single-crystal pieces of aluminium, aiming to reduce or eliminate defects such as dislocations through repeated, small-amplitude, cyclic deformation, rather than heat-based annealing. They found that repeated small displacements tend to dislodge the dislocations from their pinned location within the crystal. These dislocations are attracted to the surface due to the crystal’s high surface-to-volume ratio, so the energy stored in the defects is reduced, and the metal’s strength increased. 

Subra Suresh, Carnegie Mellon’s President and Professor Emeritus of Materials Science and Engineering, said, ‘This work demonstrates how cyclic deformation, under certain controlled conditions, can lead to the removal of defects from crystals of small volume. In addition to pointing out how these mechanisms of cyclic deformation can be very different from those seen in larger-volume materials, this work also offers new avenues for eliminating defects from crystals without the need for thermal treatment or shape change.’

‘The article appears to hold out the prospect of removing the need for heating, which could damage delicate components and structures’, said Mike Cox, Chair of the IOM3 Structure and Property of Materials Committee. ‘The concept of matrix strengthening in nano- to micrometre cross-sections by dislocation starvation has been around since 2006, although the cyclic tensile stress (low amplitude) is new as a means of removing dislocations,’ he added, citing the 2006 paper Nanoscale gold pillars strengthened through dislocation starvation, by Stanford University, USA researchers, Greer and Nix, which was published in Physical Review B.  

The MIT researchers claim that the process could help in the production of strong parts for nanotechnology applications, such as mechanical nanosensors, nanoelectromechanical systems and nanorobots. 

Cox, however, has reservations about the applicability of the technique. ‘The example is in pure aluminium, which is a metal with high stacking fault energy (250mJ/m2). This means dislocations are not confined to specific planes and can therefore cross-slip to overcome obstacles, thereby allowing the dislocations greater mobility. Would this be as successful in, for example, copper, which is also an FCC metal but whose stacking fault energy is low (90mJ/m2), making cross slip more difficult?’ 

‘It seems the technique is unlikely to be applicable to poly-crystalline materials, owing to the presence of grain boundaries which will hinder the mobility of the dislocations,’ he adds. ‘Alloys with substitutional or interstitial solutes will also hinder the mobility of dislocations.’

Cox considers precipitation strengthening of alloys through this technique unlikely, and also questions its applicability in multi-phase structures and situations where heterogeneous plastic flow needs to be investigated. 

‘The work contributes to the science of dislocation in metallic materials, but the applicability of this technique to other materials and structures needs to be demonstrated. In the early stages of any technique, there are obviously a great many questions that the science community will ask.’ 

To view the paper in full, visit bit.ly/1PxCtCd