Abstract
For aluminium alloys, precipitation strengthening is controlled by age-hardening heat treatments, including solution treatment, quenching, and ageing. In terms of technological applications, quenching is considered a critical step, because detrimental quench-induced precipitation must be avoided to exploit the full age-hardening potential of the alloy. The alloy therefore needs to be quenched faster than a critical cooling rate, but slow enough to avoid undesired distortion and residual stresses. These contrary requirements for quenching can only be aligned based on detailed knowledge of the kinetics of quench-induced precipitation. Until the beginning of the 21st century, the kinetics of relevant solid-solid phase transformations in aluminium alloys could only be estimated by ex-situ testing of different properties. Over the past ten years, significant progress has been achieved in this field of materials science, enabled by the development of highly sensitive differential scanning calorimetry (DSC) techniques. This review presents a comprehensive report on the solid-solid phase transformation kinetics in Al alloys covering precipitation and dissolution reactions during heating from different initial states, dissolution during solution annealing and to a vast extent quench-induced precipitation during continuous cooling over a dynamic cooling rate range of ten orders of magnitude. The kinetic analyses are complemented by sophisticated micro- and nano-structural analyses and continuous cooling precipitation (CCP) diagrams are derived. The measurement of enthalpies released by quench-induced precipitation as a function of the cooling rate also enables predictions of the quench sensitivities of Al alloys using physically-based models. Various alloys are compared, and general aspects of quench-induced precipitation in Al alloys are derived.
Highlights
The microstructure at the end of the solution treatment is a result of the structure after casting and the coarsening caused by the solution treatment
Fciogoulrieng84r.at(eas) fCroomntianbuoouust 3D×SC10c−o4 otolin3gKc/usrcvaens nofowIncboenaelp7p1l8ie;d(bin) haarredlniaebssleparnofialleysainsdmsepthecoifidcusing pdrierceipctit,aitniosnithueaDt SdCepexnpdienrgimonenthtse.cBoaosliendg orantea[1ty22p]i.cal temperature interval for the cooling of light metal alloys from solution treatment, this is equivalent to cooling durations ranging from several weeks up to a few minutes
Based on a typical temperature interval for the cooling of light metal alloys from solution treatment, this is equivalent to cooling durations ranging from several weeks up to a few minutes
Summary
Aluminium alloys are the second most important metallic materials. In 2014, the global production of steel (about 1670 Mt) amounted to about 90% of global metal production [1,2]. The metrological methods required to achieve this dynamic range have been developed, and currently allow for quantitative evaluation covering the whole range of heating and cooling rates of technological and physical relevance The consideration of this very broad dynamic width is associated with a large range of associated micro- and nano-structural features, i.e., the dimensions of quench-induced precipitates can range from several tens of μm to just a few nm. In metals which allow precipitation hardening, the relevant solid-solid phase transformations typically only amount to a small percentage of the total atomic fraction (excluding Ni-based super-alloys), and their detection requires very sensitive measurement setups. Since only precipitation will take place during cooling, DSC still is the most promising method for comprehensive analysis of the kinetic behaviour of quench-induced precipitation
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