Jitka Pelcová

Microstructure and Phase Changes Due to Heat Treatment of Squeeze Cast Mg–Sc–(Ce)–Mn Alloys

The time dependence of the electrical resistivity of MgScMn and MgScCeMn alloys due to isothermal annealing at 500 and 600 °C confirms the results of thermodynamic calculations that a homogenisation heat treatment (T4) and consequently a full age hardening (T6) is impossible. The peak age hardening of the as-cast material (T5) is due to the precipitation of fine dense Mn 2 Sc particles in both materials accompanied by the precipitation of Mg 12 Ce particles in Ce containing alloys. This microstructure accounts for the excellent creep resistance of these materials which is superior to the high temperature creep resistant alloy WE43. The decrease in electrical resistivity during isochronal annealing in the temperature range 300-400 °C (MgScMn) and 300-480 °C (MgScCeMn) has been shown by TEM observations to be due to the same precipitation processes. TTT diagrams drawn from peak hardness values obtained by the isothermal heat treatment also confirm the complex precipitation process in MgScCeMn alloys. Short pseudo-homogenisation heating at 600 °C preceding the isochronal annealing causes another earlier resistivity decrease at 200-300 °C in quaternary alloys corresponding to a different precipitation sequence.

Phase transformations due to isochronal annealing of Mg – rare earth – Sc–Mn squeeze cast alloys

Abstract Magnesium alloys containing rare earths (Gd, Y, Ce, ≈ 3–10 wt.%), Sc (< 1 wt.%) and Mn (≈ 1 wt.%) were squeeze cast. Electrical resistivity and hardness responses to isochronal annealing were measured in the range 20–580°C and phase changes responsible were identified by transmission electron microscopy. In alloys containing Gd and Y simultaneous precipitation of three phases takes place, namely the metastable c-base-centred orthorhombic phase (Mg–Y, Mg–Gd) in the form of prismatic plates, the thermally stable hexagonal Mn2Sc phase in the form of fine basal discs and a metastable hexagonal coherent phase, containing Mn and Y or Gd, in the form of very thin basal plates (missing in the Mg alloy with 10 wt.% Gd). In the Ce-containing alloy the Mg12Ce tetragonal phase, present after casting in various forms, transforms into a rod-like form. Simultaneously, fine Mn2Sc discs precipitate on basal planes and the grain boundary eutectic transforms to the equilibrium phases.

Phase transformations in creep resistant MgYNdScMn alloy

Abstract Mg-4 wt.% Y-2 wt.% Nd-1 wt.% Sc-1 wt.% Mn alloy was squeeze-cast. It benefited from the addition of Nd which decreased the solubility of Y in Mg, as is known from WE alloys. Isochronal annealing of the alloy showed relative resistivity changes similar to those obtained in other Mg–R.E.–Sc–Mn alloys (R.E. – rare-earth element). The resistivity decrease observed is due to the complex precipitation process, namely a combination of decomposition sequences in the Mg–Y–Nd, Mg–R.E.–Mn, and Mn–Sc systems. The increase in the resistivity on isochronal annealing up to temperatures over 500 °C is due to the re-solution of the phases in the first two systems. The Mn2Sc phase coarsens only during this annealing treatment. The creep resistance of the alloy at high temperatures is better than that of low Sc content Mg alloys with Yand Gd in the T5 condition and also than that of WE alloys in the T6 condition.

Structure and Properties of AlMg3 Alloy with Sc and Zr

The present contribution deals with the effect of thermal and mechanical treatment on structure, mechanical and physical properties of as cast, cold and hot worked AlMg3 type alloy produced by conventional casting, as well as by powder metallurgy. The properties of the AlMg3 alloys with additions of Sc and Zr in the weight concentration ranges 0.14 – 0.29% and 0.05 – 0.12% respectively were studied and compared with those of the alloys without Sc and Zr additions. The anti-recrystallization effect, phase development and mechanical properties investigated in the course of isothermal as well as isochronal annealing were studied by using optical and transmission electron microscopy, electrical resistometry and hardness measurements. The parameters of technological procedure of optimum age hardening by the Al3(ScxZr1-x) phase are presented. This optimum can not be reached if the as cast alloy is exposed to temperatures higher than 350°C. Even the small additions of Sc such as 0.15 wt. % ensure the anti-recrystallization effect of Al3(ScxZr1-x) phase.