Period Stacking Order Phase Research Articles

Bimodal grain structure, phase transformation, and mechanical properties of a Mg-Gd-Y-Zn-Zr alloy during hot extrusion

In this study, we investigated the evolution of grain structures and second phases of a Mg-9.5Gd-4Y–2Zn-0.3Zr alloy during two-stage deformation that took place in the metal during hot extrusion, as well as their effects on its mechanical properties. We found that the alloy exhibited a bimodal grain structure consisting of equiaxed dynamic recrystallized (DRXed) grains and elongated unDRXed grains. As the extrusion process progressed, DRX occurred on a large scale, resulting in an increase in the proportion of DRXed grains from 17.8% to 93.2%, and a reduction in average grain size from 80.3 μm to 9.41 μm. The grain orientation also underwent a transition from a single orientation of an extrusion direction (ED) parallel to <0001> (i.e. ED//<0001>) to mixed orientations of ED//<01–10> and ED//<0001>. The primary mechanism of dynamic recrystallization (DRX) during extrusion involved particle-stimulated nucleation (PSN) accompanied by both continuous and discontinuous DRX. The Mg-9.5Gd-4Y–2Zn-0.3Zr alloy mainly contained intergranular 18R-long period stacking order (LPSO) phases and intragranular 14H-LPSO. As the extrusion process progressed, 18R-LPSO changed from blocky to lamellar to accommodate deformation, and β-Mg5(Gd,Y) phase particles dynamically precipitated along DRXed grain boundaries. The 14H-LPSO phase was found to be primarily distributed within coarse unDRXed grains. As deformation via extrusion progressed, the quantity of 14H-LPSO gradually decreased. Ultimately, with the gradual dissolution of Mg5(Gd,Y), 14H-LPSO reprecipitated within the recrystallized grain boundaries. As the extrusion process progressed, both the yield strength (YS) and elongation (EL) increased from 132.6 MPa to 287.6 MPa and from 10.8% to 15.5%, respectively. This was mainly attributed to the significant reduction in grain size and second phase size.

Phase transformation during solid-solution and aging treatment of a Mg-Gd-Y-Zn-Zr alloy containing LPSO and W phases

In this study, solid-solution and aging treatments were conducted on a hot-drawn Mg-9.5Gd-4Y–2Zn-0.3Zr alloy wire, which included long period stacking order (LPSO) phases and a Mg3Zn3RE2 (W phase) with a diameter of 4 mm. Subsequently, the phase transformations within the alloy were meticulously analyzed. Following solid solution treatment, the W-Mg3Zn3RE2 phase in the edge region almost completely dissolved, leaving behind blocky RE-rich phases. Conversely, the core region retained more of the original LPSO phase, resulting in significant microstructural differences between the core and the edge. The aging treatment induced a more pronounced hardening response in the core region, achieving a peak hardness of 136.33 HV at 96 h. By contrast, the edge regions exhibited a weaker aging response. Specifically, edge region 1 showed minimal fluctuations in hardness without significant hardening, while edge region 2 reached its peak hardness of 103.13 HV at 96 h. The differential hardening effects induced by aging treatment primarily stemmed from the formation and distribution of precipitate phases. Both the core region and edge regions initially precipitated β-Mg5Gd at the grain boundaries, with stacking faults (SFs) forming within the grains. However, dense and uniform precipitation of the β′-Mg7Gd phase also occurred within the core region. Therefore, controlling the precipitation of the W phase was crucial for tuning the mechanical properties of the Mg-9.5Gd-4Y–2Zn-0.3Zr alloy.

Microstructure and Properties of Mg-Gd-Y-Zn-Mn High-Strength Alloy Welded by Friction Stir Welding.

