Year

2012

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

Severe plastic deformation (SPD) as a technique to process ultrafine grained (UFG) materials has attracted significant interest over recent years. Equal channel angular pressing (ECAP) and accumulative roll bonding (ARB) are two of the most important SPD techniques. Despite that many studies have been conducted on both techniques, some research areas have still not been explored deeply, such as the influence of low pressing temperature on ECAP, the deformation behaviour of the same material in monotonic material sheets or laminated composite sheets during ARB and the evolution of point defects during SPD. The present work investigates the microstructure, mechanical properties, texture, and vacancy-type defects evolution in detail in UFG aluminium alloys processed by ECAP and ARB.

The UFG AA1050 billets were processed by ECAP at both room temperature (RTECAP) and cryogenic temperature (CT-ECAP) to study the effect of cryogenic temperature on the properties of the materials. Substantial grain refinement was achieved after both RT-ECAP and CT-ECAP deformation. The average grain size after CT-ECAP is comparable but slightly smaller than after RT-ECAP and the CTECAP processed samples have a higher hardness and tensile strength than the RTECAP processed samples deformed to the same strain. A high fraction of vacancytype defects were detected in both RT-ECAP and CT-ECAP deformed samples by positron annihilation lifetime spectroscopy (PALS). The increased hardness of the CT-ECAP processed samples can be attributed to the existence of bulk mono- and di-vacancies in the samples. The textures developed after CT-ECAP are completely different from those after RT-ECAP. Three fibres f1C, f2C and γC can be used to characterise the texture components developed for CT-ECAP processed samples.

The UFG AA1050 sheets were processed by ARB. The properties of AA1050 after ECAP and ARB deformation to the same level of strain were compared. The grain size of the AA1050 after ARB is smaller than those after RT- and CT-ECAP to the same strain, while the hardness and tensile strength are higher, indicating that the ARB process is more efficient in grain refinement and strengthening compared with ECAP. The positron lifetime of the ARB processed AA1050 is smaller than those for the RT- and CT-ECAP processed AA1050 due to the elevated deformation temperature during ARB.

The UFG AA1050 sheets, AA6061 sheets and AA1050/AA6061 composite sheets were processed by ARB. The properties of AA1050 and AA6061 deformed in the monotonic material sheets and the composite sheets were compared and it has been found that the grain size and the hardness values of the AA1050 and AA6061 layers in the composite sheets are similar to those in the corresponding monotonic sheets. The tensile strengths of the composite sheets agree well with the rule of mixture. The positron lifetimes of the ARB processed AA1050/AA6061 composites are also in between those of the ARB processed AA1050 and AA6061 sheets.

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