Ivana Stulíková

Structural aspects of high performance Mg alloys design

Magnesium–gadolinium binary alloys exhibit good mechanical properties and high creep resistance comparable to or better than commercial WE type (Mg–Y–Nd–Zr) alloys. Combining scandium and manganese with a particular rare earth element (R.E.–Gd, Y, Ce) has a beneficial effect on the creep behaviour of complex Mg–R.E. alloys, at lower R.E. contents than in WE type alloys. They stabilise high creep resistance up to high temperatures (above 300°C) by precipitation of the stable phase Mn2Sc and by precipitation of basal plates of a Mn and R.E.-containing hexagonal phase.

Microstructure evolution in isochronally heat treated Mg-Gd alloys

High thermal stability and good mechanical properties are crucial for the wider future application of magnesium alloys. One of the most promising directions is the alloying of Mg with heavy rare earth elements such as Gd, Dy, Tb, etc. Three squeeze cast Mg–Gd binary alloys (up to 15 wt% Gd) have been investigated after a solution heat treatment by isochronal annealing up to 500 °C using electrical resistivity and hardness measurements. The microstructural development during this treatment, responsible for the observed changes, was observed by transmission electron microscopy. The decomposition of α′-Mg supersaturated solid solution in Mg–14.55 wt% Gd with increasing heating temperature is as follows: β″ (D019) metastable phase β′ (c-b.c.o.) metastable phase β(Mg5Gd f.c.c.) stable. Peak hardening is achieved by a heat treatment resulting in precipitation of β′ phase in the shape of fine plates parallel to all three 21-1-0 planes of the α-Mg matrix. At higher temperatures (above 280 °C) coarsening occurs and only one orientation of β′ plates remains. The decomposition of Mg–4.47 wt% Gd and Mg–9.33 wt% Gd alloys differs from that of Mg–14.55 wt% Gd by the absence of the β′ phase. Um das Anwendungsfeld fur Magnesiumlegierungen zu erweitern, ist die Entwicklung von Legierungen mit hoher thermischer Stabilitat bei gleichzeitig guten mechanischen Eigenschaften erforderlich. Eine der vielversprechendsten Entwicklungen ist das Zulegieren von schweren Seltenen Erden wie z. B. Gd, Dy und Tb. Nach einer Homogenisierungsbehandlung wurden drei presgegossene binare Mg–Gd-Legierungen (bis zu 15 Gew.% Gd) mit Hilfe elektrischer Widerstandsmessungen und Hartemessungen hinsichtlich ihres Verhaltens bei isochronen Gluhungen bis zu 500 °C untersucht. Die wahrend dieser Warmebehandlung eingetretenen mikrostrukturellen Veranderungen wurden mittels transmissionselektronenmiskroskopischer Untersuchungen verfolgt. Mit der Gluhungstemperaturerhohung bilden sich in einer Mg–14,55 Gew% Gd-Legierung folgende Phasen: β″ (D019) metastabile Phase β′ (c-b.c.o.) metastabile Phase β (Mg5Gd f.c.c.) stabil. Die hochsten Hartewerte sind mit einer Warmebehandlung erreichbar, die zur Bildung der β′-Phase in Form feiner Plattchen parallel zu allen drei 21-1-0-Ebenen der Mg-Matrix fuhrt. Bei hoheren Temperaturen (uber 280 °C) bleibt nur eine Orientierung der β′-Phase erhalten und es tritt eine Vergroberung ein. Die korrespondierenden Entmischungsvorgange in einer Mg–4,47 Gew.% Gd- und einer Mg–9,33 Gew.% Gd-Legierung ist durch die Abwesenheit der β′-Phase gekennzeichnet.