Mg-Gd-Y-Zn-Mn (MVWZ842) is a kind of high rare earth magnesium alloy with high strength, high toughness and multi-scale strengthening mechanisms. After heat treatment, the maximum tensile strength of MVWZ842 alloy is more than 550 MPa, and the elongation is more than 5%. Because of its great mechanical properties, MVWZ842 has broad application potential in aerospace and rail transit. However, the addition of high rare earth elements makes the deformation resistance of MVWZ842 alloy increase to some extent. This leads to the difficulty of direct plastic processing forming and large structural part shaping. Friction stir welding (FSW) is a convenient fast solid-state joining technology. When FSW is used to weld MVWZ842 alloy, small workpieces can be joined into a large one to avoid the problem that large workpieces are difficult to form. In this work, a high-quality joint of MVWZ842 alloy was achieved by FSW. The microstructure and properties of this high-strength magnesium alloy after friction stir welding were studied. There was a prominent onion ring characteristic in the nugget zone. After the base was welded, the stacking fault structure precipitated in the grain. There were a lot of broken long period stacking order (LPSO) phases on the retreating side of the nugget zone, which brought the effect of precipitation strengthening. Nano-α-Mn and the broken second phase dispersed in the matrix in the nugget zone, which made the grains refine. A relatively complete dynamic recrystallization occurred in the nugget zone, and the grains were refined. The welding coefficient of the welded joint exceeded 95%, and the hardness of the weld nugget zone was higher than that of the base. There were a series of strengthening mechanisms in the joint, mainly fine grain strengthening, second phase strengthening and solid solution strengthening.

Regulating the microstructures and creep behaviors of an LPSO-containing Mg alloy via heat treatment

The microstructures and creep behaviors of a hot-extruded Mg-Gd-Y-Zn-Zr alloy containing long period stacking order (LPSO) phases prepared by different heat treatment procedures were investigated in this work. Three samples were made, including Sample #1, which was in the as-extruded state; Sample #2, which was preserved at 500 °C for 12 h; and Sample #3, which was preserved at 500 °C for 12 h and 460 °C for 10 h. The average grain sizes of the heat treated samples were much larger than those of the as-extruded samples. The second phases included mainly blocky 18R LPSO phases, lamellar 14H LPSO phases and needlelike ZnZr compounds in the three samples, but Sample #2 had much fewer 14H LPSO phases than the other two samples. The grain boundary migration resulting from the small grain size induced the lowest creep resistance in Sample #1. In contrast, the hardening effects generated from the pyramidal <c+a> dislocations and the dynamic precipitation of dense β’ phases dramatically retarded the cross-slip, resulting in the highest creep resistance in Sample #2. Although the aging treatment introduced numerous 14H LPSO phases in Sample #3, they delayed the dislocation movements to a limited degree, and the creep resistance was inferior to that of Sample #2. Therefore, the creep resistance ultimately decreased in the order of Sample #2 > Sample #3 > Sample #1. It is suggested that the microstructures and creep behaviors of LPSO-containing Mg alloys can be well regulated through appropriate heat treatment, and this work can hopefully offer important guidance for the design of highly creep resistant Mg-Gd-Y-Zn-Zr alloys.

The role of zinc addition on the microstructural evolution and mechanical properties of wire-arc directed energy deposited Mg-Gd-Y-(Zn)-Zr alloys

