Fundamental understanding of the classification of microstructural evolution during large strain, high Z deformation is essential in order to produce bulk nanostructured materials.

Our group have previously examined the large strain high Z deformation leading to the formation of ultrafine ferrite grains and reported that (a) there exists a critical strain for the formation of ultrafine ferrite grains, (b) their volume fraction increases with an increase in strain and (c) the size of the ultrafine ferrite is determined by the Z parameter and is independent of strain.

Here, we report the dependence of the critical strain on the combined effect of strain rate and temperature on the formation of ultrafine grains.

d=100Z^-0.16 (μm)

Extensive microstructural and micro-textural analysis was conducted on specimens deformed in a wide range of Z parameter subjected to strains in the range of 1-4. Based on the boundary maps, the grains were categorized as high angled ones (with misorientation greater than 15 degrees) and low angled ones (with misorientation less than 15 degrees).

The outcome of this work is a processing map on the axes of strain and Z parameter with the microstructural manifestations clearly marked on the map. The microstructures can be essentially classified in to three clear regions as (a) elongated grains representing work hardened microstructure, (b) newly generated ultrafine grains along with elongated grains, and (c) newly generated ultrafine grains. It was also noted that the strain for obtaining newly developed ultrafine grains increases as the Z parameter increases. In other words, higher strain should be imposed for obtaining ultrafine grains at lower temperatures or higher strain rates. The mechanisms of formation of ultrafine grains are grain subdivision and dynamic recovery in high Z regions and they are grain subdivision and dynamic recrystallization in low Z regions [1]. Following correlations are obtained.

Critical strain for 99% recrystallization ε=0.15Z^0.1

Critical strain for starting recrystallization ε=0.007Z^0.15

Z=exp(Q/RT)

Q=240Kj/mol

This kind of processing map [2] is very useful for the optimization of the processing parameters (such as strain, strain rate and temperature) for obtaining desired ultrafine microstructures in large strain deformed components.

References:

1. S.V.S. Narayana Murty, S. Torizuka, K. Nagai, N. Koseki and Y. Kogo: “Classification of microstructural evolution during large strain high Z deformation of a 0.15 carbon steel”, Scr. Mater. 52 (2005) 713-718.
   
   
2. S.V.S. Narayana Murty, S. Torizuka and K. Nagai, “Microstructural evolution during simple, heavy warm compression of a low carbon steel: Development of a processing map, Materials Science and Engineering A (in press, available on online 29 Sept.2005)