Fig. 4A), the morphology of the interfacial phase changed further and a high volume fraction of a particle-like phase with the composition of 48.4 ± 1.4 at.-% Ni, 50.1 ± 1.2 at.-% Al, and 1.5 ± 0.4 at.-% Mg was detected (Fig. 4E). This phase was also found to be AlNi IMC phase. It should also be noted that formation of the AlNi phase consumed almost all of the Al content of the melt near the steel-FZ interface. Thus, no β-Mg17Al12 was observed near the interface compared with the central part of the FZ. Solidification of the Remaining Melt between the AlNi IMC Phase and Steel At the bottom of the joint, AlNi IMC first crystallized from the liquid close to the interface and then supersaturated α- Mg solid solution containing 10.6 ± 3.6 at.-% Ni, 3.2 ± 1.7 at.-% Al, and 2.9 ± 1.5 at.-% Fe (dark regions in Figs. 4B and 8) solidified from the liquid during cooling along with the AlNi phase. In some locations between the bottom and middle of the interface, some gray lamellar phases, as shown in Figs. 8C and 9, were also observed between the AlNi IMC layer and the steel. Figure 9A shows the position of AlNi precipitates with respect to the steelfusion zone interface in the prepared sample during focused ion beaming for TEM analysis. Figure 9B, C shows TEM images of this lamellar (plate-like) phase. According to EDS analysis, the white lamellae corresponded to α-Mg and the dark lamellae containing 27.6 ± 7.2 at.-% Ni and 72.3 ± 7.3 at.-% Mg represented the Mg2Ni stoichiometric intermetallic compound (also confirmed by SADP analysis). Based on these results, these two phases next to each other are the Mg-Mg2Ni lamellar eutectic. Formation of the Mg-Mg2Ni lamellar eutectic was not uniform and continuous along the interface. As shown in Fig. 8A, B, in some locations between the steel and the AlNi IMC layer, Mg2Ni crystallized in the form of a lamellar gray phase and in other locations it was not seen and a dark solid solution of Mg containing small amounts of Ni, Al, and Fe was formed. In the middle portion of the interface, the AlNi phase crystallized first in the liquid (Fig. 4D). Then, dark α-Mg solid solution containing 5.8 ± 2.1 at.-% Ni, 1.2 ± 0.3 at.-% Al, and 3.1 ± 0.5 at.- % Fe formed during cooling along with AlNi phase. Finally, at the top of the joint, the AlNi phase precipitated heavily in the liquid along the interface and then the remaining liquid solidified during cooling in the form of α-Mg solid solution (containing 2.4 ± 0.6 at.-% Ni, 0.3 ± 0.1 at.-% Al, and 3.4 ± 0.3 at.-% Fe) along with and among AlNi particles (see Fig. 4E). Upon moving from the bottom to the top of the interface, the Fe content of the remaining liquid between AlNi IMC and the steel increased from 2.9 to 3.4 at.- % due to more diffusion of Fe from the steel side to the FZ at higher temperature. In contrast, Al and Ni showed opposite behaviors due to an increase in the thickness of AlNi IMC from the bottom to the top portion of the joint (from 5 to 30 μm). Transition Layer Based on the TEM analysis, the AlNi phase did not grow epitaxially on the steel substrate, but instead nucleated and grew in the liquid adjacent to the interface and was surrounded by either α-Mg + Mg2Ni eutectic phases or just α-Mg phase close to the interface. On the other hand, while it may appear in Fig. 8 that all of the electroplated Ni had melted and gone into solution in the liquid and the α-Mg may have nucleated and grown epitaxially from the steel surface, it is well known that Mg and Fe are an immiscible couple. From a crystallographic point of view, it is not possible for magnesium to nucleate on steel due to the very large lattice mismatching of Fe and Mg (Ref. 4). Therefore, another layer or phase must be responsible for bonding between the steel and fusion zone. Further high-magnification microstructural analysis of the steel-fusion zone interface was performed by TEM to find an explanation for the observed interfacial phases. Figure 10A shows a TEM image of the steel-fusion zone interface. A continuous nano-interlayer (50–200 nm thick) phase was observed along the interface, which was bonded to the steel side on one side and to the fusion zone on the other side. Higher magnification of this JANUARY 2013, VOL. 92 6-s WELDING RESEARCH A B C Fig. 8 — SEM images of the steel-FZ interface show the solidification morphology of remaining melt between the IMC layer and steel side: A — Position A in Fig. 4A near bottom side; B — position B in Fig. 4A; C — Mg- Mg2Ni eutectic phases. A B C Fig. 9 — A — TEM sample attached to a copper grid; B, C — TEM images of the lamellar phases formed along the steel-FZ interface.
Welding Journal | January 2013
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