14H Long Period Stacking Ordered Phase Research Articles

A new insight into LPSO phase transformation and mechanical properties uniformity of large-scale Mg-Gd-Y-Zn-Zr alloy prepared by multi-pass friction stir processing

A large-scale fine-grained Mg-Gd-Y-Zn-Zr alloy plate with high strength and ductility was successfully prepared by multi-pass friction stir processing (MFSP) technology in this work. The structure of grains and long period stacking ordered (LPSO) phase were characterized, and the mechanical properties uniformity was investigated. Moreover, a quantitative relationship between the microstructure and tensile yield strength was established. The results showed that the grains in the processed zone (PZ) and interfacial zone (IZ) were refined from 50 µm to 3 µm and 4 µm, respectively, and numerous original LPSO phases were broken. In IZ, some block-shaped 18R LPSO phases were transformed into needle-like 14H LPSO phases due to stacking faults and the short-range diffusion of solute atoms. The severe shear deformation in the form of kinetic energy caused profuse stacking fault to be generated and move rapidly, greatly increasing the transformation rate of LPSO phase. After MFSP, the ultimate tensile strength, yield strength and elongation to failure of the large-scale plate were 367 MPa, 305 MPa and 18.0% respectively. Grain refinement and LPSO phase strengthening were the major strengthening mechanisms for the MFSP sample. In particularly, the strength of IZ was comparable to that of PZ because the strength contribution of the 14H LPSO phase offsets the lack of grain refinement strengthening in IZ. This result opposes the widely accepted notion that IZ is a weak region in MFSP-prepared large-scale fine-grained plate.

Activation of pyramidal II slips at room temperature in Mg–Zn–Y 18R and 14H long-period stacking ordered phases

The 18R and 14H long-period stacking ordered (LPSO) phases were generally considered that only the basal and non-basal <a> slips and kink banding could be activated during quasi-state deformation at room temperature. Here, we confirm that the pyramidal II <c+a> slips can also be activated in the 18R LPSO phase by quasi-state nanoindentation and in the 14H LPSO phase by cold roll with a rolling direction parallel to the (0001) plane. Due to the interaction between the Zn6Y8 atomic clusters in the LPSO phases and the pyramidal II <c+a> dislocations, the pyramidal II <c+a> slips are localized in the well-defined slip bands while the chemically ordered structure of Zn6Y8 atomic clusters is disrupted to be disordered. Additionally, the interaction also induces Zn/Y atoms segregation at the two edges of the slip band. Our results unravel the origin for the excellent room temperature plasticity in the LPSO phases.

Microstructure evolution during superplastic deformation process and its impact on superplastic behavior of a Mg-Gd-Y-Zn-Zr alloy

In this work, the microstructure evolution during superplastic deformation process and its impact on superplastic behavior of a peak-aged wrought Mg-10Gd-3Y-1.5Zn-1Zr (wt%) alloy were investigated by comparing the microstructure before and after high temperature tensile test (HTTT). The results show that except for the sample deformed at 400 °C with the strain rate of 1 × 10 −3 s−1, all the other samples exhibit superplasticity when deformed at temperatures between 400 °C and 475 °C with the strain rate from 1 × 10−3 s−1 to 5 × 10−3 s−1. The dominant superplastic deformation mechanism of the alloy is grain boundary sliding (GBS) controlled by grain boundary (GB) diffusion. In addition, three main changes of the microstructure are confirmed after HTTT compared with the peak-aged alloy, i.e., the disappearance of the Mg3Gd phase, the emergence of the Mg24Y5 phase, and the various volume fraction of 14H long period stacking ordered (LPSO) phase. Moreover, when deformed at 450 °C with the strain rate of 5 × 10−3 s−1, this alloy obtains the highest superplastic elongation (972%). This is due to the largest number of precipitated 14H LPSO and Mg24Y5 phases, as well as the fragmentation of the Mg24Y5 particles, which can delay the grain boundaries separation and improve the ability of the microstructure to accommodate more dislocations.

Formation of long-period stacking-ordered (LPSO) structures and microhardness of as-cast Mg-4.5Zn–6Y alloy

Long-period stacking-ordered (LPSO) structures formed in both the α-Mg component and the interdendritic component in as-cast Mg-4.5Zn–6Y-0.5Zr (wt.%) alloy were studied from micro to atomic scales. Inside the α-Mg component, stacking-ordered structure took three forms: single unit, scattered cluster, and a single layer of short range 18R structures. Chemical ordering of clusters of Zn and Y formed in the short range scale but not in the long range. Inside the interdendritic component LPSO-layered structure took two forms: 18R and 14H LPSO. Long-range 18R formed at locations with high concentrations of Zn and Y. 14H LPSO phase formed at the edge of any layer of 18R LPSO phase, sandwiched between 18R and α-Mg band. We concluded that the formation of LPSO had occurred in two distinct steps. First, Zn and Y atoms were segregated into two adjacent atomic layers of the original hexagonal close-packed matrix. Then, the nucleation of stacking fault occurred, followed by the growth of a stacking fault, which attracted more Zn and Y atoms into four constitutive close-packed layers, forming a new segment of an LPSO structural unit.