Resistivity Changes Due to Precipitation Effects in Fibre Reinforced Mg–Al–Zn–Mn Alloy

The change of electrical properties of alumina short fibre reinforced Mg-Al-Zn-Mn alloy AZ91D during isochronal annealing up to 300°C is discussed. The Saffil fibres were incorporated into the magnesium alloy by direct squeeze casting. The fibre distribution is random planar parallel to the flat faces of the dc four-point resistivity specimens machined from the solution treated castings. A sharp drop of resistivity between 140 and 260°C is explained by the formation of incoherent β-phase particles. Some practical recommendations concerning the use of alumina short fibre reinforced AZ91 alloy are made on the basis of the results obtained.

Microstructure Changes in Isochronally Annealed Alumina Fibre Reinforced Mg–Ag–Nd–Zr Alloy

The commercial alloy QE22 (Mg–Ag–;Nd–Zr alloy) was reinforced by 22 vol% δ-Al2O3 short fibres applying the squeeze cast technology. Precipitation effects were studied in this material after a preceding solution heat treatment by isochronal annealing up to 300 °C by means of electrical resistivity, hardness and reversible stress relaxation measurements. The annealing response of the properties was compared to the annealing response of the unreinforced matrix alloy. The microstructure changes were studied in detail by transmission electron microscopy. A sharp drop of resistivity between 180 and 280 °C was found on normalised resistivity annealing curves of both reinforced and unreinforced specimens due to the redistribution of solutes. In composites the fibres act as nucleation centres in the precipitation process promoting e.g. precipitation of Al2Nd or Ag compounds. The Al content in the matrix is enhanced due to the decomposition of the preform binder. The evolution of the particle population inside the grains involves the formation of new Al2Nd-like cubic particles between 120 and 180 °C. Above 180 °C these particles are continuously substituted by hexagonal β-phase and/or tetragonal Mg12Nd particles. This process finishes at 300 °C by the transformation of all new particles to semicoherent Mg12Nd precipitates. The precipitation process in grain interiors of the unreinforced alloy is different involving only change of the morphological features of tetragonal semicoherent Mg12Nd particles existing in the alloy already in the initial state after solution heat treatment. Die kommerzielle Legierung QE22 (Mg–Ag–Nd–Zr) wurde im Presgiesverfahren mit 22 Vol.-% δ-Al2O3 Kurzfasern verstarkt. Die Untersuchung des Ausscheidungsverhaltens wahrend isochroner Warmebehandlungen bis 300 °C nach vorangegangenem Losungsgluhen erfolgte uber die Bestimmung der Anderung des elektrischen Widerstandes, der Harte und der reversiblen Spannungsrelaxation. Die Auswirkung der Warmebehandlung auf die Eigenschaften der verstarkten Proben wurden mit den Eigenschaften ebenfalls warmebehandelter unverstarkter Proben verglichen. Transmissionselektronenmikroskopie diente zur detaillierten genauen Beschreibung der Gefugeanderungen. Zwischen 180 und 280 °C ergibt sich fur beide Materialien ein starker Abfall des elektrischen Widerstands infolge der Umverteilungen der Legierungsatome. Im Verbundwerkstoff kommt es im Verlauf der Warmebehandlung zur bevorzugten Keimbildung auf den Fasern, was zur Ausscheidung z. B. von Al2Nd oder Ag Verbindungen fuhrt. Der Al Gehalt im Verbundwerkstoff ist als Folge der Zersetzung des Al2O3-Binders des Faserformkorpers erhoht. Im Korninneren bilden sich zwischen 120 und 180 °C Al2Nd-ahnliche, kubische Ausscheidungen. Oberhalb von 180 °C werden diese Partikel kontinuierlich durch β-Phase bzw. tetragonale Mg12Nd ersetzt. Dieser Prozes endet bei 300 °C mit der Umwandlung der neugebildeten Ausscheidungen in teilkoharentes Mg12Nd. In der unverstarkten Legierung ist der Ausscheidungsprozes im Korninneren hauptsachlich durch Anderungen der Mg12Nd Morphologie charakterisiert.

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.

