M.Sc Thesis Department of Materials Science and Engineering

Galit Atiya


Precipitation Sequence and Phase Evolution in Mg-Nd Alloy
Containing Zn and Zr

Supervisor: Prof. Bamberger Menachem
Full thesis text - English Version   Full Thesis text


Abstract

Mg - Rare Earth (RE) alloys have are very attractive in various applications due to their high specific strength. The decreasing solubility of RE elements in Mg matrix with temperature results in remarkable age hardening response during aging at ~175°C-200°C. In the present research a magnesium alloy containing Nd,Zr and Zn (Mg-3.1Nd-0.45Zr-0.25Znwt.%) was investigated.

The microstructure of the alloy was investigated in the as-cast, solution treated(ST) and aged (175°C) conditions using microhardness-tests, optical microscopy, XRD analysis and various electron microscopy techniques. The crystal structure and orientation-relationships (ORs) of co-existing phases were characterized. It was found, that in the as-cast samples part of magnesium atoms in BCT Mg12Nd phase are probably substituted by zinc atoms and the suggested formula: Mg12-xZnxNd (x~0.3). After ST, the Mg12-xZnxNd phase almost fully dissolved, and small tetragonal Zn2Zr3 rod-like particles heterogeneously precipitated. Zn2Zr3 particles are elongated along their [001] direction with the following OR: Mg║Zn2Zr3,Mg║Zn2Zr3.            
At the first stage of aging (up to 8 days) the metastable phase b''(Mg3Nd)HCP (DO19 structure) formed. This structure was proven by using the Exit Wave Reconstruction method. The b'' precipitates are fully coherent with the matrix, and have the following ORs:    β''║Mg and  β''║Mg.

In the second stage of aging (16?32 days), b'' transform to metastable b'(Mg3Nd) precipitates with FCC (DO3) structure. The b' precipitates are semi-coherent with the matrix and have the following ORs:  β'║Mg, β'║Mg and β'║Mg, β'║Mg .

In the third stage of aging, the b' transform into a stable incoherent b Mg12Nd / Mg12-xZnxNd phase in the grain boundary region in the over-aged samples. The Zn2Zr3 rods serve as additional nucleation sites for Mg-Nd precipitates. The heterogeneous nucleation occurs in two ways: precipitates nucleate on the basal planes (habit planes Mg ) and on the side planes (habit planes Mg ) of the Zn2Zr3 rods resulting in the formation of H-shape particles observed in the peak-aged sampled. This study demonstrated that the formation of Zn2Zr3 rod-like particles during solution treatment enhanced the age hardening response by the formation of H-like particles providing efficient barriers for dislocation movement.