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Minor additions of Sc are effective in controlling the recrystallization resistance of 5xxx, 2xxx, and 7xxx aluminum. The addition of Sc to aluminum results in the rapid precipitation of homogeneously distributed Al 3 Sc dispersoids, which are coherent with the matrix and have the L1 2 structure. The presence of Al 3 Sc dispersoids increases the recrystallization resistance of wrought alloys. The higher coarsening rate of Al 3 Sc compared to that of Al 3 Zr may limit its applications as a single ancillary addition. When both scandium and zirconium are used in the same alloy, Al 3 (Sc 1-x , Zr x ) dispersoids form. These dispersoids are more effective recrystallization inhibitors than either Al 3 Sc or Al 3 Zr. The Al 3 (Sc 1-x , Zr x ) dispersoids precipitate more rapidly than Al 3 Zr but have a slower coarsening rate than Al 3 Sc. Furthermore, the distribution of Al 3 (Sc 1-x , Zr x ) is significantly more homogeneous than Al 3 Zr. It was also established that alloys containing up to 3.5Mg showed improvement in recrystallization resistance when both Sc and Zr were present. Several morphologies of Al 3 Sc and Al 3 (Sc 1-x , Zr x ) were also observed.

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Improvements to an Ni-(5-6 wt.%)B coating have resulted in exceptional hardness and wear resistance that meet or exceed many of the performance standards set by hard chromium. Currently, no other bath plating technologies offer properties competitive with hard chromium, including electroless nickel-phosphorous derivatives and older versions of Ni-B. In this article, the improved Ni-B is discussed in terms of microstructural features, performance properties, coating methodology, and applications.

Surface 3 of alloy AA6111-T4, even deeper into the weld material, showed similar results.Finally, we note that we did not perform a complete investigation of the base metal. However, some preliminary results indicate that the 6111 base metal had a much stronger texture than the 5182. This difference could be another reason why the 6111 material had a stronger texture in the columnar grain region of the weld.From these results, we can conclude that a strong cube texture forms in laser-welded AA5182-O and AA6111-T4 alloys and that the strength of the texture depends on the particular alloy and the depth through the weld zone. In particular, we note that the columnar grains that form on either side of the weld centerlines and appear to grow out from the parent metal into the liquid are highly textured, with a Ͻ001Ͼ direction parallel to the growth direction. This result is in agreement with studies of solidification in aluminum alloys that reported that a cube direction is a preferred growth direction. [15] Given that mechanical properties are strongly dependent on texture, [16] one would expect this texture could have a strong effect on the local mechanical response of the welds.The authors thank R. Mishra, for his critical review of the manuscript, and J. Cross, for his assistance in metallographic sample preparation and microhardness measurements. It is a pleasure to acknowledge stimulating discussions with P. Krajewski and M. Verbrugge. The work at Brown University was supported by GM through the GM Collaborative Research Lab at Brown University.

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a eutectic reaction, and it is indeed observed that the eutectic reaction in hypereutectic Al-Si alloys does not start spontaneously when Al has been nucleated. Instead, Al dendrites develop and grow without establishing cooperation when impinging on primary Si crystals. Evidently, the eutectic reaction has to await the nucleation of a Si crystal with the ability to grow as a plate. For the cubic Si, this can occur by the formation of parallel twin planes, which make possible the classical growth mechanism using re-entrant angles. If this is the growth mechanism of the eutectic Si plates, then the fragments giving rise to new plates must contain the active twin planes. As an alternative, it has been proposed that the appearance of slightly new directions of the Si plates is not caused by fragments breaking off but by low-angle branching during growth of Si through a displacement twinning mechanism. [7] Even in that case it is required that the parallel twin planes be preserved.

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