Wire-arc directed energy deposition (WA-DED) demonstrates significant potential in the rapid and free forming of high-strength magnesium rare-earth (Mg-RE) alloys. This highlights a growing demand for tailored wire alloy compositions to promote engineering applications of this technology. In this work, the role of zinc addition on WA-DED processed Mg-Gd-Y based alloys has been thoroughly investigated by utilizing Mg-6Gd-3Y-0.5Zr (wt.%, GW63K) and Mg-6Gd-3Y-0.5Zn-0.5Zr (wt.%, GWZ631K) alloy wires as model materials. The microstructure evolution of these two alloys during deposition and heat treatment is revealed by multi-scale microstructure characterization techniques. The influences of Zn addition on mechanical properties of these deposited alloys are investigated comparatively. Obtained results indicate that addition of Zn element into Mg-Gd-Y based alloys promotes phase transformation between eutectic phases and long period stacking order (LPSO) phases during the WA-DED deposition process owing to the multiple thermal cycling. More importantly, the addition of Zn element increases the tendency of solidification shrinkage of the deposited metal. During high-temperature solid solution treatment, the presence of Zn is conducive to improving the thermal stability of the as-deposited grain boundaries and thus suppressing grain coarsening. After optimized T6 treatments, the strength of GW63K alloys increases to 374±7MPa, and is obviously higher than that (315±6MPa) of GWZ631K alloys. The addition of Zn element weakens the precipitation strengthening effect. This can be attributed to the formation of LPSO phases, which consumes a substantial amount of RE elements, and in turn decreases the density of aging precipitated nanoscale β´ phases. This work promotes the understanding of the non-equilibrium solidified microstructure evolution of multi-component Mg-RE alloys and guides the optimization of alloy composition of WA-DED specific wire materials.

Direct observation and characterization of a novel long period stacking ordered phase in GH4169 alloy

As is widely recognized, the Long Period Stacking Ordered (LPSO) phase is an important strengthening phase in the rare earth magnesium alloys. The LPSO phase not only enhances the strength of the alloy, but also improves its plasticity, thereby effectively addressing the strength-ductility trade-off phenomenon. Therefore, the preparation of metallic materials containing LPSO phase provides a new idea for designing a new generation of high-strength and high-toughness alloys. In present study, A novel nano-sized LPSO phase is observed and characterized for the first time in GH4169 nickel-based alloy. The results show that the stacking sequence of the LPSO phase is ABCBCBA, which indicates that this LPSO phase has a 6H structure. Meanwhile, the orientation relationships between LPSO phase and the Ni matrix are (11–1)γ//(0006)LPSO and [011]γ//[2-1-10]LPSO.

Microstructure and mechanical properties of extruded and rolled Mg–Y–Zn–Ni–Co alloy

The as-cast Mg–2Y-0.5Zn-0.5Ni-0.5Co alloy (MYZNC, at%) underwent composite hot working, extrusion and rolling to further improve the mechanical properties, and the corresponding microstructure and mechanical properties were also investigated. The extruded and rolled MYZNC alloy (MYZNC-R) consisted of α-Mg, a long period stacking order (LPSO) phase comprising lamellar and fine block 18R LPSO phase as well as a thin strip 14H LPSO phase, along small amounts of Mg2Y, MgY(Co,Zn,Ni)4 and Mg3(Co,Zn,Ni,Y) particle phases. After composite processing, the dispersion and amounts of LPSO phase in the alloy was significantly enhanced, accompanied by a substantial introduction of low angle grain boundaries (LAGBs) during rolling process. The MYZNC-R alloy exhibited the high mechanical properties, with an ultimate tensile strength (σb) of 350 MPa, a yield strength (σ0.2) of 305 MPa and an elongation to failure (εf) of 10.6 %. In comparison to the extruded MYZNC alloy, both σb and σ0.2 demonstrated significant improvements by 25 % and 35.6 %, respectively. The elongation to failure only decreased by mere 1 %, indicating that the alloy had a good strength-plasticity. The high tensile strengths of MYZNC-R alloy were mainly attributed to grain refinement, LPSO phase strengthening and texture strengthening. The good ductility of MYZNC-R alloy was associated with the coordinated deformed LPSO phase and the formation of a high-volume fraction (∼76.5 %) of LAGBs.