CONCENTRATION DEPENDENCE OF THE CRITICAL RESOLVED SHEAR STRESS OF CADMIUM-SILVER ALLOY SINGLE CRYSTALS AT LOW TEMPERATURES

The increase of the critical resolved shear stress of cadmium single crystals by addition of silver has been investigated in the temperature range of 77 K to 199 K. At all temperatures the critical resolved shear stress increases withc2/3, wherec is the atomic concentration of silver as solute, and it decreases with increasing temperature. The concentration dependence of the critical resolved shear stress is explained according to the theory ofLabusch (phys. stat. sol.41 (1970), 659).

Microstructure and Thermal Stability of Ultra Fine Grained Mg-Based Alloys Prepared by High Pressure Torsion

In the present work we studied microstructure of ultra fine grained (UFG) pure Mg and UFG Mg-based alloys. The initial coarse grained samples were deformed by high pressure torsion (HPT) using pressure of 6 GPa. Such deformation leads to formation of UFG structure in the samples. The severe plastic deformation results in creation of high number of lattice defects. Therefore, we used positron annihilation spectroscopy (PAS) for defect characterizations. PAS represents a well developed non-destructive technique with high sensitivity to open volume defects like vacancies, vacancy clusters, dislocations etc. In the present work we combined PAS with TEM and XRD to obtain complete information about microstructure of the UFG samples studied. We have found that microstructure of HPT-deformed Mg contains two kinds of regions: (a) ”deformed” regions with UFG structure (grain size 100-200 nm) and high number of randomly distributed dislocations, and (b) ”recrystallized” regions with low dislocation density and grain size of few microns. It indicates some kind of dynamic recovery of microstructure already during HPT processing. On the other hand, homogenous UFG structure with grain size around 100 nm and high density of homogeneously distributed dislocations was formed in HPT-deformed Mg-9.33 wt.%Gd alloy. After characterization of the as-deformed microstructure the samples were subsequently isochronally annealed and the development of microstructure with increasing temperature and recovery of defects were investigated.

Structure and stability of microcrystalline MgCa alloy

Abstract Microcrystalline ribbons of MgCa alloys (the calcium content increasing from 1 to 10 wt.%) were prepared by melt spinning. The thermal stability of the ribbons was studied by step-by-step isochronal annealing from room temperature to 300 °C by means of electrical resistivity and microhardness measurements. Pronounced resistivity changes were observed between 120 and 300 °C depending on the calcium content. Electron and light microscopy studies were performed in order to identify the microstructural processes. It was found that the resistivity changes are due to changes in the form, size and volume fraction of the particles of Mg 2 Ca compound. The grain size equals approximately 1 μm and remains constant up to 300 °C in all the materials investigated.

Microstructure and mechanical properties of the AA6082 aluminium alloy with small additions of Sc and Zr

Abstract The effect of Sc and Zr addition on microstructure and hardness development in a commercial as-cast and solution-treated (530 °C/45 min) AA6082 alloy was studied. The electrical resistivity decrease and hardness increase in the temperature range 180 – 300 °C are caused by precipitation of β′′ and/or β′ needles of the Mg – Si system. Precipitation and morphology changes of Al – Mn – Fe system phases in AA6082 and AA6082-ScZr alloys influence the resistivity significantly, but have a negligible effect on hardness. The initial hardness of Sc and Zr containing AA6082 alloys is higher mainly due to the presence of Al3(Sc, Zr) particles. This difference is less pronounced in the peak-hardening state. Natural ageing of solution-treated alloys influences microstructure development in both alloys when annealed subsequently up to 240 °C.

Recovery during stress relaxation in Cd-Ag alloy single crystals

Stress relaxations in a series of cadmium-silver alloy single crystals containing up to 0·25 at% Ag were carried out at temperatures of 77 K, 145 K and 200 K. The samples were deformed in tension at a strain rate of 1×10−4 s−1. When the specimens deformed at 145 K and 200 K were reloaded after stress relaxation, the deformation started with the flow stress which was lower than that before stress relaxation. The observed differences in the stresses could be due to a variation in the internal stress. The activation volume should be estimated from the part of stress relaxation in which the internal stress remains practically constant.