Effect of Zr Content on the Distribution Characteristic of the 14H and 18R LPSO Phases

Abstract Zirconium (Zr) is an essential element in Mg-Zn and Mg-Zn-Y system magnesium alloys. In this study, an interesting phenomenon that the content of Zr element could influence the size and the morphology of the long period stacking ordered (LPSO) phases, which has never been reported by previous works before. The Mg98.5-xZn0.5Y1Zrx (x =0, 0.1, 0.2 and 0.3 at. %) magnesium alloys were fabricated by directional solidification, and the effects of the Zr content on the distribution characteristics of the bulk LPSO phases (18R) and the lamellar LPSO phases (14H) were investigated. The directional solidification technology showed good controllability in LPSO phase’s distribution, and the morphology of LPSO phases in Mg98.5-xZn0.5Y1Zrx (x =0, 0.1, 0.2 and 0.3 at. %) alloys were observed clearly. The results showed that the amount and the morphology of the 14H and 18R LPSO phases within grains continuously decreased with the Zr content increasing. The continuous 14H lamellar structure changed to discontinuous. In addition, Zr element exhibited purification ability on the grain boundaries and refined effect on the 14H and 18R LPSO phases. This can be attributed to the influence of Zr atoms on stacking fault energy (SFE) and the attraction of Zr atoms to Mg atoms.

Effect of Zn addition on microstructure and mechanical properties of cast Mg-Gd-Y-Zr alloys

Abstract The effects of Zn addition on the microstructure and mechanical properties of Mg-10Gd-3Y-0.6Zr (wt.%) alloys in the as-cast, solution-treated, and peak-aged conditions were investigated. Experimental results reveal that the microstructure of the as-cast alloy without Zn consists of α-Mg and Mg24(Gd,Y)5 phases, and the alloy with 0.5 wt.% Zn consists of α-Mg, (Mg,Zn)3(Gd,Y) and Mg24(Gd,Y,Zn)5 phases. With the addition of Zn increasing to 1 wt.%, the Mg24(Gd,Y,Zn)5 phase disappears and some needle-like stacking faults distribute along the grain boundaries. Moreover, the 18R long-period stacking ordered (LPSO) phase is observed in the as-cast alloy with 2 wt.% Zn. After solution treatment, the Mg24(Gd,Y)5 and Mg24(Gd,Y,Zn)5 eutectic phases are completely dissolved, and the (Mg,Zn)3(Gd,Y) phase, needle-like stacking faults and 18R LPSO phase all transform into 14H LPSO phase. Both the suitable volume fraction of 14H LPSO phases and the fine ellipsoidal-shaped β′ phases make the peak-aged alloy with 0.5 wt.% Zn exhibit excellent comprehensive mechanical properties and the UTS, YS and elongation are 338 MPa, 201 MPa and 6.8%, respectively.

Role of LPSO Phase in Crack Propagation Behavior of an As-Cast Mg-Y-Zn Alloy Subjected to Dynamic Loadings.

In this work, the role of long period stacking ordered (LPSO) phase in the crack propagation behavior of an as-cast Mg95.5Y3Zn1.5 alloy was investigated by dynamic four-point bent tests. The as-cast Mg95.5Y3Zn1.5 alloy is mainly composed of Mg matrix, 18R LPSO phase located at the grain boundaries and 14H LPSO phase located within the Mg matrix. The alloy exhibits excellent dynamic mechanical properties; the yield stress, maximum stress and strain to failure are 190.51 ± 3.52 MPa, 378.32 ± 4.26 MPa and 0.168 ± 0.006, respectively, at the strain rate of ~3000 s−1. The LPSO phase effectively hinders dynamic crack propagation in four typical ways, including crack tip blunting, crack opening inhibition, crack deflection and crack bridging, which are beneficial to the mechanical properties of the alloy under dynamic loadings.