Plastic contribution via DRX induced by kink and twin in a hot compressed Mg-Gd-Zn-Mn alloy with 14H LPSO

The plastic deformation behavior in Mg alloy with long period stacking ordered phases (LPSO) has not fully understood yet. To understand the interaction between twins, kink and dynamic recrystallization in a Mg alloy with long period stacking ordered phases, the plastic forming behavior under compression at elevated temperatures of the Mg-4Gd-1Zn-0.5Mn alloys containing lamellar long period stacking order phases were investigated. The lamellar long period stacking ordered phases hindered the occurrence of grain boundary migration, thus delaying dynamic recrystallization. The dominated deformation mechanisms of the compressed alloy were slip and discontinuous dynamic recrystallization accompanied by the occurrence of kink bands at low deformation rates. The twinning was the domination deformation mechanisms of the alloy at high deformation rates, a few recrystallized grains distributed along the boundaries of the twins and it was an obvious twin-induced dynamic recrystallization mechanism. The dissolving of long period stacking ordered phases reduced the grain boundary migration resistance of dynamic recrystallization and thus promoted the nucleation of the recrystallized grains at 500 °C. The broken small-sized lamellar long period stacking ordered phases became effective heterogeneous nucleation sites and thus promoted DRX. The parallel and beak-like kink bands are clearly observed in the microstructures of the deformed alloys. The severe kink behavior improves the plastic formability of the alloy.

Investigations on microstructure and mechanical properties of cast Mg-Gd-Y-Zn-Zr-Nd alloy

Different solution treatments were carried out on the semi-continuously cast Mg-9.55Gd-2.27Y-1.28Zn-0.55Zr-0.11Nd (wt.%) alloy at temperatures of 480 °C, 500 °C, and 520 °C. It was found that the different solution treatment temperatures led to different phase constitutions of the alloy. The Mg3RE-type eutectic phases transform into long period stacking order (LPSO) phases with various morphologies after solution treatment. Increasing the solution temperature resulted in larger grain size, greater number of lamellar LPSO phases, and segregation of Zr element. Block LPSO phases also shifted to rod LPSO phase. After ageing treatment, the special spatial structure consisting of prismatic β′ precipitates, basal γ′ precipitates, and lamellar LPSO phases hindered dislocation movement effectively, leading to high strength.However, the accumulation of massive dislocations promoted the premature formation of cracks at the grain boundary and LPSO/Mg matrix interface, resulting in a rapid decrease in ductility of the alloy. After solution treatment at 480 °C for 12 h and aged at 200 °C for 48 h, the Mg-9.55Gd-2.27Y-1.28Zn-0.55Zr-0.11Nd alloy exhibited favorable comprehensive mechanical properties at room temperature: ultimate tensile strength (UTS) of 383 MPa, tensile yield strength (TYS) of 269 MPa, and fracture elongation (EL) of 5.1%.

Investigation on Thermodynamic Stability of 10H and 14H Long Period Stacking Ordered Phase in Mg-Zn-Y Alloy

In this paper, enthalpies of formation and electronic structures of long period stacking ordered phases in ternary Mg-Zn-Y alloys are calculated by first-principles. Through calculations of the enthalpies of formation, it was found that enthalpies of formation of 10H phase and 14H phase are negative, and the enthalpy of formation of 10H phase less than that of 14H phase. It shows that both phases can form stable structure. The energy of transformation calculations show that 14H phase is more stable than 10H phase. The analysis of electronic structure shows that the difference between the structural stability of 10H phase and 14H phase is due to the different number of bonding electrons at the lower energy level of Fermi level. More bonding electrons determine that 14H phase has better stability than 10H phase.

Effect of lamellar LPSO phase on mechanical properties and damping capacity in cast magnesium alloys

The effect of the lamellar long period stacking order (LPSO) phase on both mechanical properties and damping capacity of magnesium alloys is still vague. After heat treatment at 540 °C for 4 h and 450 °C for 10 h, the Mg-10Gd-2Y–1Zn-0.5Zr-0.2Nd (wt%) alloy with the lamellar LPSO phase is prepared to study the influence of lamellar LPSO phase on damping capacity and mechanical properties. Combined transmission electron microscopy (TEM) and scanning electron microscopy (SEM) equipped with electron back-scattering patterns (EBSD) system, it is surprisingly found that the growth direction of the lamellar LPSO phase on basal plane is nearly parallel to the slip direction of basal dislocations, which is benefit from dislocations movement on basal plane. The dislocations move on basal plane easily, lead to a relative high damping capacity. The alloy with LPSO phases shows higher ultimate tensile strength (UTS) and elongation (EL), increased from 234 MPa to 2.6% to 254 MPa and 4.6%, respectively. The lamellar LPSO phase has an attribution to UTS and EL due to its kink deformation mechanism. Therefore, the lamellar LPSO phase can be used as a functional phase to enhance the damping capacity and mechanical properties at the same time, and providing new ideas about preparing high-strength and high-damping magnesium alloy.

