Search

The recovery (softening) in magnesium alloys with gadolinium (yttrium group REM) and in these alloys additionally alloyed with samarium (cerium group REM) is studied after age hardening and subsequent short-term annealing performed at a slightly higher temperature. The aging-induced hardening is found to decrease at a high annealing temperature, at which the recovery occurs. The recovery of the magnesium–gadolinium alloys decreases with increasing samarium content in them. Small samarium additives are shown to increase the strength properties and, simultaneously, to decrease the recovery of the Mg–Gd alloys during accidental short-term heating when the alloys are used as lightweight structural materials in practice.

The parameters of reversion after hardening aging of magnesium alloys containing two rare earth metals gadolinium (yttrium group) and samarium (cerium group) were determined at various ratios of their contents. The reversion was observed at short annealing at 250 and 300°С beginning with 5 min after preliminary aging at 200°С up to maximum hardening and consisted of significant softening of the alloys then. Measurements of the electrical resistance indicated that during softening the reverse dissolution of the rare earth metals in magnesium-based solid solution takes place with diminution of the quantity of hardening particles precipitated during aging beforehand. The reverse dissolution of the rare earth metals in solid magnesium increases with increase in the annealing temperature after aging from 250 to 300°С and with increase in the gadolinium to samarium ratio in the alloys.

The phenomenon of post-aging retrogression in magnesium alloys containing two rare-earth metals belonging to different groups, i.e., yttrium Y (the yttrium group) and samarium Sm (the cerium group), is investigated. It is shown that retrogression may occur in alloys with different proportions of yttrium and samarium aged at 200°C for maximum hardness and then annealed at somewhat higher temperatures (250 and 300°C). The process is possible due to decomposition of the supersaturated magnesium solid solution. The degree of the retrogression is shown to depend on the temperature and duration of the post-aging annealing and on the yttrium- to-samarium proportion in the alloys.

The joint effect of samarium and holmium on the properties of an IMV7-1 Mg–Y–Gd–Zr alloy in two different states, namely, homogenized and homogenized and subsequently aged at 200°C to the peak strength, has been studied at testing temperatures of 20 and 250°C. Samarium and holmium do not change the behavior of the Mg–Y–Gd–Zr alloys during aging; at the same time, the presence of ~4% Sm in the alloys accelerates the decomposition of the magnesium solid solution and increases their strength properties, and the presence of 2 to 5% Ho in the IMB7-1 alloys containing 4% Sm leads to a decrease in their strength properties.

The kinetics of decomposition of supersaturated magnesium solid solution and of the accompanying hardening of the latter at 200°C in cast alloy IMV7-1 of the Mg – Y – Gd – Zr system alloyed with two rare-earth metals, i.e., holmium and ytterbium, is studied. Holmium and ytterbium are introduced into IMV7-1 by substituting two other rare-earth metals, i.e., yttrium and gadolinium. It is shown that the presence of holmium and ytterbium instead of yttrium and gadolinium does not change the decomposition behavior of the supersaturated solid solution but decelerates this process and lowers the effect of age hardening.

The aging-induced recovery of binary magnesium alloys with various rare-earth metals (Y, Nd, Sm, Gd, Tb, Dy, Ho), in which the decomposition of the supersaturated magnesium solid solution is accompanied by substantial hardening, is studied. It is found that annealing of the alloys at 250°C, which were preliminary aged at 200°C to a peak hardness, leads to their softening; in this case, the hardness continues to decrease as the annealing time increases. Electrical resistivity measurements show that, as a result of annealing at 250°C, recovery takes place and is accompanied by partial dissolution of the decomposition products in the magnesium solid solution; after that, its decomposition continues. The degree of recovery in the alloys with yttrium-group rare-earth metals (Y, Gd, Tb, and Ho) is found to be higher than that for the alloys with cerium-group rare-earth metals (Nd and Sm); within the lanthanide series, the degree of recovery increases with the atomic number of a rare-earth metal.

Kinetics and phase transformations are studied for the previously unconsidered decomposition of supersaturated solid solution in magnesium-based alloys Mg–Dy–Sm. The decomposition of solid solution slows when there is an increase in % Dy: % Sm ratio in the alloys. Mg–Sm and Mg–Dy alloys grow harder when Dy and Sm are added to them, respectively. Products of decomposition in ternary Mg–Dy–Sm alloys are the same as in binary alloys Mg–Dy and Mg–Sm but contain combinations of Dy and Sm.

The mechanical properties of as-cast Mg–Dy–Sm alloys additionally alloyed with zirconium to refine grains in the as-cast state have been studied. The dysprosium and samarium contents are close to their mutual solubility in a magnesium-based solid solution at a homogenization temperature of 515°C, and their total content is 4–12 wt %. The mechanical properties of the alloys have been determined in the states homogenized after casting and aged after homogenization. It is found that the strength properties of the alloys increase to σu ≈ 290 MPa and the plasticity decreases to lower values at a relative elongation δ ≈ 3% as the total content of the rare-earth metals in the alloy increases. The Mg–Dy–Sm–Zr alloys are significantly hardened upon aging, and the age hardening effect increases with the rare-earth metal content in them.

