| Abstrakt: |
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 [Formula: see text] 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 [Formula: see text] 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 μ m. 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. |
