These stages occur during the incubation process followed by artificial aging. The above mechanism has been well documented and validated by others,31-33 al.34
carried out extensive transmission electron microscopy
(TEM) and high-resolution transmission electron micros- copy (HRTEM) studies to understand the crystallography of the intermediate phases during the precipitation reaction as proposed by Edwards et al.6
The summary of this work is
presented below where the crystallography of precipitates forming subsequent to the clustering stages was determined.
SSSS Clusters of Si atoms and Clusters of Mg atoms Dissolution of Mg clusters GP Zone (GP-I) β” (GP-II) β’ + B’ + U1 + U2 β (Stable Mg2
Si phase) Where,
GP Zones (GP-I) à Mgx β” (GP-II) à Mg5 U1 (Type A) à MgAl2
à Mg1.8
Al5-xSi6 à Primarily Monoclinic à Monoclinic
Si6 Si2 à Trigonal
U2 (Type B) à MgAlSi à Orthorhombic β’
β’ (Type C) à MgSi>1 à Mg2
β
Si à Hexagonal à Hexagonal
Si à FCC
Although, the precipitation sequence during aging of the Al-Si-Mg wrought alloys have been well documented, not much is known about the sequence and kinetics of the pre- cipitation reactions that occur during the incubation and arti- ficial aging in A356.2 with a high Si to Mg atom ratio of ~4. Further, there is no information about the effect of duration of incubation on the precipitation sequence during the artifi- cial aging process.
Sequence of Precipitation During Incubation
Individual and co-clusters of Mg and Si atoms in the matrix of the primary Al phase were believed to form during the in- cubation treatment of Al-Mg-Si alloys at room and interme- diate approx.(70C)[158F] temperatures. Clusters are zones in the Al matrix which have a higher number of Mg and/or Si solute atoms compared to the random distribution (SSSS) of these atoms. Murayama et al.35,36
reported that after a pro-
long period of incubation at room temperature, individual- clusters of Si and Mg atoms and co-clusters (GP Zone I) of these atoms could be formed. These clusters are typically about 2nm in dimension with low solute concentration with an undefined morphology. Typically, clusters do not have a distinct crystal structure. Individual and co-clusters of Si and Mg atoms formed during incubation at room tempera- ture could not be detected using HRTEM.36
GP zones II or β” (Mg5
co-clusters of Si and Mg atoms in the Al matrix grow and coarsen, and they have a specific crystal structure as defined by Vissers et al.34
Si6 These are ordered solute clusters with the International Journal of Metalcasting/Fall 2011 ) precipitates are formed when the
as well. Vissers et
size and solute content large enough to give an image con- trast in the HRTEM. The β” precipitates are believed to be generated during incubation at room temperature or at a pre- aging stage at 70C (158F).37
shaped and have a monoclinic structure, oriented along the <100> direction of the aluminum matrix.33,34,38
tion in Al-Mg-Si alloys during the artificial aging treatments at temperatures above 120C (248F). There are two known mor- phologies of β’ precipitate: needle-shaped and rod-shaped. It was shown that the needle-shaped morphology forms first fol- lowed by the transition to the rod-shaped morphology.9
in the primary Al matrix.31-36 Sequence of Precipitation During Artificial Aging Formation of β’- Mg1.8
Si6 The
exact composition of β’ precipitate is not unique. The EDS results of Maruyama et al.9
oms range between 1.75 and 1.78. Vissers et al.34
indicate the ratio of Mg to Si at- suggested
a ratio of 1.8 for Mg to Si atoms. The needle shaped mor- phology has a monoclinic structure and is oriented along the <100> direction of the aluminum matrix.31,34
morphology has a hexagonal crystal structure and is oriented along the <100> direction of the aluminum matrix .9,34 al.39
The rod-shaped Dutta et
showed typical TEM bright field images of rod-shaped β’
precipitates; these have a large aspect ratio and have a promi- nent shape effect on the strength of the alloy component.
cording to Maruyama et al.9 β is 2.10.
The β precipitates are the final stable phases in the precipita- tion reaction sequence of Al-Mg-Si alloys. They are formed subsequent to the β’ precipitates during artificial aging treat- ment. β precipitates are in the form of plates, which are ori- ented along the <100> direction of the aluminum matrix. Their crystal structure is akin to the CaF2
structure, and ac- the ratio of Mg to Si atoms in
Characterization Techniques to Detect Precipitation Reaction Sequence
There are many advanced characterization techniques such as Three Dimensional Atom Probe (3DAP),9,35,36 TEM and Differential Scanning Calorimetry (DSC)6,33,39
HRTEM, to
observe and quantify the formation of the various phases during the precipitation reaction sequence of Mg and Si at- oms in Al. However, none of these techniques are in-situ methods wherein the sequence of the precipitation reaction can be observed. Typically, the formation of individual- clusters of Si begins instantly after quenching of the alloy subsequent to solution treatment. Electrical resistivity mea- surement is one of the more popular methods in precipita- tion reaction studies.37,40
comparable or higher than the signal during measurements of electrical resistivity during incubation,37 technique less reliable.
However, sometimes the noise is thus, making the
19
of precipitation reaction during incubation at room tempera- ture ends with the formation of the β” (Mg5
The β” precipitates are needle- The sequence ) precipitates
Si is the first stage of precipitation reac-
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