Critical Strain for Dynamic Recrystallisation. The Particular Case of Steels

The knowledge of the flow behavior of metallic alloys subjected to hot forming operations has particular interest for metallurgists in the practice of industrial forming processes involving high temperatures (e.g., rolling, forging, and/or extrusion operations). Dynamic recrystallisation (DRX) occurs during high temperature forming over a wide range of metals and alloys, and it is known to be a powerful tool that can be used to control the microstructure and mechanical properties. Therefore, it is important to know, particularly in low stacking fault energy materials, the precise time at which DRX is available to act. Under a constant strain rate condition, and for a given temperature, such a time is defined as a critical strain (εc). Unfortunately, this critical value is not always directly measurable on the flow curve; as a result, different methods have been developed to derive it. Focused on carbon and microalloyed steels subjected to laboratory-scale testing, in the present work, the state of art on the critical strain for the initiation of DRX is reviewed and summarized. A review of the different methods and expressions for assessing the critical strain is also included. The collected data are well suited to feeding constitutive models and computational codes.

Local Accommodation Processes of Superplastic Grain Boundary Sliding: Their Direct Observation in Two-Dimensional Grain Boundary Sliding

Accommodation processes are crucial for grain boundary sliding in superplasticity though few have been reported on their positive experimental evidences. The present study achieved two-dimensional grain boundary sliding in ODS ferritic steel which had elongated and aligned grain structure and got direct observations of accommodation processes without the surface effect of floating grains: 1) In Region II, diffusional accommodation was confirmed through observing the change in marking-line spacing, which indicates volume inflow and outflow at grain boundaries. 2) Between Regions II and III, dislocation accommodation inside of the mantle region, as proposed by Gifkins, was confirmed through observing curves of marking lines near grain boundaries. 3) In Region III, dislocation accommodation inside of the core region, as proposed by Ball and Hutchison, was confirmed through observing slip bands and sub-boundaries passing through a grain. It is, then, derived that superplasticity relies not on a single mechanism but on several diffusional and dislocation accommodations contributing depending on strain rate condition.

Fatigue Behavior of A356 Cast Aluminum Alloy Microstructurally Modified by Friction Stir Processing under Low Strain Rate Condition

The fatigue behavior of cast aluminum alloy, A356-T6, microstructurally modified by the friction stir processing (FSP) was investigated. The FSP conditions were set to be the tool rotational speed of 500 rpm and traveling speed of 200 mm/min, in which the strain rate was relatively low. Plane bending fatigue tests have been performed using the as-cast and friction stir processed (FSPed) specimens. Fatigue strengths in the finite life region and the fatigue limit of the FSPed specimens were highly improved compared with the as-cast ones resulting from the elimination of casting defects by the FSP. However, the crack growth rates of the FSPed specimens were faster than those of the as-cast ones due to the softening of the material by heat input during the FSP. The effects of FSP with low stain rate were discussed based on the microstructural consideration.