Tomáš Kekule

Influence of Natural Ageing on Precipitation Processes during Isochronal Annealing in MgGd Alloys

The Mg-10 wt. % Gd and Mg-15 wt. % Gd alloys produced by squeeze casting were solution treated at 500 °C for 8 hours and subsequently naturally aged for more than 2 months. Electrical resistivity of both materials measured at 77 K decreases, if the alloys are kept at room temperature after quenching from the solution temperature. This change accompanied by a microhardness increase almost saturates after 2 months and is caused most probably by solute atoms clustering. Phase transformations and microhardness changes were investigated during isochronal annealing in both naturally aged alloys in comparison to just solution treated ones. Electrical resistivity changes measured at 77 K were used to characterize microstructure development. Transmission electron microscopy was performed at selected states heat treated in the identical way. The Mg15Gd supersaturated solid solution isochronally annealed up to 500 °C immediately after the solution treatment decomposes into following successive phases: β ́ ́ (D019) metastable → β ́ (cbco) metastable → β (Mg5Gd) stable. All three possible orientation relationship modes of the metastable β ́ (cbco) phase existed at lower temperatures (up to 280 °C) but only one mode persists up to 330 °C. Precipitation of the β ́ (cbco) phase has not been observed in the Mg10Gd alloy annealed isochronally immediately after the solution treatment. The natural ageing does not change the precipitation sequence but concentration of Gd atoms involved in individual precipitation processes is influenced in both alloys. Peak hardening increases after natural ageing in the Mg15Gd alloy, shifts to higher temperatures and the temperature region of peak hardening extends.

Annealing Effects in Conventionally Cast and Homogenized Al-Zn-Mg(-Sc-Zr) Alloy

Al-based alloys are very preferred for automotive manufacture to produce lightweight vehicles. The positive effect of Sc,Zr-addition on the mechanical properties in Al-based alloys is generally known. Microstructure, mechanical, electrical and thermal properties of the conventionally cast and homogenized Al–Zn–Mg alloy with and without Sc,Zr-addition during isochronal annealing were studied. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared to microstructure development that was observed by optical microscopy and transmission and scanning electron microscopy. It was observed that the Sc,Zr-content in the alloy after casting is not homogeneously distributed but concentrated in randomly localized matrix regions and together with Zn and Mg in the particles at grain boundaries. However, the hardening effect after annealing above 280 °C lightly reflects the Sc,Zr-addition. The distinct changes in resistivity and microhardness as well as in heat flow of the alloys studied are mainly caused by formation/dissolution of the Guinier-Preston zones and subsequent precipitation of the metastable particles from the Al–Zn–Mg system. The eutectic Zn,Mg-containing phase partly disappeared during the annealing above ~ 390 °C. Melting of the Zn,Mg-containing phase was observed at ~ 475 °C. The decomposition sequence of the supersaturated solid solution of the studied alloys is compatible with the decomposition sequence of the Al–Zn–Mg system minimally up to ~ 380 °C.

Development of Microstructure and Properties of Mg-Y-(Nd)-Zn Alloys during Heat and Mechanical Treatment

This work is focused on development of microstructure and properties of Mg-Y-Zn and Mg-Y-Nd-Zn alloys during heat and mechanical treatment. In the as-cast state both alloys exhibit almost equiaxed grains with little larger size in Mg-Y-Zn alloy and grain boundaries decorated by different structures - long period ordered structure (LPSO) was detected in Mg-Y-Zn alloy and eutectics of Mg3Nd type structure in alloy with Nd addition. A high density of stacking faults is evident in both alloys. Both alloys were repeatedly isochronally heat treated from room temperature up to 440 °C. Resistivity and microhardness measurement was performed after each heating step. Stacking faults persist both annealings in both alloys and microhardness development shows no remarkable differences. LPSO in Mg-Y-Zn alloy disappears after the first annealing and was again detected after repeated annealing up to 340 °C. After the whole treatment no grain growth appeared. Differential scanning calorimetry measurement was performed at both repeatedly heated alloys up to 540 °C. There are three exothermic peaks in DSC curves of Mg-Y-Zn alloys that can be ascribed to embedding solute atoms in stacking faults, LPSO development and transformation and coarsening of grain boundary particles. DSC curves of Mg-Y-Nd-Zn alloy exhibit two exothermic peaks that probably correspond to precipitation of basal plates of γ ́and γ phase. Measurement of microhardness was performed after sequential deformation of both alloys in the as-cast state. The alloys were cold rolled in steps of 0,9 % thickness reduction up to cracks formation. Strengthening of both alloys is very similar but formation of cracks in the alloy with Nd addition begins after a lower reduction (about 11 %) compared to Mg-Y-Zn alloy (about 15 %).

