Strain reversal during HPT on the evolution of hardness and microstructure has been reported by a few researchers who have proved significant differences in microstructure and mechanical properties [5, 16]. It has been reported that disc subjected to strain reversal show limited increase in the hardness at the centre with increasing strain values, which is justified upon microstructure observation at the centre where grain refinement is limited [8]. A study on strain path change during cold rolling has shown differences in behaviour of microstructure and texture in Al and Al-Mg alloy [17]. There has been no studies focussing strain path change in HPT for Al-Mg alloys. The present study is determined to clarify these issues by studying microstructure and microhardness evolution during strain reversal of Aluminium-Magnesium (Al-2.5%Mg) alloy during HPT. It is envisaged that strain reversal would greatly affect the microstructure-property evolution in Al-2.5%Mg having lower propensity for dynamic recovery which essentially remains the main motivation for the present study.
Experimental
Al-2.5%Mg alloy was used for the present study. The as-received Al-2.5%Mg block (160mm (length) × 60mm (width) × 10mm (thickness)) was cold rolled to ~80% reduction in thickness and annealed in air furnace at 673K for one hour. The above material was used as the starting material for further processing. Disks with the diameter of 10mm were then cut from the sheet using EDM wire cut equipment. A total of 10 disks were cut for further HPT processing. These disks having 2 mm starting thickness were manually grinded to ~1.5mm thickness using SiC grit papers with grit size of 500, 1000 and 1200 respectively. The disks were then designated according to the strain value or the number of rotations as per the given chart.
The disks were then deformed by HPT to the desired strain levels as shown in Table 1.
Table 1: Designation of the samples according to the strain