Structural features and mechanical properties of Mg-Y-Zn-Sn alloys with varied LPSO phases

The present work explored the microstructure and mechanical properties of Mg-Y-Zn-Sn alloys with varied long period stacking ordered (LPSO) phases, including 18R and/or 14H LPSO phases, which were obtained by controlling the Y content in alloys. The results show that the lamellar-shaped 14H LPSO phase can greatly refine the average grain size of alloys from 60 μm in as-annealed state to 3 μm in extrusion state through the combined effects of kink bands, interspaces and dense stacking faults (SFs) occurred during dynamic recrystallization (DRX), while the existence of dense 18R LPSO phase can only reduce the grain size from 20 to 10 μm for an obviously restricted effect in DRX process. It is interesting to find that the alloy containing 18R LPSO phase exhibits higher strength but lower ductility in both as-annealed and as-extruded states than that with lamellar-shaped 14H LPSO phases, and the relevant mechanism has been discussed.

Coexistence of 14H and 18R-type long-period stacking ordered (LPSO) phases following a novel orientation relationship in a cast Mg−Al−RE−Zn alloy

Coexistence of 14H and 18R-type long-period stacking ordered (LPSO) phases was observed in a cast Mg−Al−RE−Zn alloy, following a novel orientation relationship of (0001)14H about 70.5° from (001)18R and [11¯00]14H//[100]18R. Transmission electron microscopy (TEM) observations reveal that this novel structure was formed because of the concurrence of the precipitation of 18R structure in the lumpy Mg4.8Al12RE7.2 phase and the heterogeneous nucleation of 14H structure on the coherent Mg/Mg4.8Al12RE7.2 interface during isothermal heat treatment at 300 °C. This paper gives an in depth understanding of the new formation mechanisms of the coexistence of both 14H and 18R-type LPSO phases.

Microstructure and mechanical property of Mg–10Gd–2Y–1.5Zn–0.5Zr alloy processed by eight-pass equal-channel angular pressing

In this work, a high-strength Mg–10Gd–2Y–1.5Zn–0.5Zr (wt%) alloy was prepared via eight passes of equal-channel angular pressing (ECAP). The microstructures and mechanical properties of as-cast and ECAP alloys were systematically investigated by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electronic universal testing machine. The obtained results indicate that the microstructure of as-cast alloy consists of α-Mg dendrite, network Mg3(Gd,Y,Zn) phase and lamellar 14H long-period stacking ordered (LPSO) phase which is precipitated near the boundary of Mg3(Gd,Y,Zn) networks. After eight-pass ECAP, the network Mg3(Gd,Y,Zn) phase is deformed and broken. However, the refined Mg3(Gd,Y,Zn) particles are not distributed uniformly in the matrix, but still aggregated at the interdendritic area. Moreover, the content of 14H lamellas increases obviously, and they become bent and kinked during severe deformation. DRX is activated in the region between Mg3(Gd,Y,Zn) particles and 14H clusters. Compression test at room temperature indicates that the ECAP alloy exhibits excellent mechanical property with compressive strength of 518 MPa and fracture strain of 21.6%. The comprehensive high strength and toughness could be ascribed to the refined Mg3(Gd,Y,Zn) particles, DRX grains and kinked 14H LPSO phase.

Preparation, Microstructure Evolutions, and Mechanical Property of an Ultra-Fine Grained Mg-10Gd-4Y-1.5Zn-0.5Zr Alloy

In this work, the microstructural evolutions and mechanical properties of an as-cast Mg-10Gd-4Y-1.5Zn-0.5Zr (wt %) alloy during successive multi-pass equal channel angular pressing (ECAP) were systematically investigated by X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, and compression test. The obtained results show that the microstructure of as-cast alloy consists of α-Mg grains, Mg3Gd island phase, few Y-rich particles, and lamellar 14H LPSO (long period stacking ordered) phase located at the grain boundaries. During ECAP, the Mg3Gd-type phase is crushed and refined gradually. However, the refined Mg3Gd particles are not distributed uniformly in the matrix, but still aggregated at the interdendritic area. The 14H phase becomes kinked during the early passes of ECAP and then broken at the kinking bands with more severe deformation. Dynamic recrystallization of α-Mg is activated during ECAP, and their average diameter decreases to around 1 μm, which is stabilized in spite of increasing ECAP passes. Moreover, nano-scale γ′ phases were dynamically precipitated in 16p ECAP alloy. Compression tests indicate that 16p ECAP alloy exhibits excellent mechanical property with compressive strength of 548 MPa and fracture strain of 19.1%. The significant improvement for both strength and ductility of deformed alloy could be ascribed to dynamic recrystallization (DRX) grains, refined Mg3Gd-type and 14H particles, and dynamically precipitated γ′ plates.

