Mao Z N, An X H, Liao X Z, et al. Opposite grain size dependence of strain rate sensitivity of copper at low vs high strain rates[J]. Materials Science and Engineering: A, 2018.

Dear Zhang Weiliang,

There are many factors in the game that affects the strain rate sensitivity of deforming metals. The microstructure plays a great role. Exactly this question in the opposite direction was studied in my Ph.D. thesis, strain rate change tests during dynamic recrystallization, and is available as an open source software here on ResearchGate.

There exist an intricate interplay between the total grain boundary area and deformation rate feeding system by energy that affects strain rate sensitivity.

In my case, it was studied on dynamic recrystallization at elevated temperatures where was switched constant state rate from value A to B and back.

The structural change of the microstructure is delayed after the abrupt changes of strain rate. I define it as structural stability of the material. The material keeps miry of the previous deformation and sometimes resist to adjust to a new strain rate.

In your case, you do not speak about DRX, it would be easier dependence as there is not present release of dislocation densities via recrystallization.

See strain rate change tests in the following references for a better understanding. Feel free to ask more.

Video: https://www.researchgate.net/publication/317013011_Self-Organization_Video_Sequence_Depicting_Numerical_Experiments_with_Cellular_Automaton_Model_of_Dynamic_Recrystallization_with_source-code_link?_sg=8n0GJ32cv8WxXoimrezvZRp187WdngRfiGXuzqklM2jlVJBRK_rFnTb54yYh7on0rOrv5rgRAZa2754NlfOWw1QI7e8SxiHWZkdcFP7k.I-eXLbrYmoqZK2PMTEPsf2_zpzx53tZJb-TJFlfq1LqDHgAbLuFfCRuW_988zzqVNsmK5YWNhFCpRbQIgYamzA

Software: https://www.researchgate.net/publication/316989956_Cellular_Automaton_Simulation_of_Dynamic_Recrystallization_Introduction_into_Self-Organization_and_Emergence?_sg=AdDC0TXldcq50yPEWu4EZlpCsOo6bAmwsVIcazrLmZVr9Szy_6DXcdWFv933PWyMGe3hEnpGbqZOy69QSLh-WX9jHaXxuNn4arusQFU1.pV5CpG_XAqU2VwOUHbVIrQ8UyenDlqu-UrhlwrkYT-oKWlVqxXK5eUdg5XCmkpHLn39mM8nrzowWMOTJs-KpPg

Ph.D. Thesis: https://www.researchgate.net/publication/311733496_Simulation_of_Dynamic_Recrystallization_by_Cellular_Automata?_sg=OYi-ioo2RfXWCZeNU7kRZlNzmUZyX24puJD8qp6AsLTLAbY6HSeblZ4GOMdHjcW5f55hxLgWAPBxldLfwzu8jCf77LIxX2u5mI81XWjC.cQ86WZ7TIH5U7oCpx0jaN4CNil7cK3joLkdKh9Rvw-d3HlFlz-DbZQ-kRxxyGynSy5FL7466N4x24tqJXMiiAA

The question is really wide. Crystal structure, phase content, acting alloying mechanisms, grain size, grain boundary amount, structure and chemical composition, etc., etc. - all these factors (apart and together) may significantly affect strain rate sensitivity of metallic material. Wide variety of atomistic models have been proposed to explain these numerous and different effects. Usually, researchers try to separate factors in order to exclude all of them, except for the studied one.

For example, if you want to study temperature dependence of strain rate sensitivity in Aluminum alloy, you need to keep chemical composition, phase content, grain size and initial heat treatment parameters unchangeable and then to apply different loading rates in the plastic zone at various temperatures. After the experiment, you need to examine the occurred microstructural changes (grain size - optical microscopy and SEM), fine structure (TEM), phase content (SEM, XRD) and try to connect them with the measured second derivative of the strain. Probably, you will be available to explain the observed phenomena. Additionally, study of the strain rate vs time at various conditions may permit you to calculate activation energy of elementary dislocation jump through plastic flow (if this mechanism is ultimate in your material).

I recommend to study basic handbooks on theory of plastic deformation in metals (including dislocation theory) as well as perform a literature survey on alternative deformation mechanisms (twins, APB, etc.).

Dislocation pile ups and pinning of dislocation movements by precipitates raise strengths as strain rate increases in metal deformation.

Hall-Petch equation applies in strength -grain size relationship and this can be superimposed on strain-rates effects. Obviously, the smaller grains will have higher strengths with increasing strain rate because more grain boundries etc.

I think it is important to firstly define what you understand under 'high strain rates'. Depending on your 'reference strain rate' you could be in the superplasticity domain, the structural mechanical domain, the crash domain, the impact domain, the shock domain or even the hyperdynamic domain (and I am not even covering all specific domains and subdomains yet). All these domains are linked to specific deformation mechanisms that sometimes are unique to one or only a few of these domains (e.g. grain boundary sliding). Depending on the active deformation mechanisms the microstructural features that have an influence on the deformation will also change.