On the limits of material grain size reduction – Small is not always beautiful

A major challenge in Mechanical Engineering is the development of high-performance structural materials with superior mechanical properties. High strength is highly desirable, since it enables elegant light-weight design, conserves natural resources and may even save costs. High ductility and high toughness are just as desirable, since they allow plastic forming operations, and they extremely improve mechanical safety of components. Unfortunately, it is difficult to obtain both high strength and high toughness simultaneously. When the focus is on high strength, in most cases a reduction in toughness must be accepted, and vice-versa.

One of the few methods to achieve both high strength and high toughness at the same time is grain size reduction. It is not surprising, that the grain size has been reduced more and more, leading to the fine-grained, ultrafine-grained and nanocrystalline materials. However, below certain grain sizes (typically around 1 µm), the plastic deformation behavior of some metallic materials may change dramatically during tensile straining. Immediately after first yielding, plastic deformation concentrates in a localized region, leading to a neck there and final fracture. The rest of the specimen stays in the elastic state; uniform plastic deformation no longer occurs. This is the knock-out criterion for most tensile forming operations, and the mechanical safety of components made from these materials is very low. Although the complete loss of uniform plastic deformation has been observed long ago, no profound explanation has yet been given.

Recently, Professor Rainer Schwab at the University of Applied Sciences from the Faculty of Mechanical Engineering and Mechatronics in Karlsruhe/Germany examined the complete loss of uniform plastic deformation in structural materials. By combining experimental evidence, micro- and macrostructural considerations, a new analytical model, and extensive finite element simulations, he can consistently explain this form of plastic instability. The research work is currently published in International Journal of Plasticity.

Starting with a survey of experimental evidence, the author clearly finds that the yield point phenomenon is the common cause for the loss of uniform plastic deformation. In the next step he uses his recently published new model of the yield point phenomenon, showing that the lower yield strength is not an inherent material property, but a function of a real upper yield strength, a real lower yield strength and a real flow curve. Further on, a novel empirical strain hardening law combined with experimental evidence and micromechanical considerations serves as a basis to set up an analytical macromechanical model. Finally, he uses the macromechanical model data as input data for extensive finite element calculations.

Professor Schwab points out, that, “when lowering the grain size, the real upper yield strength increases more than the observed lower yield strength. This leads to a point, where the observed lower yield strength exceeds the tensile strength. Then the material becomes plastically instable in tensile straining, and loses its ability to deform uniformly. It is fascinating for me to see, that experiments, macro- and micromechanical considerations, analytical calculations and finite element simulations fit together well and confirm the model”.

In summary, the research work of Professor Schwab successfully presents a fundamental understanding of the complete loss of uniform plastic deformation in structural materials. The study provides vital information that will not only advance future research work but also the design and development of various high-performance materials for numerous applications.