The influence of minor boron on the precipitation behavior of LPSO phase and dynamic recrystallization in the Mg94Y2.5Zn2.5Mn1 alloys

The influence of minor boron on the microstructure and mechanical properties of solution-treated and as-extruded Mg94Y2.5Zn2.5Mn1 alloys has been investigated in this paper. The mechanisms of long period stacking ordered (LPSO) phase transformation during solid-solution treatment and dynamic recrystallization (DRX) in the process of extrusion were analyzed. Experimental results demonstrate that the boron addition effectively promotes the precipitation of 14H LPSO phase and reduces the area fraction and average size of the W phase particles in the solution-treated Mg94Y2.5Zn2.5Mn1 alloy. Furthermore, boron delays the dynamic recrystallization process due to increased 14H LPSO phase during extrusion process. And two kinds of DRX were observed in as-extruded alloys. Finally, the boron addition improves the mechanical properties of both the solution-treated and as-extruded Mg94Y2.5Zn2.5Mn1 alloys. Especially, the as-extruded Mg94Y2.5Zn2.5Mn1 alloy with 0.003wt% boron addition exhibited optimal mechanical performance with ultimate tensile strength (UTS) and elongation of 389.7MPa and 24.9%, respectively.

Stretching the engineering strain of high strength LPSO quaternary Mg-Y-Zn-Al alloy via integration of nano-Al2O3

In the present study, an attempt is made for the first time to reinforce long-period stacking ordered (LPSO) MgY1.06Zn0.76Al0.42 (at.%) alloy with 0.5, 1.0, and 1.5 vol% of nano-Al2O3 particles to form nanocomposites. Microstructure characterization revealed the ability of nano-Al2O3 in inhibiting the formation of 14H LPSO phases in the nanocomposites during solidification. Homogenization at 723 K (450 °C) for 2 h led to the subsequent precipitation of fine Mg-Y-Zn-Al precipitates (≤1 µm) in the nanocomposites. The fine Mg-Y-Zn-Al precipitates and nano-Al2O3 particles were established to be active in promoting dynamic recrystallization (DRX) of α-Mg via particle-simulated nucleation during extrusion, which was responsible for weakening the basal texture in the nanocomposites and improving failure strain. As a result, failure strain was significantly increased from 10.8 % in the monolithic alloy to beyond 15 % in the nanocomposites with the highest strength among nanocomposites achieved in NC5 (nanocomposite reinforced with 0.5 vol% of nano-Al2O3 particles).

Negative Strain-Rate Sensitivity of Mg Alloys Containing 18R and 14H Long-Period Stacking-Ordered Phases at Intermediate Temperatures

The strain-rate sensitivity of a Mg-10Dy-1Zn (wt pct) alloy containing different long-period stacking-ordered (LPSO) phases has been investigated in the strain rate range of 10−3 to 1 s−1 from room temperature to 673 K (400 °C). Both alloys containing 18R-LPSO and 14H-LPSO phases show negative strain-rate sensitivity (–0.02 to –0.01) at intermediate temperatures [423 K to 523 K (150 °C to 250 °C)]. The serration behavior of the Mg-10Dy-1Zn alloy containing 18R-LPSO phase is related to dynamic strain aging. However, the appearance of serrated flow in the Mg-10Dy-1Zn alloy containing 14H-LPSO phase is mostly rooted in the formation of microcracks in \( \left\{ {10\overline{1} 2} \right\} \) planes.

Formation of profuse long period stacking ordered microcells in Mg–Gd–Zn–Zr alloy during multipass ECAP process

A cast Mg97.1Gd1.8Zn1Zr0.1 (at.%) alloy having long period stacking ordered (LPSO) structures was continuously processed at 375°C by equal channel angular pressing (ECAP) for 1, 4, 8, 12 and 16 passes. With increasing ECAP passes, more and more rectangular microcells with a width of 0.25–0.5μm and a length of 0.5–1.5μm were produced adjacent to the β-phase particles and dynamic recrystallization (DRX) grains. These microcells were actually sandwichs consisting of 14H LPSO phase and α-Mg nano-slices, resulting from the shear effect and high internal temperature of the billet generated by continuous ECAP process. After 16 passes, the ultimate tensile strength of 387MPa and the elongation of 23% were obtained due to the special microstructure in the alloy.

