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In this paper, in situ electron back scattered diffraction combined with scanning electron microscopy and digital image processing techniques were used to study the microscopic deformation compatibility of AZ31 Mg alloy rolled sheets with strong basal texture during biaxial tension at room temperature. The results firstly quantitatively showed that the distribution of microscopic strain in AZ31 rolled sheet was inhomogeneous during formation. Strain concentrations happened in some regions even at early stage of deformation. Short distorted bands also appeared in these regions and extended to connect each other with increasing macroscopic strain. The appearance of distorted bands play an important role in strain compatibility since the materials lack easy deformation modes at room temperature. Besides slips and twins, the grain distortion should also be an indispensable mechanism in accommodating plastic deformation in some parts in distorted bands. According to the analysis from the Schmid factor, the geometric compatibility factors and slip trace identification, the comprehensive effect of strong basal texture and biaxial tensile stress state not only led to the quite low SFs for prismatic , pyramidal slips and tensile twinning, but limited the compatibility between slip systems and tensile twins. This mechanism was the main reason for the lack of easy deformation modes and the appearance of distorted bands, which further led to the inhomogeneous strain distribution at room temperature. Therefore, the formability of Mg alloys rolled sheets with strong basal texture was weakened at room temperature.

The influence of Al (0.5wt.%, 1wt.%, 2wt.%) on the microstructural evolution and creep resistance of Mg-2.85Nd-0.92Gd-0.41Zr-0.29Zn (El21) alloy was systematically investigated. The creep results revealed that the additions of 0.5wt.% and 1wt.% Al significantly decreased the creep rate of El21 by more than an order of magnitude, whereas 2wt.% Al in El21 led to the reduction of creep properties. Microstructural analyses indicated that the additions of 0.5wt.% and 1wt.% Al led to significant grain coarsening due to the consumption of Zr via the formation of Al2Zr3 and Al2Zr phases. In contrast, the addition of 2wt.% Al caused distinct grain refinement, resulting from the additional formation of lumpy Al2RE in the centre of α-Mg grains. Additionally, the increase of Al content in the El21 gradually led to the disappearance of the Mg3RE phase and left Al2RE as the only dominant phase. The main Al–Zr phase was also changed from Al2Zr3+Al2Zr to Al2Zr phase. Creep data analysis showed that the dominant creep mechanism was dislocation creep for all alloys, which was in agreement with the EBSD and TEM characterizations. The enhanced creep resistance via the addition of 0.5wt.% and 1wt.% Al was ascribed to the high area fraction of intermetallic phases and the additional formation of the thermally stable Al2RE phase. El21 + 0.5Al has better creep resistance than El21+1Al, which was attributed to its stronger dynamic precipitation strengthening from γ precipitates. The deteriorated creep properties caused by adding 2wt.% Al in El21 alloy arose from the bimodal inhomogeneous distribution of grains and the laminar Al2RE phase. Such microstructure might cause significant stress concentrations and could not effectively impede dislocation motion or reinforce the grain/dendritic boundaries during creep, thus deteriorating the creep properties of El21+2Al.

Microstructure and texture evolutions during tensile and compression deformation of an as-extruded Mg–11Gd–4.5Y–1Nd–1.5Zn–0.5Zr (wt%) alloy have been investigated using in-situ synchrotron radiation diffraction and subsequent microscopy. The alloy consists of 〈View the MathML source101¯0〉 fiber texture, {View the MathML source112¯0}[0001] and {View the MathML source112¯0}〈View the MathML source101¯0〉 texture components prior to deformation. The texture evolves from [0001] to 〈View the MathML source101¯0〉 in tension, but from 〈View the MathML source101¯0〉 to [0001] in compression. The evolution of texture is attributed to the activity of the tensile twinning and non-basal 〈a〉 type slip. The tendency of texture evolution depends on the favorable texture component for the activation of above deformation modes. The grain refinement, Mg5(Gd, Y, Nd) and LPSO phases, and the texture contribute to the improvement in strength. The authors thank Dr. Jan Bohlenand Dr. Sangbong Yi for their fruitful discussion. Prof. Florian Pyczak and Mr. Uwe Lorenzare acknowledged for the provision of access to the TEM facilities at Helmholtz-Zentrum Geesthacht. The access to the beam line PETRA III at DESY, Hamburg, Germany was provided under the proposal number I-20130153,andtheworkofN. Schellis greatly appreciated. This work is supported by the National Key Technologies R&D Program(2012BAE01B04,2012DFH50100,KGFZD-125-13-021,201001C0104669453). Zijian Yu would like to thank the Chinese Academy of Sciences and German Academic Exchange Service (CAS-DAAD)scholarship program (Grant Number: A/12/94735)for the financial support.

In previously published research, creep resistance of commercial alloy Elektron21 (El21) and El21 + 1% AlN/Al nanocomposite were predominantly investigated in as-cast condition, little work focused on creep resistance following heat treatment. In this work, El21 and its nanocomposite with and without T6 treatment (520 °C for 8 h and 200 °C for 16 h) were prepared to reveal the influence of heat treatment on their microstructural evolutions and creep properties. Different intermetallic particles and precipitates that formed in El21 and El21 + 1% AlN/Al with different states were characterized using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). Creep tests were performed over a stress range of 80–140 MPa at 240 °C. Creep results showed that the application of T6 treatment could improve the creep resistance of El21, but deteriorate that of El21 + 1% AlN/Al. This is attributed to the reduced amount of γ'' and β′ precipitates in El21 + 1% AlN/Al (T6) after ageing, resulting from the formation of plate-like Al2(Nd, Gd) (Al2RE) precipitates. It is also found that after T6 heat treatment, El21 (T6) had a lower minimum creep rate with a shorter duration of secondary creep stage than El21 + 1% AlN/Al (T6) at high creep temperatures due to the overageing of precipitates and the thermal stability of the Al2RE particles. El21 + 1% AlN/Al nanocomposites, either in the as-cast or T6 condition, show a much longer duration of secondary creep than NP-free El21. The responsible mechanism was attributed to the addition of AlN NPs and the formation of particulate/plate Al2RE phase.