Microstructure and yield phenomenon of an extruded Mg-Y-Cu alloy with LPSO phase

A yield phenomenon was firstly reported in an extruded Mg-6.8Y-2.5Cu alloy and the corresponding microstructure was also investigated in this work. The cast alloy is mainly composed of α-Mg, 18R long period stacking order (LPSO) phase, eutectic phase (Mg20Cu4Y1), and Mg2Cu phase. The 18R LPSO phase at the dendritic grain boundary transforms into the 14H LPSO phase in the grain interior during homogenization. After extrusion, the grain size of the homogenized alloy is remarkably refined to –3.69 μm and the second phase is significantly broken and distributed in the extrusion direction. Tensile testing curves of the extrude alloy at room temperature indicate that the yield strength and ultimate tensile strength increase while the elongation of the alloy decreases with increasing strain rate. Interestingly, a yield plateau forms and gradually decreases with increasing strain rate. The yield phenomenon is related to the dislocation multiplication and the interaction between the movable dislocations and solute atoms.

On the microstructure and RE-texture evolution during hot tensile deformation of Mg-Gd-Y-Zn-Zr alloy

This study was conducted to investigate the correlation between the microstructure/texture evolutions and the high temperature flow behavior of Mg-8.3Gd-3.6Y-1.6Zn-0.5Zr (wt. %) alloy containing long period stacking order (LPSO) phases. Toward this end, the hot tensile tests were conducted at 350 and 400 °C on two different starting microstructures: (i) as-extruded containing blocky LPSO, and (ii) as-annealed, which contained both blocky and lamellar LPSO phases. The peak stresses of the annealed microstructures at 350 and 400 °C, i.e. 200 and 80 MPa, were almost twice that of extruded ones at the same temperatures, i.e. 117 and 39 MPa, respectively. The annealing treatment effectively strengthened the alloy and decreased the ductility value. This was justified relying on the grain growth, deformation twinning and texture evolution, but a special emphasize was paid on the effect of lamellar and blocky LPSO phases. It was found that the blocky LPSO could assist the dynamic recrystallization through activation of particle stimulated nucleation (PSN) mechanism and effectively contributed in the formation of rare earth (RE)-texture component. This provided high Schmid factors for operation of basal slip and improved the ductility values. On the contrary, the fine lamellar LPSO effectively increased the strength of the alloy via texture hardening but reduced the ductility. The lamellar LPSO played a key role in the formation and strengthening of the <10-10> fiber texture, retarded the recrystallization process and suppressed the formation of RE-texture component.

Analysis of the In Situ Crack Evolution Behavior in a Solid Solution Mg-13Gd-5Y-3Zn-0.3Zr Alloy.

The low plasticity of high strength Mg-Gd-Y alloy has become the main obstacle to its application in engineering. In this paper, the origin, propagation and fracture processes of cracks of a solution of treated Mg-13Gd-5Y-3Zn-0.3Zr alloy were observed and studied with scanning electron microscopy (SEM) in an in situ tensile test to provide theoretical references for the development of a new high-performance Mg-Gd-Y alloy. The results showed that there was still some bulk long period stacking order (LPSO) phase remaining in solid solution Mg-13Gd-5Y-3Zn-0.3Zr alloy. Most importantly, it was found that the locations of micro-cracks vary with the different solution treatment processes, mainly including the following three types. (1) At 480 × 10 h and 510 °C × 10 h, much bulk LPSO phase with higher elastic modulus remains in the alloy, which can lead to micro-cracks in the LPSO phase due to stress concentration. (2) At 510 °C × 13 h and 510 °C × 16 h, the phase structure of bulk LPSO changes, and the stress concentration easily appears at the LPSO/α-Mg interface, which leads to micro-cracks at the interface. (3) At 510 °C × 19 h and 510 °C × 22 h, the grain size increases, and the stress concentration is obvious at the grain boundary of coarse grains, which leads to the formation of micro-cracks.

