Tailoring microstructure of Mg–Zn–Y alloys with quasicrystal and related phases for high mechanical strength

Abstract

The occurrence of a stable icosahedral (i-) phase, which is quasicrystalline with an icosahedral (fivefold) symmetry, on the equilibrium phase diagram of Mg–Zn–RE (RE = Y, Gd, Tb, Dy, Ho or Er) alloys opened up an interesting possibility of developing a new series of magnesium alloys for structural applications. Alloys based on the i-phase have been studied for the past 14 years. Ultra-high strengths combined with good ductility have been shown. Here we show two strategies for tailoring microstructures for very high strengths in Mg–Zn–Y alloys. One of them involves strengthening by a fine distribution of rod-like precipitates, where the matrix grain size is not critical. The alloy is solutionized at a high temperature of 480 °C to dissolve a large part of the i-phase, followed by a high temperature extrusion (∼430 °C) and a low temperature ageing to reprecipitate phases with fine size distribution. At first, phase transformations involved in this procedure are described. The closeness of the structure of the precipitates to the i-phase is brought out. By this procedure, tensile yield strengths of over 370 MPa are obtained in grain sizes of 20 μm. In another strategy, the alloys are chill cast and then extruded at low temperatures of about 250 °C. Ultra-fine grains are produced by enhanced recrystallization due to presence of the i-phase. At the same time nano-sized precipitates are precipitated dynamically during extrusion from the supersaturated matrix. Ultra-high tensile strengths of up to 400 MPa are obtained in combination with ductility of 12 to 16%. Analysis of the microstructure shows that strengthening by the i-phase occurs by enhanced recrystallization during extrusion. It produces ultra-fine grain sizes to give very high strengths, and moderate texture for good ductility. Fine distribution of the i-phase and precipitates contribute to strengthening and provide microstructre stability. Ultra-high strength over a very wide range of grain sizes is thus demonstrated, by utilizing different strengthening effects.

Lattice Correspondence and Growth Structures of Monoclinic Mg4Zn7 Phase Growing on an Icosahedral Quasicrystal

“…(a) Here, we also note of a report of an icosahedral phase growing on a Mg 4 Zn 7 phase precipitate [35]. In this case, the icosahedral phase also shows streaked diffraction spots (in the direction of the growth, perpendicular to a fivefold plane).…”

“…The same study also determined the composition of the decagonal phase to be about Mg 35 Zn 60 Y 4.4 , while the Mg 4 Zn 7 phase did not show any yttrium. In dilute Mg-Zn-Y/RE alloys, however, no decagonal phase is reported.…”

A Critical Review of Mg–Zn–Y Series Alloys Containing I, W, and LPSO Phases

“…[54] Because of the icosahedral symmetry I-phase (Mg 3 YZn 6 ) has fivefold symmetry axes, and owing to the quasi-periodicity the distance between reciprocal points along a reciprocal direction is scaled by the irrational number, t ¼ 1þ ffiffi [54,60] The I-phase can exhibit a variety of orientation relationships with hexagonal matrix. [34,59,61] In such a configuration, the I-phase shows a plate-like shape on the basal plane, such as in Figure 4a since the identical icosahedral twofold and the hexagonal planes create an interface in a low energy state. [34,59,61] In such a configuration, the I-phase shows a plate-like shape on the basal plane, such as in Figure 4a since the identical icosahedral twofold and the hexagonal planes create an interface in a low energy state.…”

“…As a result, the atomic bonding between the W-phase and Mg matrix is very weak because of the limited symmetry of the crystal lattice structure of W-phase and the incoherence of the interface between them. [59] be expressed as 111 [64] W-phase can be transformed into I-phase at the interface by reacting with the matrix. The crystallographic orientation relationship between W-phase and I-phase can [11] Fig.…”

“…4. [59,64] (b) A circular I-phase particle in Mg-4.2Zn-0.8Y alloy and an FFT pattern from the particle is shown in (c).…”