Modification of the recently developed creep-resistant Mg-3Gd-2Ca (wt.%) magnesium alloy using Nd and Mn remarkably improved the creep resistance at both 180 °C and 210 °C . The modified Mg-2Gd-1Nd-2Ca-0.5Mn alloy after solid solution treatment exhibited outstanding creep resistance that is superior to the commercial creep-resistant Mg alloy, EV31, but contained less RE addition. The microstructural observations revealed that partial replacement of Gd with 1 wt% Nd did not enhance the effect of dynamic precipitates significantly. But further analysis by atom probe tomography verified the stronger co-segregation between Nd solute atoms and other solute atoms than that of Gd and Ca in the Mg solid solutions, leading to a higher solid solution strengthening effect on the creep resistance. The addition of 0.5 wt% Mn led to the formation of polygonal-shape α-Mn precipitates, which served as heterogeneous nucleants for dynamic precipitates, refining their size and increasing the number densities. As a result, the creep properties of newly developed Mg alloys were strengthened by a combination of improved solid solution strengthening by Nd and increased precipitation hardening by Mn addition.

The present work studies the effects of solid solution Gd on the texture and lattice strain evolution of an extruded Mg15Gd alloy under uniaxial compression. In situ experiments were carried out using high energy X-ray diffraction on samples of the investigated materials with three different orientations. The original textures of the pure Mg and the Mg15Gd alloy exhibit basal planes that are preferentially parallel and perpendicular to the extrusion direction (ED), respectively. The c/a ratio of the Mg15Gd alloy decreases with increasing Gd content in the solid solution, leading to a different deformation behavior compared with pure Mg under the compressive load. The addition of Gd enhances the slip and twinning modes. However, prismatic slip is activated earlier in the Mg15Gd alloy due to the lower c/a ratio.

Effects of 0.25% Al addition and the high shearing dispersion technique (HSDT) on the microstructure and creep resistance of Mg-2.85Nd-0.92Gd-0.41Zr-0.29Zn (Elektron21, El21) alloy were investigated. Compressive creep testes were performed at 240 °C over a stress range between 80 and 140 MPa. The results indicate that the creep resistance of El21 was significantly improved by about one order of magnitude with the addition of 0.25% Al and HSDT than that of El21 without Al and HSDT. Microstructural characterizations show that the grains were coarsened from 80.1 ± 5.0 to 167.0 ± 5.7 μm due to the loss of grain refiner Zr by the chemical reaction of Al with Zr. With the addition of 0.25% Al to El21, the grain morphology was changed from equiaxed to typical dendrite. The morphology of intermetallic Mg3RE was modified from network to a pronounced dendritic structure. Owing to the employment of HSDT, the dendrite arm spacing of primary α-Mg was refined from 74.0 ± 6.4 to 56.2 ± 1.6 μm in Al-containing El21 alloy, the dominant Al–Zr compound changed from Al2Zr3 to Al2Zr phase. A small amount of Al2Nd phase was formed. Creep data analysis indicates that the dominant controlling mechanism for H-El21A alloy is dislocation climb during creep deformation. Its superior creep resistance was mainly attributed to the pronounced and dense dendritic structure of Mg3RE intermetallic in the α-Mg hindering the grain boundary sliding and dislocation movement. In addition, the relative homogeneous dispersion of Al2Zr phase and Al2Nd phase with a high melting point can also act as efficient reinforcements to inhibit the dislocation movement and benefit the creep properties.

The development of creep-resistant magnesium alloys that avoid the use of rare-earth alloying elements is an important area of research. The creep response of Mg–Al–Ca alloy containing barium (DieMag422) was compared to that of commercially available creep resistant magnesium alloys AE42 and MRI230D. The creep tests were performed between 175 °C and 240 °C at stresses between 60 MPa and 120 MPa. From the temperature and stress dependence of the minimum creep rate, the apparent activation energy Q c and the stress exponent n for creep were calculated. The concept of a threshold stress was applied. True stress exponents n t close to 5 were calculated. Microstructural investigations and phase analysis were performed on the as-cast materials as well as after creep. Fine precipitates could be identified that justified application of the concept of threshold stress. The DieMag422 alloy shows an improvement in creep resistance at low stresses compared with the other two alloys AE42 and MRI230D.

A Mg–11Gd–4.5Y–1.5Zn–1Nd–0.5Zr alloy with high-strength and heat-resistance has successfully been prepared by hot extrusion and subsequent ageing. It exhibits an ultimate tensile strength of 473 MPa, 0.2% proof stress of 373 MPa and elongation to failure of 4.1% at room temperature. At 250 °C, this alloy shows an ultimate tensile strength of 369 MPa, 0.2% proof stress of 316 MPa and elongation to failure of 6.3%. Its good mechanical properties and thermal stability are attributed to the dispersion of large volume fraction of 14H-LPSO phase, small α-Mg grains, basal-fiber texture and dense distribution of precipitates at the grain boundaries and inside the grains.