Accelerated degradation rate of high-strength Mg-4Y-1Zn alloy by Cu addition for degradable bridge-plug applications

Abstract To find good candidate materials for degradable bridgeplugs used in shale oil or gas exploitation, a novel hot extruded Mg-4Y-1Zn-1Cu alloy with long period stacking order (LPSO) phase and excellent mechanical properties and rapid degradation rate was fabricated. Compared with the Mg-4Y-1Zn alloy, the compressive properties of Mg-4Y- 1Zn-1Cu alloy are dramatically enhanced by adding Cu, with a compressive strength of 480 MPa, which can be attributed to the formation of fine-grains and the strengthening LPSO phase distributed within the substrate. Furthermore, Mg-4Y-1Zn-1Cu alloy shows a rapid degradation rate as demonstrated by immersion tests and polarization curves, with about 30 times higher corrosion rate than Mg- 4Y-1Zn alloy, which can be attributed to the 14H-type LPSO phase containing Cu with high corrosion potential and high fraction grain boundaries.

Accelerated degradation rate of high-strength Mg-4Y-1Zn alloy by Cu addition for degradable bridge-plug applications

Abstract To find good candidate materials for degradable bridgeplugs used in shale oil or gas exploitation, a novel hot extruded Mg-4Y-1Zn-1Cu alloy with long period stacking order (LPSO) phase and excellent mechanical properties and rapid degradation rate was fabricated. Compared with the Mg-4Y-1Zn alloy, the compressive properties of Mg-4Y- 1Zn-1Cu alloy are dramatically enhanced by adding Cu, with a compressive strength of 480 MPa, which can be attributed to the formation of fine-grains and the strengthening LPSO phase distributed within the substrate. Furthermore, Mg-4Y-1Zn-1Cu alloy shows a rapid degradation rate as demonstrated by immersion tests and polarization curves, with about 30 times higher corrosion rate than Mg- 4Y-1Zn alloy, which can be attributed to the 14H-type LPSO phase containing Cu with high corrosion potential and high fraction grain boundaries.

A Review on Developments in Magnesium Alloys

In order to facilitate the understanding of the current research efforts and directions, this article first introduces the anomalous/problematic features of magnesium, and presents the recent approach of stacking fault energy (SFE)-based alloying element selection to lessen or eliminate these problem. SFE computations via ab initio techniques necessitate an understanding of the free electron density distribution around atoms in a solid solution. Therefore, the assessment of the role of atoms by also considering the possibility of short range order (SRO) formation rather than a random solid solution has been revisited. Two possible types of SRO have been indicated. The relevant electronic interactions between the host Mg and the alloying element atoms are more clearly incorporated in, a generally less known model, by Miedema’s based on atomic level thermodynamics rather than in Hume-Rothery rules. This more successful approach has also been addressed here. An evaluation founded on these premises, introducing the relatively more recent magnesium alloy systems, has been given in terms of their achievements towards healing the problematic features of magnesium alloys. The spectrum of alloy systems discussed ranges from doping of magnesium to dilute alloy systems, and to some rich alloy systems that offer remarkable properties. Among the first category, an unorthodox addition, doping with oxygen, and its implications has been presented. The dilute alloy systems and their compositional design based on short range order (SRO) and SFE together with their potentials have been reviewed. Among the rich alloy compositions, the most interesting precipitate systems, i.e. the ones involving order and intermetallic formations, long period stacking order (LPSO) phases, and quasi-crystals have been discussed. Among all the alloying elements, one that deserves particular attention, with its implications such as being economical, offering environmentally friendly magnesium metallurgy, as well as remedial effects on the shortcomings of engineering properties, calcium and a closely related issue of CaO addition, have been scrutinized. This article also makes an attempt to point out the future directions throughout the text, whenever possible.

