Significance of stable and metastable phases in high temperature creep resistant magnesium–rare earth base alloys

Smola, Bohumil; Stulı́ková, Ivana; Pelcová, Jitka; Mordike, B. L.

doi:10.1016/j.jallcom.2003.10.099

Cited by 119 publications

(27 citation statements)

References 20 publications

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“…In the present alloy, the dislocation density increases dramatically at the early stage, and after 10 and 16 turns, the saturated dislocation density reaches ~3.2×10 14 m -2 . The increased dislocation density for the present Mg-Gd-Y-Zn-Zr alloy is thought to be related to the combined addition of Gd, Y and Zn elements which decreases the stacking fault energy (SFE) [38,39]. Increased width of stacking faults makes dislocation cross-slip and climb more difficult, which makes dynamic recovery slow and difficult, leading to an increased dislocation density and high strain hardening effect in the first several HPT turns.…”

Section: Discussionmentioning

confidence: 92%

Evolution of microstructure and mechanical properties of an as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion

Sun

Qiao

et al. 2017

Materials Science and Engineering: A

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Abstract:The novel Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy with high content of rare earth elements has been processed successfully by high pressure torsion (HPT) starting from an as-cast condition. HPT processing was conducted at room temperature for a range of turns from 1/8 to 16, and the evolutions of microstructure and microhardness were investigated.The average grain size decreases from ~85 μm in the as-cast condition to ~55 nm when the equivalent strain reaches ~6.0, and remains almost constant on further strain increase. Meanwhile, the coarse netlike Mg3(Gd,Y) second phase structures are gradually broken into fine dispersed particles and the dislocation density increases. The microhardness of the alloy increases with increasing strain, and when the equivalent strain reaches ~6.0, the microhardness reaches a saturated value of about 115 HV, which is higher than that obtained by conventional extrusion / rolling of this alloy. The full range of possible mechanisms of hardening are analysed and this reveals that hardening is primarily due to the pronounced grain refinement, which is substantially 2 stronger than that for HPT-processed conventional Mg alloys, and to the homogeneously distributed fine second phase particles and the high dislocation density.

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Section: Discussionmentioning

confidence: 92%

Evolution of microstructure and mechanical properties of an as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion

Sun

Qiao

et al. 2017

Materials Science and Engineering: A

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“…[1][2][3][4][5][6] The Mg-GdNd alloy with minor addition of Zr as the grain refiner [4,5] is one of the Mg-RE alloys with the excellent strength and creep-resistance at elevated temperatures. For the advanced development of the Mg-Gd-Nd-Zr alloys, knowledge of the phase equilibria in the ternary Mg-Gd-Nd system is of fundamental importance, as the Mg-Gd-Nd system is one of the most important sub-system in the Mg-Gd-Nd-Zr system.…”

Section: Introductionmentioning

confidence: 99%

Solid-State Phase Equilibria of the Mg-Gd-Nd System at 500 °C

Huang

et al. 2015

J. Phase Equilib. Diffus.

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The phase equilibria at 500°C of the Mg-Gd-Nd system were investigated via a combination of a diffusion couple technique and the equilibrated alloys method, by means of the electron probe microanalyses and x-ray diffraction technique. It was revealed that continuous solid solutions of (Gd,Nd)Mg and (Gd,Nd)Mg 3 form between GdMg and NdMg and between GdMg 3 and NdMg 3 , respectively. No ternary compound was found. 3 three-phase equilibria, i.e. (Mg)+Nd 5 Mg 41 +GdMg 5 , Nd 5 Mg 41 +GdMg 5 +(Gd,Nd)Mg 3 and (Gd,Nd)Mg 3 +GdMg 2 +(Gd,Nd) Mg, were well determined. In addition, the homogeneity ranges of the binary phases in the MgGd and Mg-Nd systems were also well measured. The metastability of the NdMg l2 and GdMg 7 phases were confirmed and the isothermal section at 500°C was constructed in the present work.

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“…Manganese appears as the next suitable candidate, although classic experiments [14,155] [12,156] showed that the binary alloys were very hard to cast and the results were inconclusive.…”

Section: Heat Treatable Alloysmentioning

confidence: 99%

Thermodynamics-based design of creep resistant Mg solid solutions using the Miedema scheme

Abaspour¹

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Most current creep resistant alloy design methods for Mg alloys aim at incremental strengthening of existing alloys such as AZ91 thorough addition of ternary and quaternary elements, or alternatively through RE's additions. The most remarkable improvements has been through either Mg-Y or recently developed Mg-Gd-Y-Zr and Mg-Y-Nd-Zr alloys, although still below the strength of MgTh alloys, which are being phase out due to Th's radioactivity.Many micromechanisms proposed to account for the observed improvements in either hardness or creep strength of Mg alloys including solid solution and/or precipitation hardening, dynamic precipitation, solute dragging/solute clustering and more importantly atomic size controlled diffusivity more often than not are little more than ad-hoc, after-the-fact explanations, valid at best for the particular system under consideration. Hence, extrapolation to other alloy-systems is either not possible or contradictory. In practice, a feasible path for systematic, either precipitationhardened or creep-resistant Mg alloys, selection and design is still lacking.The aim of the present work was to use atomic level thermodynamical arguments that suggest that extending the athermal regime through short range order (SRO) is a most feasible path to increasing the creep strength of Mg alloys. Potential solutes were sorted out and ranked based on their tendency to developing SRO using the Miedema's phenomenological scheme. The predictions were corroborated by experiments involving dilute binary alloys as well as through a collection of data from the literature.Monotonic compression and stress relaxation tests were carried out on specimens of 6 cast binary alloys with (at.%) 2.5 Al, 0.6 Sn, 2.2 Zn, 0.8 Gd, 1.3 Y and 0.9 Nd, and of a similarly cast AZ91D alloy for reference. The solute concentration in the binary alloys was kept deliberately low to limit precipitation hardening effects during the testing. Compression testing was carried out at 298K (25°C), 373K (100°C) and 453K (180°C) The relative tendency or lack of it, of most solutes to develop SRO rationalizes a number of observations in current multicomponent Mg alloys while it disputes the viability of several other micromechanisms often considered.The feasibility of strengthening HPDC Mg alloys through a combination of percolating eutectics and residual solute in solution, or, more generally, age hardenable alloys through either homogeneously nucleated, coherent with the Mg matrix, precipitates, or via internally ordered precipitates mimicking Mg-Th alloy system, is also considered by making parallels with either the Al-Zn or the Al-Cu alloy systems. Examples of these approaches were discussed using data from the literature, such as experiments on alloys combining Mn and Sc to introduce order in the soft Mn precipitates, or the use of Na to provide finely dispersed, homogeneously nucleated clusters with the aim of refining the subsequent precipitation in alloys such as Mg-Sn. III Declaration by authorThis thesis is composed of my or...

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Significance of stable and metastable phases in high temperature creep resistant magnesium–rare earth base alloys

Cited by 119 publications

References 20 publications

Evolution of microstructure and mechanical properties of an as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion

Evolution of microstructure and mechanical properties of an as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion

Solid-State Phase Equilibria of the Mg-Gd-Nd System at 500 °C

Thermodynamics-based design of creep resistant Mg solid solutions using the Miedema scheme

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