Precipitation Processes in Hot-Rolled Al-Mn-Sc-Zr Alloy

The effect of hot rolling on mechanical and electrical properties, microstructure and recrystallization behaviour of the AlMnScZr alloy was studied. The mould-cast alloy and the alloy after hot rolling at 300 °C was studied during step-by-step quasilinear annealing from 200 °C up to 600 °C with heating rate 100 K/h followed by subsequent isothermal annealing at 600 °C/5 h. Precipitation reactions were studied by electrical resistometry, differential scanning calorimetry and hardness measurements. Transmission electron microscopy and electron backscatter diffraction examination of specimens quenched from temperatures of significant resistivity changes were used to identify microstructural processes responsible for these changes. Only occasional irregular sharp-edged polygonal particles of the AlMnFeSi system were found in the as-prepared state of the mould-cast alloy. The as-prepared state of the hot-rolled alloy was characterized by a dispersion of fine coherent Al3Sc and/or Al3(Sc,Zr) particles and furthermore a fine (sub) grain structure was observed. The hardening effect in the alloys is due to presence and/or precipitation of the Sc,Zr-containing particles with L12 structure. The distinct resistivity changes of the alloys are mainly caused by precipitation of Mn-containing particles. Two-stage development of the Al6Mn phase (in (sub) grain interiors and at (sub) grain boundaries) in the hot-rolled alloy was observed. The presence of Sc,Zr-and Mn-containing particles has an anti-recrystallization effect that prevents recrystallization minimally up to 600 °C and annealing of 1 hour in the hot-rolled alloy. The apparent activation energy for the Al3(Sc,Zr)-phase and Al6Mn-phase precipitation was also determined. The activation energy values obtained in the hot-rolled AlMnScZr alloy are comparable to those observed in the hot deformed AlMnScZr alloys prepared by powder metallurgy.

Phase Transformations and Recrystallization in Cold-Rolled Al–Mn, Al–Sc–Zr and Al–Mn–Sc–Zr Alloy

The effect of cold-rolling on mechanical and electrical properties, microstructure and recrystallization behaviour of the AlMn, AlScZr and AlMnScZr alloys was studied. The materials were investigated during isothermal annealing at 300, 400, 500 and 550°C and during step-by-step quasilinear annealing from 200°C up to 600°C with a heating rate of 100 K/h followed by subsequent isothermal annealing at 600°C/5 h. Precipitation reactions were studied by electrical resistometry and (micro) hardness measurements. The microstructure development was investigated by electron microscopy and electron backscatter diffraction examinations. The hardening effect is due to uniformly distributed Al3Sc and/or Al3(Sc,Zr) particles. The distinct changes in resistivity of the alloys above ∼ 300°C are mainly caused by precipitation of Mn-containing particles. It has a negligible effect on hardness. Phase transformations in the AlMn and AlMnScZr alloys are highly enhanced by cold rolling. The precipitation is dependent on the deformation degree – the more deformation the more intensive precipitation of the Mn-containing particles. The combination of Mn, Sc and Zr additions to Al substantially suppresses recrystallization at 550°C. A partial recrystallization was observed in the AlScZr alloy and AlMnScZr alloy after annealing 550°C/60 min and 550°C/760 min, respectively. The decomposition sequence of the supersaturated solid solution of the AlMnScZr alloy is compatible with the decomposition sequence of the AlScZr system accompanied and/or followed by the formation of Mn-containing particles.

Annealing Effects in Mg-Y-Zn Alloys Prepared by Powder Metallurgy and by Squeeze Casting

Response to isochronal annealing up to 440°C was investigated in squeeze cast Mg2Y1Zn alloy and in the same alloy prepared by powder metallurgy and extrusion at 280°C (PM). Electrical resistivity measurements at 77 K and at room temperature after each annealing step and differential scanning calorimetry performed at various heating rates characterized phase changes proceeding during the heat treatments. Transmission and scanning electron microscopy and optical microscopy revealed ribbons of a long-period ordered structure and a relatively high density of stacking faults in grain interiors of the cast alloy having the grain size of ~50 μm. Well pronounced subgrains were observed in the PM prepared alloy. Secondary phase particles decorate grain boundaries in this alloy. Electrical resistivity response of the cast alloy to isochronal annealing up to 440°C shows three precipitation processes, whereas one significant process was revealed in the PM alloy. Activation energies of precipitation processes were determined. Microhardness exhibits good thermal stability in the whole temperature range in the cast alloy and up to 360°C in the PM alloy.

Response of Hot-Extruded Al-Mn-Sc-Zr Alloy to Annealing with Constant Heating Rate

The effect of hot extrusion at 350°C on microstructure, thermal, electrical and mechanical properties of the AlMnScZr alloy was studied. The samples of the cast and of the hot-extruded alloys were annealed from 20°C up to 600°C. Transmission and scanning electron microscopy and electron backscatter diffraction examinations of specimens quenched from temperatures of significant resistivity changes were used to identify microstructural processes responsible for these changes. The cast as well as hot-extruded alloy is characterized by a dispersion of fine coherent Al3Sc and/or Al3(Sc,Zr) particles, and furthermore the fine (sub) grain structure was observed in the hot-extruded alloy. Microhardness HV1 and resistivity values reflect different microstructure of the alloys accordingly. The distinct resistivity changes of the alloys are mainly caused by precipitation of Mn-containing particles. The apparent activation energy for the Al6Mn-phase precipitation in the hot-extruded alloy was also determined. The obtained results agree with those observed in the alloys prepared by powder metallurgy studied in our previous work.