Metrology for length-scale engineering of materials

Reducing the weight of materials used in industry and transport, whilst retaining or improving their strength, would reduce energy consumption and CO₂ emissions. Conventional understanding of material strength is built on the principle that a small and a large beam, made of the same material, will equally fail under the same stress. This assumption underlies Finite Element Analysis (FEA) models used in engineering design. However, in reality, a small beam would be stronger and this ‘size effect’, or ‘length-scale effect’, can change the strength of a material by up to a factor of ten. A material’s strength is also dependent on its temperature and microscopic composition, but a limited understanding existed on how these combine to make up overall strength.b FEA models were therefore unable to determine the true improvements that could be made to material performance with length-scale engineering. New design rules, based on validated data, would enable industries to exploit these size effects. Increasingly important as nanotechnology continues to provide new opportunities for producing smaller-sized materials with potentially major strength improvements.

The project developed new methodology and instrumentation to address these measurement challenges. Two new models for finite element code used in FEA were developed and validated through comparison with experimental results. The first combined size effects, such as indentation size, grain size and ‘dislocation work-hardening’, which is the strengthening of a material by plastic deformation. The second model simulated combined plasticity size effects.

To test material strength a new Micro-Electro-Mechanical System (MEMS) nano-indentation system was built, based on an Atomic Force Microscope (AFM). New probes for this were developed using single crystal diamond, demonstrating advantages over the traditional silicon styli used. The system was then experimentally validated, demonstrating an extended and traceable measurement range over conventional techniques.

A high-temperature indentation facility was constructed, capable of testing materials up to 900^oC in vacuo, and used to carry out experiments on copper at temperatures up to 500^oC.

New test methods were also developed to give information on the length-scale dependence of materials, including the use of spherical indentation, length stress-strain mapping and macro-scale indentation.

A comprehensive testing matrix was then carried out across the length-scales of a range of materials, including a large programme of measurements, modelling and analysis on CuCrZr alloys.

Project results have fed a new work item drafted for ISO 14577-part 7 to provide guidelines for instrumented indentation testing at elevated temperature. The new models, algorithms, software and data on material properties at the nano- and microscale, both at room and elevated temperatures, will shape a more unified understanding of size effects on material strength.

As well as significantly improving component lifetime and performance, engineering lighter, stronger and more wear-resistant materials for transport or industry would help reduce energy usage and CO₂ emissions.