Period Stacking Order Structure Research Articles

Directional heat treatment technique to prepare columnar grains in a TiAl alloy: Microstructure evolution and deformation behavior

Directional TiAl alloys show excellent comprehensive mechanical properties at high temperature (HT), and their notable attributes in high-temperature deformation resistance offer them promising application potential as structural materials in the aero engine industry. Directional heat treatment (DHT) is an effective local solid phase transformation technology in alloy processing and microstructure control. The directional Ti47Al2Nb2Cr alloys with columnar grains microstructure were fabricated via multiple DHT technology, where the HT deformation performance and corresponding deformation mechanism were both investigated. Results show that both the length in axial and width in radial are increasing with the increase of DHT time, together with the reduce of columnar grains amount, ultra-large columnar grains of 58.6 mm in length and 4.8 mm in width are even obtained, which makes it possible to realize the directional single crystals fabrication further, and the DHT-TiAl alloy may possess a tensile strength of 611.7 MPa at 800 ℃. For the first time, the mechanism of columnar grain growth is considered from the point of thermodynamics via analyzing the driving force of different DHT regions, which is thought to be greatly related to the temperature gradient, and the rule of preferred orientation choosing for columnar grains growth is discussed as well, then we defined the possible conditions of controlling columnar grains steady growth. Eventually, the detailed deformation mechanism of the multiple-DHT treated TiAl alloy was analyzed by transmission electron microscopy (TEM), which demonstrates that the twin intersections together with stress-induced γ variants within γ matrix, and the well-known long period stacking order (LPSO) structure 6 R and 9 R characterized by periodic stacking fault both found near α2 phase and γ twins/pseudo-twins further improved the plastic deformation accommodation and interface compatibility of DHT-TiAl alloy. This observed microstructure transformation and corresponding mechanical performance improvement demonstrate the potential of DHT technology in microstructure improving and properties strengthening for TiAl alloys during practical manufacturing process.

Hydrogen generation performances and electrochemical properties of Mg alloys with 14 H long period stacking ordered structure

Hydrolysis of Magnesium with water is an excellent way to produce H2 at high rate, especially when alloyed with nobler elements that accelerate its corrosion. In this work, the hydrogen generation properties of four Magnesium based alloys with Long Period Stacking Ordered (14 H-LPSO) structures (Mg91Cu4Y5; Mg91Ni4Y5; Mg91Ni4Gd5 and Mg91Ni4Sm5) are studied through standard electrochemical methods. The hydrolysis experiment on coarse powder are coherent with the electrochemical measurements performed on polished surface. The rare earth (Y, Gd or Sm) and transition metal (Ni or Cu) involved in the LPSO are found to have a significant influence on the corrosion and hydrolysis properties of alloys with 14 H-LPSO structures.

Effects of Laser Surface Remelting on Microstructure and Corrosion Properties of Mg-12Dy-1.1Ni Alloy

Effects of laser surface remelting (LSR) on microstructure and corrosion properties of as-cast Mg-12Dy-1.1Ni (wt.%) alloy were investigated. The as-cast alloy was mainly composed of α-Mg, lamellar Mg12DyNi phase with an 18R-long period stacking order (LPSO) structure and Mg24Dy5. After LSR treatment, fine and compact 18R-LPSO phase reprecipitated on dendrite boundaries as a continuous network. In addition, the volume fraction of the LPSO phase remarkably increased to 34.9% and the grain size of as-cast alloy was refined to ca. 4 μm. Electrochemical and immersion tests indicated that the LSR-treated alloy exhibited a lower weight loss rate (2.8 ± 0.2 mg/cm2/h) and corrosion current density (160.1 ± 20.7 μA/cm2, and of 37% reduction after LSR) than the as-cast alloy in 0.1 M NaCl solution. The improved corrosion resistance of the LSR alloy was mainly ascribed to the grain refinement and continuous distribution of 18R-LPSO phase on dendrite boundaries.

Corrosion of Mg Alloys with Long Period Stacking Ordered Structure

Corrosion of Mg Alloys with Long Period Stacking Ordered Structure

Effect of heat treatment on microstructure evolution and mechanical properties of selective laser melted Mg-11Gd-2Zn-0.4Zr alloy

