Introduction

The international symposium is to be held in Tokyo under the auspices of Japan Society of Promotion of Science (JSPS) as a commemorative event of 50-year anniversary of the 133rd Committee of University-Industry Cooperation of JSPS. The organizers solicit contributions by active researchers in this field all over the world, so that the symposium be quite fruitful in serving for the further development of science and technology of fine-grained materials.

Since E. O. Hall (1951) and N. J. Petch (1953) proposed the following equation

σ_y = σ₀ + kd^‒1/2

for the grain-size (d) dependence of the yield strength (σ_y) of steels, the equation has been shown to be valid for a variety of crystalline solids, and this equation is known as the Hall-Petch law or the Hall-Petch relation. The same equation has been shown to hold for the flow stress and fracture strength of metallic materials. Therefore, grain refining has been used as a powerful tool of strengthening and toughening of crystalline materials without changing the composition. Since 1990s, however, it has been found that when grain size decreases below, say, 20 nm, the Hall-Petch law becomes no more valid and the strength even decreases with decreasing the grain size, now termed as the inverse Hall-Petch behavior.
Nearly 60 years have passed since the Hall-Petch relation was established. At this stage, it seems quite meaningful to review the present state of the art of the Hall-Petch and the inverse Hall-Petch relations from experimental and theoretical viewpoints to understand the role of grain boundaries in the mechanical properties of polycrystalline materials and to carry out the optimum design of structural materials.

・Hall-Petch relation
・Inverse Hall-Petch relation
・Superplasticity
・Strength of fine-grained steels
・Strength of fine-grained non-ferrous metals
・Strength of fine-grained ceramics
・Grain boundary segregation and its effects on strength

---