Effect of heat treatment on LPSO morphology and mechanical properties of Mg–Zn–Y–Gd alloys

The mechanical properties and microstructure were investigated under different Zn content and heat treatment conditions in a Mg–Zn–Y–Gd cast alloy. A part of the long period stacking order (LPSO) phases transformed to W -Mg 3 Zn 3 RE 2 phases with an increase in Zn content from 0.9 at.% to 1.8 at.%, and the ultimate tensile strength ( UTS) increased from 229 MPa to 248 MPa. With solution treatment at 480 °C, the content of the LPSO phase and strength sharply decreased in the Mg-1.8Zn-0.8Y-0.8Gd alloy, whereas this change was not significantly observed in the Mg–0.9Zn–0.8Y–0.8Gd alloy. After solution treatment, the elongation significantly improved and the UTS sharply decreased in both alloys. The lamellar and filminess LPSO phases were observed with aging treatment at 200 °C. Moreover, the strengthening efficiency of lamellar and filminess LPSO phases was lower than that of the block LPSO phases. Therefore, the UTS of the T6 state was lower than that of the as-cast alloy.

The Role of Long Period Stacking Ordered Phase in Dynamic Recrystallization of a Mg-Gd-Y-Zn-Zr Alloy during Multi-Directional Forging Process.

The Mg–Gd–Y–Zn–Zr alloy containing a long period stacking ordered (LPSO) phase was subjected to multi-pass deformation by means of a multi-directional forging process, and the microstructure evolution and the influence of the LPSO phase on its dynamic recrystallization (DRX) were studied. The results showed that multi-directional forging can effectively refine the grain with the DRX fraction increased, and DRXed grains lead to the decrease of the texture intensity, which can significantly improve the mechanical properties of the alloy. The different morphologies of the LPSO phase have different degrees of promotion relative to DRX behavior. The lamellar LPSO phase with kinks promoted dislocation plugging, where there could be a potential nucleation site for DRX grains. The fragmented lamellar LPSO phase promoted the DRX process through the particle-stimulated nucleation mechanism, and the block-shaped phase was more prone to stress concentration, which promoted DRX. These effects resulted in continuous grain refinement and a more uniform microstructure.

Enhanced mechanical properties and degradation rate of Mg–Ni–Y alloy by introducing LPSO phase for degradable fracturing ball applications

Abstract In this work, as-cast Mg–Ni–Y alloys were proposed to develop a feasible material for fracturing balls, and their mechanical performance and corrosion behavior were systematically investigated. Long period stacking order (LPSO) phase was firstly introduced to improve both the mechanical properties and degradation rate of magnesium alloys. With the increase of LPSO phase, the compressive strength was improved significantly, while the elongation of the alloys decreased owing to the relatively brittle nature of LPSO phase. Due to the higher corrosion potential of LPSO phase, the LPSO phase can accelerate the corrosion process by providing more micro-couples. However, the LPSO phase would serve as the corrosion barrier between the corrosion medium and the matrix when the contents of LPSO phase are too high in Mg92.5Ni3Y4.5 and Mg87.5Ni5Y7.5 alloys. As-cast Mg97.5Ni1Y1.5 alloy with satisfactory mechanical properties and rapid degradation rate was successfully developed, exhibiting a high degradation rate of 6675 mm/a (93 °C) in 3 wt.% KCl solution and a favorable ultimate compressive strength of 410 MPa. The degradation rate of Mg97.5Ni1Y1.5 alloy is 2–5 times of the current commercial magnesium alloy fracturing materials.