טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAbd El Majid Suzan
SubjectMicrostructural Characterization and Phase Evolution of Aged
Mg-Gd and Mg-Gd-Nd Alloys with Additions of Zn, Y
and Zr
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Emeritus Menachem Bamberger
Dr. Alexander Katsman


Abstract

The investigated alloys were: Mg-6Gd-3.7Nd-0.3Zn-0.18Y-0.15Zr and Mg-4.7Gd-0.37Zn-0.14Y-0.09Zr (wt. %). Specimens cut from the cast were solution treated at 540°C for 24hr, quenched in water, and then aged at 175°C in an oil bath for various periods of time up to 125 days, followed by water quenching.

The as-cast alloys contained primary a-Mg-matrix, eutectic-like structures, cuboid-like phases and Zr-rich clusters. The eutectic structures were α-Mg β-Mg5RE, where the FCC β phase was Mg5(Gd0.5Nd0.5) in Mg-6Gd-3.7Nd alloy. The cuboid shaped phases have a FCC structure with a≈0.54nm and and is a GdH2-type compound, the compositions were Gd0.7(YxMg1-x , x?0.5)0.3 in Mg-5Gd alloy, Y0.75Gd0.25 in Mg-6Gd-3.7Nd alloy.

During ST, the eutectic structures in the Mg-6Gd-3.7Nd alloy were melted and formed globular shaped eutectic regions during quenching; the composition of the eutectic structure was Mg5(GdxNd1-x, x≈0.8). The composition of the cuboid-like phase in Mg-6Gd-3.7Nd alloy was Gd4(YxNd1-x, x?0.5), while it was found to be stable during ST in Mg-5Gd alloy, and appeared to be stable during subsequent aging.

The four-stage precipitation sequence in Mg-6Gd-3.7Nd and Mg-5Gd alloys was found: α-Mg (S.S.S.S.) → β'' (Mg3RE, DO19)→ β' (Mg7RE, BCO) → β1 (Mg3RE, FCC) → β (Mg5RE, FCC).

The first stage is the formation of a metastable phase β'', that have a platelet-like shape of a few tens of nanometers in habit planes  and are fully coherent with the Mg matrix.

During the second stage, the β'' precipitates transformed to β' precipitates, which have a platelet-like shape of a few hundreds of nanometers in habit planes , growing along <0001>α-Mg and are coherent with the Mg matrix.

During the third stage, the nucleation of  β1 was associated with β'  precipitates, with the following orientation relationship between β' and β1 precipitates: . Further aging resulted in transformation of  all β'  precipitates to the β1 precipitates, which have a platelet-like shape of hundreds of nanometers in habit planes , growing along three directions of  and are semi-coherent with the Mg matrix.

In the fourth stage of aging, the β1  precipitates transformed to a stable incoherent β precipitates.

Hardening occurred with three-peaks in the Mg-Gd-Nd alloy and two-peaks in Mg-Gd alloy. Additional first peak in the Mg-Gd-Nd alloy was connected with the formation of β'' containing mainly Nd. The second peak corresponds to β'' precipitates with Nd and Gd, and the third peak corresponds to the formation of β' and β1 precipitates.