Kinetics of isothermal and non-isothermal precipitation in an Al-6 at.% Si alloy

Abstract

A novel theory which describes the progress of a thermally activated reaction under isothermal and linear heating conditions is presented. It incorporates nucleation, growth and impingement, and takes account of temperature dependent solubility. The model generally fits very well to isothermal calorimetry and differential scanning calorimetry data on precipitation in an Al-6at%Si alloy. Analysis of the data shows that two processes occur in this precipitation reaction: growth of large silicon particles and growth of pre-existing small nuclei. Determination of the sizes of Si precipitates by transmission electron microscopy indicates that interfacial energy contributions are small and have a negligible influence on solubility.

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

“…This can be explained by the suppression of diffusion processes. As only diamond cubic Si-rich phase (a = 0.542 nm) is likely to occur [1,15], -9 -the occurrence of two reaction peaks for alloy Al-0.72Si within the analysed cooling rate range may seem surprising. For the alloy containing a lower mass fraction of Si only one reaction could be detected in the same cooling rate range.…”

“…Due to the limited DSC data at very low cooling rates for Al-0.26Si, |ΔHRT sat | for this alloy was estimated from value |ΔHRT sat | = 11.3 J/g for Al-0.72Si using the Si content proportion of both alloys. The enthalpy change of precipitation per mole Si from the Al-rich matrix was experimentally studied in [15,38] and was derived to be about 54 kJ mol -1 . Taking this value into 370 account, theoretically |ΔHRT sat | ≈ 13.2 J/g for Al-0.72Si and |ΔHRT sat | ≈ 4.8 J/g for Al-0.26Si result, respectively.…”

“…Thus, binary alloys of the Al-Si system are analysed in a first step here. Al-Si is an ideal model system for the investigation of precipitation kinetics, since exothermic reactions are always due to precipitation of Si [15] and solubility of Al in the Si phase is low [16,17]. Furthermore, knowledge on influence of single alloying elements on mechanical properties is considered to be important in order to set up a new plasticity model for commercial aluminium alloys.…”

VPPA welds of Al-2024 alloys: Analysis and modelling of local microstructure and strength

“…[25,8,45], and experimentally verified for a number of precipitation reactions in heat treatable Al based alloys [46,47,48];…”

“…The transformed fraction of precipitates during ageing can be described by the Starink-Zahra (SZ) model for nucleation and growth [45][46][47][48]:…”