SRAS++ single crystal elasticity matrix measurement in polycrystalline materials

Many materials are polycrystalline, that is they are made up internally of lots of individual crystals of irregular shapes that have grown together. The class of polycrystalline materials includes nearly all metals and is therefore highly relevant to advanced engineering. The elasticity (three dimensional stiffness) of a individual crystals needs to be described by a six by six elasticity matrix, Cij rather than a single number. While Cij has 36 members, at most only 21 are independent and in most materials only around 1/2 dozen members are independent or non-zero. The elasticity of a material, Cij , is a fundamentally important in understanding not only the material's mechanical properties but other behaviours because it is directly influenced by the atomic arrangement inside the crystals.

It is surprising to discover that there is no simple way to measure Cij in polycrystalline materials. Until the technique developed in this proposal, SRAS++, was invented, the only practical way to do this was to prepare a single crystal of the material and then measure Cij using that. However, preparing a single crystal of many of these materials is extremely difficult, slow and expensive. Furthermore, if a crystal is specially grown it is not truly representative of the real, bulk, polycrystalline material because the preparation conditions will be very different from the bulk. However, if the crystal is isolated from the bulk (a very difficult task) this damages the original specimen making it a poor technique to use on valuable materials or as a way to assess real products.

In this proposal, we will develop a newly invented technique, SRAS++. This is based on a laser ultrasound imaging technique, SRAS, which uses surface acoustic waves to interrogate the crystals (grains) in the material. In SRAS++, measurements on many grains and at many different angles are combined together to allow a solution for common elasticity measurement, Cij , to be extracted. SRAS++ will be an extremely quick and easy measurement and it will learn from samples it has previously seen. For a new sample it will take a few minutes to measure Cij and for something that has been previously scanned, Cij will be able to be be measured in real time making it perfect to track and monitor this important measurement through processes that transform the materials properties, like heat treatments.

Because of it's unique capability SRAS++ will become an important measurement in materials science and aid the discovery and development of important new materials. It also has great potential in industry to monitor processes and assess the condition and state of components, especially safety critical parts. To maximise this impact We have partnered with an internationally leading team of materials scientists and advanced industries to deliver five challenging science themes to demonstrate the utility and potential of this new measurement technique. We will develop a dedicated instrument for SRAS++ which will be 10-100 times faster than the existing SRAS machines. We will increase the spatial resolution of this machine by exploiting new laser technology that has become available which will allow us to work on a wider variety of materials. This dedicated machine, along with a program of work to improve the SRAS++ solver, will allow us to push the already world leading sensitivity of the technique so that we can see smaller and smaller changes in the material proprieties which, in turn, will allow use to extract more science from the samples and monitor more processes in more detail.