The microstructure, the aging kinetics, and the strength properties of Mg–Y–Gd–Zr cast alloys, in particular, a samarium-alloyed IMV7-1 alloy, at room and high (250, 300°C) temperatures after homogenization without and with subsequent aging are studied. Alloying with samarium accelerates the decomposition of the supersaturated magnesium solid solution and enhances the properties of the Mg–Y–Gd–Zr alloys.

The effect of dysprosium added in the amounts such that it does not form an individual phase in equilibrium with solid magnesium on the decomposition of the supersaturated magnesium solid solution in Mg–Sm alloys is studied. The presence of dysprosium in Mg–Sm alloys is found to retard the decomposition of the supersaturated magnesium solid solution and to increase the hardening effect upon aging. When these alloys are aged, dysprosium is partly retained in the magnesium solid solution and partly enters into the compositions of the phases that form during the decomposition of the solid solution and are characteristic of Mg–Sm alloys.

Magnesium-base alloys attract a great attention as light structural materials, where economy of weight is of great importance. According to the last investigations, the highest strength properties of these alloys, can be obtained, when they contain some of the rare-earth metals. All of them belong to the yttrium group. Existence of the rare-earth metals in magnesium alloys enable to form in their structures small hard particles, which increase significantly strength of the alloys. The hard particles appear after the special heat treatments and should have the certain measurements and amounts. In this article the main results of some investigations are described, where the best conditions for the heat treatments of the magnesium alloys with high strength were established. The highest strength in the alloys was observed, when the hard particles were nano-scale in them with certain disposition in the structure.

The phase compositions of solid Mg–Sm–Dy alloys corresponding to the magnesium-corner region of the phase diagram are studied by optical and scanning electron microscopy, electrical resistivity measurements, and electron microprobe analysis. The obtained results allowed us to determine the joint solubility of samarium and dysprosium in solid magnesium at 500, 400, and 300°C; it decreases with decreasing temperature. The magnesium solid solution is found to be in equilibrium only with the Mg41Sm5 and Mg24Dy5 compounds, which are in equilibrium with the magnesium solid solution in the binary Mg–Sm and Mg–Dy systems.

The changes in the structure and properties of the high-strength magnesium alloy IMV7-1 of a Mg-Y-Gd-Zr system are studied after heating at various temperatures. The alloy is hot-worked before annealing to obtain wrought semifinished products. The high stability of a hot-worked structure in annealing is established. It indicates the possibility of using the alloy as a light structural material at elevated (up to 300°C) temperatures. As the annealing temperature increases to 400°C and the holding time is one or two hours, there is only a slight decrease in the hardness, characterizing the strength properties of the alloy. With a further increase in the annealing temperature, a sharp fall in the hardness is observed. Investigation of the structure shows recrystallization in the alloy after annealing at temperatures up to 400°C. After annealing at higher temperatures of 450 and 500°C, the abrupt fall in the hardness is accompanied by complete disappearance of the elongated deformed grains which exist in the hot-worked state and by a significant grain growth. After recrystallization, the alloy retains the ability to be significantly strengthened upon aging, resulting in decomposition of the magnesium-based supersaturated solid solution.

The effect of severe plastic deformation and subsequent aging on the structure and properties of cast and hardened magnesium alloys containing samarium and yttrium is studied. Severe plastic deformation leads to additional hardening both before and after aging. Hardening is achieved by the formation of nanocrystalline structure in quenched alloys or submicrocrystalline structure in cast alloys. Severe plastic deformation results in the rapid decomposition of a supersaturated magnesium solid solution and additional hardening due to the formation of nano-sized particles of hardening phases.

Mg - Al - Ca alloys positioned in the Mg - Al 2 Ca quasi-binary section are studied. It is shown that alloys of this system bearing the Al 2 Ca compound as a hardening phase can be used for developing structural alloys with elevated strength at 170°C and that the strength characteristics of the Mg - Al - Ca alloys can be improved by adding manganese.

Magnesium alloys Mg-Al-Ca-Mn having 1 wt % Al, up to 1.8 wt % Mn, and up to 1.6 wt % Ca are studied by optical microscopy, electron microprobe analysis, and electrical resistivity and microhardness measurements. The phase equilibria corresponding to the Al, Ca, and Mn concentration ranges under study were determined and sections of the isothermal tetrahedrons of the Mg-Al-Ca-Mn phase diagram in the Mn-rich region at 450 and 300°C have been constructed. As was found, manganese does not affect the decomposition kinetics of the magnesium supersaturated solid solution in ternary Mg-Al-Ca alloys but increases their hardness.

The effect of severe plastic deformation by high-pressure torsion on the structure and properties of aged magnesium alloys containing 2.8–5.5 wt % Sm (the maximum solubility of samarium in solid magnesium is 5.8 wt %) are studied. The severe plastic deformation leads to substantial strengthening caused by the formation of a submicrocrystalline structure along with strengthening caused by the decomposition of a supersaturated magnesium-based solid solution.