UBC Theses and Dissertations
Study of concurrent recovery and precipitation on the mechanical behaviour of Al-Mg alloys with small additions of Cu Medrano, Sebastian
Aluminum-magnesium alloys are commonly used as wrought products in the automotive industry. Cold forming of such alloys leads to strengthening by work hardening but some of this strength can be lost by exposure to elevated temperatures leading to recovery. Such softening by recovery occurs when car body panels are subjected to the industrial paint bake cycle (160-200 C for 30 min). It has been previously shown that small (<< 1 wt%) additions of Cu to Al-Mg alloys can suppress softening under simulated paint bake conditions owing to the formation of small solute clusters. Two mechanism(s) could control this phenomenon; precipitation hardening and precipitation induced suppression of recovery. Through this thesis, it was shown that precipitation hardening due to solute clusters / GBP zones is the dominant effect, it being significant enough to counterbalance strength loss due to recovery. Using a combination of techniques, with a particular emphasis on atom probe tomography, the precipitation hardening in solution treated (undeformed) samples could be quantitatively related to clusters / GBP zones. A quantitative evaluation of the effect of pre-deformation on the formation of clusters / GPB zones was also obtained showing that dislocations negatively affect the strengthening owing to the effect of rapid vacancy loss to dislocations. This conclusion was reached thanks to a technique developed as part of this thesis that allows one to quantitatively assess, separately, the effects of precipitation hardening and forest hardening from a detailed analysis of the work hardening response upon yielding.
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