The Mg-11Gd-2Zn-0.4Zr (GZ112K, wt.%) alloys fabricated by selective laser melting (SLM) still contain some hard and brittle β-(Mg,Zn)3Gd eutectic phase with an area fraction of 4.86% despite the ultra-fast cooling rate of SLM process, so post heat treatment is necessary. The effects of different solution conditions and aging heat treatment on microstructure and mechanical properties of the SLMed GZ112K alloys were systematically analyzed. The results show that the optimized solution condition (400 °C × 12 h) for the SLMed GZ112K alloys can not only transform hard and brittle β-(Mg,Zn)3Gd eutectic phase into relatively softer lamellar 14H type long period stacking order (LPSO) structure leading to the improved elongation (EL) of the SLM-T4 alloys, but also retain the fine grains of the SLMed GZ112K alloys to a large extent with a small average grain size of 3.1 μm in the SLM-T4 state contributing to the high yield strength (YS) of the SLM-T4 and SLM-T6 GZ112K alloys. After peak aging heat treatment, basal γ′ precipitates or lamellar 14H-LPSO structure and dense prismatic elliptical β′ precipitates coexist in the SLM-T6 GZ112K alloys. As a result, the SLM-T6 GZ112K alloys exhibit YS of 343 ± 9 MPa, ultimate tensile strength (UTS) of 371 ± 4 MPa and EL of 4.0 ± 0.05%, while those of the SLMed GZ112K alloys are 332 ± 3 MPa, 351 ± 5 MPa and 8.6 ± 1.0%. The ultra-high YS of SLM-T6 GZ112K alloys results from the fine grain strengthening, the precipitation strengthening from β′ aging precipitates, the secondary phase strengthening from LPSO structure and X phase and the extra composite strengthening from the coexistence of basal and prismatic precipitates. The findings indicate that appropriate post heat treatment is an effective scheme for modifying microstructure and mechanical properties of the SLMed alloys.

Deformation-induced dissolution of long-period stacking ordered structures and its re-precipitation in a Mg-Gd-Zn-Mn alloy

In this paper, the dissolution, refinement and re-precipitation of long period stacking order structure and solute-segregated stacking faults (LPSO structure/SFs) in an Mg-Gd-Zn-Mn alloy fabricated by hot extruding were studied. The results demonstrate that the refinement of LPSO structure/SFs was dominated by the misalignment mechanism and re-dissolution mechanism. Dislocation slips on the LPSO structure/SFs, as well as the rapid diffusion of solute atoms was responsible for the re-dissolution of LPSO structure/SFs. As the LPSO structure/SFs dissolved, a region with supersaturated solute atoms was formed and resulting in re-precipitation of SFs in continuous dynamically recrystallized grains. Finally, the evolution model of LPSO structure/SFs during continuous dynamic recrystallization was proposed.

Microstructural Characterization of Martensite with Long Period Stacking Order Structure in Hf–Co–Pd Alloy

Microstructural Characterization of Martensite with Long Period Stacking Order Structure in Hf–Co–Pd Alloy

Recent Progress on Corrosion Behavior and Mechanism of Mg–RE Based Alloys with Long Period Stacking Ordered Structure

Mg alloys containing long period stacking ordered (LPSO) structures possess excellent mechanical properties and corrosion resistance, exhibiting great application potential in biodegradable implants. The corrosion process of different LPSO phases in the Mg alloys need to be furtherly clarified in different solution environments. In this paper, we mainly reviewed the influencing factors on the corrosion behavior of LPSO-containing Mg alloys, including the alloying elements, processing technologies, and the types, volume fractions and distributions of LPSO phases. Recent researches are well summarized with an emphasis on their corrosion mechanism. Special attention is given to the key issues for LPSO-containing Mg alloys as biomedical implants, with their biodegradation behavior comprehensively discussed. The motivation is to provide theoretical support for the possible application of LPSO-containing Mg alloys in the biomedical field.

Effect of Al-Sr master alloy on the formation of long period stacking ordered phase and mechanical properties of Mg-Gd-Zn alloy

The rapid formation of profuse fine Long Period Stacking Ordered Structure (LPSO) lamellae is especially desired for Mg-Gd-Zn alloys as type II wherein LPSO phases are difficult to form in as-cast state and prolonged heat treatments are necessary. The aim of this work is to investigate the effect of various Al-10 Sr master alloy additions (0, 0.6, 1.8, in wt. %) on LPSO formation in as-cast Mg-14.02Gd-2.33Zn alloy and the microstructural evolution during subsequent heat treatment and ECAP processing as well as their mechanical behavior variations. Minor addition of 0.6 wt. % Al-Sr master alloy introduces abundant 18R and 14H-LPSO phases in as-cast Mg-Gd-Zn alloy, and accelerates the microstructural evolution into high density of well-aligned long fiber-like 14H nanolamellae only through short-time solid solution and 1-pass ECAP processing as well as low-temperature aging. While a higher level of 1.8 wt. % (Al-Sr) addition produces short rod-like 14H particulates and considerable hard Al2Gd, network (Mg, Al)3Gd intermetallic compounds deteriorating the mechanical property. Consequently, processed Mg-Gd-Zn-0.6(Al-Sr) alloy exhibits a superior mechanical property due to synergistic effects of long fiber-like LPSO phase strengthening and grain refinement strengthening and so on. This paper provides an effective and economical method to fabricate high-performance Mg-Gd-Zn alloys reinforced with LPSO phases.

