Quasi-static and Impact Nanoindentation to Study Deformation Mechanisms in Metals

Nanoindentation is an attractive technique, allowing study of local mechanical characteristics in a versatile and cost-effective manner. The two MML indenters in the Gordon Laboratory represent the current state of the art. Hot and cold stages have been developed, allowing operation at up to ~800 C and down to -170 C. Impact indentation can be carried out so as to generate (transient) local strain rates ranging up to about 10^4 s^-1. One of the two systems can be operated in vacuum or under controlled atmosphere, greatly reducing the incidence of specimen oxidation and diamond tip erosion at high temperature, and also eliminating condensation at low temperature. Furthermore, a suite of FEM modelling routines has been developed, allowing quasi-static material constitutive relations, and residual stress levels in particular specimens, to be inferred from nanoindentation data, with good degrees of reliability and accuracy.

The proposed work will involve development of these capabilities, particularly relating to impact mode indentation. This will require enhancement of the current modelling suite, and comprehensive characterisation of the dynamics of indenter motion. This should allow the extraction of strain rate sensitivity information, over ranges of strain rate for which conventional testing presents severe difficulties. These techniques will be applied to "reference" materials, such as pure copper, and also to alloys of particular interest. These include (depleted) uranium, which will be supplied by AWE. Such alloys can exhibit pronounced superelastic deformation at relatively low strains and methodology will be developed for the extraction of parameters characterising such behaviour. The effect of ageing on these alloys will also be investigated, which will be facilitated by the capacity for heat treatments to be carried out in situ on the indenter stage (inside the vacuum chamber). AWE will fund a PhD studentship, which will be oriented particularly towards these alloys and the role of superelastic deformation in their overall mechanical behaviour.

Beneficiaries
1) Commercial firms supplying nanoindenters. MicroMaterials Limited (MML) is the UK market leader. Since the work will be carried out on MML machines (albeit ones that have been modified and enhanced since their purchase), transfer of scientific and technological information will be particularly effective.
2) Organizations with an interest in expanding the range of information obtainable via nanoindentation. There are many such bodies. The development of methodology for extraction of material response data at high strain rate, and over a wide range of temperature, will be of particular interest. AWE has recognized that such testing could play an important role for them.
3) Organisations with an interest in new information about the nanoindentation response of certain types of materials, including those in which martensitic transformations can be stimulated by imposed strain (and are hence likely to exhibit shape memory behaviour). Correlations will be established with microstructural development in certain such alloys, and the effect of ageing at elevated temperature. AWE will supply the material and fund the associated PhD studentship. They expect to derive substantial benefit. Furthermore, there are many other (industrial and research-oriented) organizations in the UK likely to benefit from improved understanding of the behaviour of such alloys, and from insights arising from these novel testing procedures.

Dissemination, Impacts and Timescales
Publication in the open literature, and a project website, will be key dissemination mechanisms. There will be close contact with the partners, fostered by quarterly meetings and by periods spent in Cambridge by Giles Aldrich-Smith (AWE) and several members of staff from MML. A nanoindentation Symposium (CAMTEC III) will be held in Cambridge (March 2014), following two earlier events (see www.msm.cam.ac.uk/mmc/index.php/events/camtec-ii). As previously, financial support will come largely from commercial organizations involved in supply of nanoindenters.
Impact will partly be in the form of wider and more varied usage of nanoindentation, with expansion in possible ranges of strain rate and temperature. Improved methodologies for extracting material property data, including close integration of FEM modeling with material response and loading system characteristics, will also promote wider usage. Such advances will be particularly relevant to suppliers of nanoindenters, including MML, bringing economic benefits to the UK. Since the work is relevant to MML's current marketing strategy, these benefits could materialise within a short time scale (~3-5 years). Benefits to AWE, and other organisations involved in testing and use of advanced alloys, will take the form of cost savings and improved versatility arising from future testing strategies, in which nanoindentation is likely to figure strongly. This may well lead to reductions in certification time for new alloys. Benefits will also flow from specific property data generated within the project. These benefits are also likely to materialise within the short to medium term.