The effect of 14H LPSO phase on dynamic recrystallization behavior and hot workability of Mg–2.0Zn–0.3Zr–5.8Y alloy

The effects of stacking sequence changes of the close-packed plane in a long period stacking ordered (LPSO) phase on the hot deformation behavior, dynamic recrystallization (DRX) evolution and hot workability of Mg–2.0Zn–0.3Zr–5.8Y alloy were investigated by compression test. A 14H LPSO phase crystal was fabricated by annealing a solidified crystal with a 18R LPSO structure at 500°C for 20h. Compression experiments were performed in temperature range of 300–500°C and strain rate range of 0.001–1s−1 using Gleeble 1500D thermo-mechanical simulator.Based on the regression analysis for the Arrhenius type equation of flow behavior, the apparent average activation energy of deformation was determined as Q=348.4KJ/mol. The kinetics of DRX evolution is calculated as XDRX=1−exp[−2.1542((ε−εc)/ε⁎)1.5119]. The DRX mechanism of the 14H LPSO phase is almost similar to that of the 18R LPSO phase. On comparison of the alloy with 18R LPSO phase, the formation of lamellar 14H LPSO phase in the Mg matrix can delay the DRX and hinder the growth of DRX grains significantly at the same deformation condition. The transformation of 18R LPSO phase to 14H LPSO phase reduced the workability of Mg–2.0Zn–0.3Zr–5.8Y at low deformation temperatures. The processing map exhibits a deformation domain of complete DRX occurring in the temperature range 454–500°C and strain rate range of 0.001–0.01s−1, which are the optimum processing parameters.

The microstructure, stability, and elastic properties of 14H long-period stacking-ordered phase in Mg–Zn–Y alloys: a first-principles study

The long-period stacking-ordered (LPSO) phases were demonstrated to be the predominant strengthening phases that can significantly improve the mechanical properties of Mg–Zn–Y alloys. However, the mechanism underlying this improvement has not been satisfyingly understood yet due to unclear characterization of the microstructures of these LPSO phases. We proposed the reliable structural model of Mg142Zn12Y16 for 14H LPSO phase formed in Mg–Zn–Y alloys with first-principles calculations in this study. The space group of Mg142Zn12Y16 is P63/mcm, with lattice dimensions a = b = 11.168 A, c = 36.408 A, α = β = 90°, and γ = 120°. And the atomic sites were given in this work. In contrast to other structural models proposed in previous studies, 14H LPSO phase was predicted to be thermodynamically stable on the basis of the present structural model, which can well account for the stability of 14H LPSO phases at higher temperature. Furthermore, the elastic constants and elastic moduli of 14H LPSO phase were also evaluated based on the present structural model. The calculated Young’s modulus of 14H LPSO phase agrees well with the available experimental values. The characteristics of 14H LPSO phase observed in experiments were successfully reproduced using the present structural model. The good match between the calculations and the experiments validates the reliability of the structural model proposed for 14H LPSO phase.

Double-peak ageing behavior of Mg–2Dy–0.5Zn alloy

Ageing behavior of Mg–2Dy–0.5Zn alloy was investigated during isothermal ageing at 180 °C. Two significant ageing peaks were observed at 36 h and 80 h, respectively. Examination of microstructure evolution during ageing revealed that 14H long period stacking ordered (LPSO) phase forms in the α-Mg matrix and its volume fraction increases, (Mg, Zn) x Dy particle phases precipitate and their size, distribution and amount vary, as ageing time increases. The LPSO strengthening and the precipitation strengthening are two main mechanisms responsible for the double-peak ageing behavior observed for the Mg–2Dy–0.5Zn alloy. The first ageing peak is mainly attributed to the precipitation strengthening of a large amount of the fine (Mg, Zn) x Dy particle phases. The second ageing peak arises mainly from the LPSO strengthening of a high volume fraction of the 14H LPSO phase.

An elevated temperature Mg–Dy–Zn alloy with long period stacking ordered phase by extrusion

A Mg–2Dy–0.5Zn (at.%) alloy was prepared by casting, heat treating at 525 °C for 10 h, extruding at 360 °C and ageing at 180 °C for 90 h. Microstructure characterization revealed that this alloy consists of the fine-grained α-Mg matrix composed nearly of the 14H long period stacking ordered (LPSO) phase and a small amount of (Mg, Zn) x Dy precipitations. Tensile tests showed that the yield and ultimate tensile strengths of this alloy can reach 245 MPa and 260 MPa at 300 °C, which are only slightly lower than those at room temperature (287 MPa and 321 MPa), the elongation of this alloy is 36% at 300 °C, which is far higher than that at room temperature (11.6%). The high volume fraction of the 14H LPSO phase in the fine-grained α-Mg grains and the high thermal stability of the microstructure during elevated temperature deformation are responsible for the excellent elevated temperature properties observed for this alloy.