Quench-induced precipitates in Al–Si alloys: Calorimetric determination of solute content and characterisation of microstructure

Abstract

The present study introduces an experimental approach to investigate mechanical properties of well-defined nonequilibrium states of Al-Si alloys during cooling from solution annealing. The precipitation behaviour of binary Al-Si alloys during the cooling process has been investigated in a wide cooling rate range (2 K/s-0.0001 K/s) with differential scanning calorimetry (DSC). To access the low cooling rate range close to equilibrium an indirect DSC measurement method is introduced. Based on the enthalpy change measured by DSC a physically-based model for the calculation of remaining solute Si amount as function of temperature and cooling rate is presented.Microstructural analyses via light optical microscopy, scanning electron microscopy, atom probe tomography 50 and X-ray diffraction have been performed to evaluate the introduced model and for information on cooling rate dependent precipitate formation. It was found that quench-induced particles of different morphology are formed during cooling. Thermomechanical analyses on clearly distinct undercooled Al-Si states show that flow stress during cooling is dependent on temperature as well as cooling rate. The mechanical behaviour is therefore influenced by solute Si content and quench-induced precipitates.

Influence of Solution-Annealing Parameters on the Continuous Cooling Precipitation of Aluminum Alloy 6082

“…An increase in the heating rate shifts this dissolution completion to higher temperatures. As derived from findings in Schumacher et al [20], it can be expected that the specific solvus temperature will be reduced by up to a few tens of Kelvin at even lower heating rates or under isothermal solution-annealing conditions. One solution-annealing temperature was thus chosen as 560 • C, which, according to the heating experiments, should be above the equilibrium solvus temperature in any case.…”

Review of the Quench Sensitivity of Aluminium Alloys: Analysis of the Kinetics and Nature of Quench-Induced Precipitation

“…Aside from the determination of the critical cooling rates, a further aspect can only be understood if almost the full range of cooling rates is considered. As discussed later, it has been shown that for any age-hardening alloy, quench-induced precipitation during slow cooling predominantly occurs at alloy-specific high temperatures (compare e.g., [117,121,133]). At faster rates, the high-temperature reactions will be suppressed to a certain extent, and maybe even suppressed completely, and quench-induced precipitation will then probably take place via a medium-or low-temperature reaction [119,133].…”

“…The lower critical cooling rate defines the fastest cooling rate at which all the alloying elements (compared to the equilibrium solubility) precipitate during cooling from solution annealing, yielding enthalpy changes on a saturation level. For an Al-0.72Si alloy, this lower critical cooling rate is about 3 × 10 −5 K/s [117,132]. To identify both critical cooling rates, they must be experimentally exceeded by at least one order of magnitude in terms of both slower and faster cooling, which requires a dynamic range of about 10 −5 K/s to 10 3 K/s.…”

“…If properly conducted, DSC allows for quantitative evaluation of the enthalpy changes related to precipitation. Since these enthalpy changes are directly proportional to the atomic fraction transformed, analysis of the volume fraction of precipitates is possible [116][117][118][119].…”

Correlation between quenching rate, mechanical properties and microstructure in thick sections of Al Mg Si( Cu) alloys

“…The critical cooling rate (v crit ) of such alloys is known to be about 10°C.s −1 and it there a general consensus that a higher cooling rate would have no signicant inuence while a lower would lead to poorer mechanical properties (Davis and Committee [18], Totten and MacKenzie [19]). However, recent investigations (Schumacher et al [20], Dutta and Allen [21]) using Dierential Scanning Calorimetry (DSC) and earlier experiments (Ryum et al [2]) tend to contests those assumptions and suggests v crit would not be a plain boundary. They suggest the existence of two borders: a Lower Critical Cooling Rate (LCCR) under which every element in solution would precipitate and an Upper Critical Cooling Rate (UCCR), above which every element would be in a solid solution state.…”