Deformation mechanisms and load distribution in multi-phase engineering materials

Doctoral thesis, 2021

While the transition to carbon neutral technologies is still in progress, it is vital to reduce the environmental impact of existing processes. The efficiency of combustion processes for e.g. power generation and aviation can be greatly improved by increasing the operating temperature. This, however, requires development of new and improved materials with increased temperature capability. Similarly, materials which enable e.g. storage of hydrogen or liquid natural gas at cryogenic temperatures can contribute to the above transition. Such high performance engineering materials are usually very complex, with many alloying elements and multiple phases. During deformation the behaviour of the phases, and the grains with different orientations within each phase, is a result of elastic and plastic interactions. Quantifying how the stresses and strains are redistributed within and among the phases is essential for the development of quantitative models capable of accurately predicting the macroscopic mechanical response from the single crystal properties.

This thesis explores the use of in-situ neutron diffraction for investigating load partitioning and deformation mechanisms in two different advanced multi-phase materials, a Ni-based superalloy and a eutectic high entropy alloy, across a wide temperature range (from 20 to 1000 K). For the superalloy, the main findings are: (i) the effect of particle size on the deformation mechanisms and load partitioning was consistent across all temperatures; (ii) plastic deformation of the strengthening phase at high stresses occurred at cryogenic temperatures, which has not been previously reported; and (iii) a strong orientation and phase dependence of the damage evolution during high-temperature deformation was observed. In the eutectic high entropy alloy transitions in the deformation mechanisms of the constituent phases were found to occur with increasing temperature, which lead to a new proposed alloy design strategy for optimising the high temperature properties. Further, the role of the phases is reversed at higher temperatures, i.e. the soft phase at lower temperature becomes the reinforcing phase when the temperature increases. The reported results will have a large impact on the development of accurate multi-scale models for property prediction, as well as development and optimization of complex materials which contribute to a sustainable society.