Formation Behavior of 14H Long Period Stacking Ordered Structure in Mg–Y–Zn Cast Alloys with Different α-Mg Fractions

Phase compositions and microstructure evolutions of three Mg–Y–Zn cast alloys during isothermal annealing at 773 K have been systematically investigated to clarify the formation behavior of 14H long period stacking ordered (LPSO) structure from α-Mg grains. The annealed microstructure characteristics indicate that the 18R phase is thermal stable in Mg86Y8Zn6 alloy where 18R serves as matrix, and 14H lamellar phase only forms within tiny α-Mg slices (less than 1% for volume fraction). The α-Mg grains in Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibit cellular shape, and 14H phase forms and develops into lamellar shape in these cellular grains after annealing. The results suggest that the presence of α-Mg grains is a requirement for the generation of 14H phase. The nucleation and growth rates of 14H lamellas are accelerated in α-Mg grains with higher concentrations of stacking faults and solute atoms. Moreover, the 14H lamellas are parallel to adjacent 18R plates in Mg86Y8Zn6 alloy, but the 14H phase precipitated in cellular α-Mg grains of Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibits random orientation relationship with surrounding 18R phase, indicating that the orientation relationship between 14H and 18R phases depends on the relationship between α-Mg grains and 18R phase.

Atomic configurations of various kinds of structural intergrowth in the polytypic M2B-type boride precipitated in the Ni-based superalloy

Abstract The heavily faulted M 2 B-type borides precipitated in the long-term aging Ni-based superalloys with B addition are comprehensively studied at an atomic scale. By means of high angle annular dark field (HAADF) imaging technique in the aberration corrected scanning transmission electron microscope (STEM), atomic configurations of various kinds of structural intergrowth in the profoundly faulted M 2 B phase are directly imaged. The polytypic M 2 B phase is mainly composed of C16, C b and C a structural variants, in which planar defects and the resultant long period stacking order (LPSO) structure are identified. In combination of the advanced spectrometry imaging technique with density functional theory (DFT) calculations, it is proposed that the polytypic feature is attributed to the conservation of the basic polyhedron of anti-square prisms.

D03+hcp mixed phase with nanostructures in Mg85Zn6Y9 alloy obtained by high-pressure and high-temperature treatments

High pressure and high temperature state of long period stacking order structure (LPSO) synchronized with chemical concentration in Mg85Zn6Y9 alloy have been investigated. A stripe-patterned nanostructure consisting of D03 and hcp phases is formed in Mg85Zn6Y9 alloy by high pressure and high temperature treatments. The nanostripe patterns are created by the phase separation of LPSO synchronized with chemical concentration. A pressure–temperature phase diagram is given in this research. LPSO is stable below 673K, the D03+hcp mixed phase dominant above 973K under the pressure up to 20GPa. Phase separation under high pressure is useful for the fabrication of nanostructured metallic materials.

Stability and formation of long period stacking order structure in Mg-based ternary alloys

Long period stacking ordered (LPSO) structure plays an important role in determining the strength and ductility of Mg-based ternary alloys. In this work, we investigated the thermodynamic stability of 10H LPSO structures in comparison with those of 14H and 18R LPSO structures. A trimer model was proposed to predict the possibility of forming LPSO structure in Mg–XL–XS (XL=Rare earth, Ca, Sr and XS=Al, Zn, Cu, Ni) with first-principles calculations. The formation of LPSO structure was determined based on the intrinsic stacking fault energy, mix energy and segregation energy. An element which decreases the intrinsic stacking fault energy is essential for the formation of LPSO structure. Decreasing the stacking fault energy favors the formation of local FCC layers, which would attract solid solutes segregating from HCP to local FCC Mg and forming L12 cluster and finally the LPSO structure.

シンクロ型 LPSO 構造の形成メカニズムと熱力学因子

シンクロ型 LPSO 構造の形成メカニズムと熱力学因子

Electronic structures of long periodic stacking order structures in Mg: A first-principles study

Abstract Long period stacking order (LPSO) structures, such as 6H, 10H, 14H, 18R and 24R, play significant roles in enhancing the mechanical properties of Mg alloys and have been largely investigated separately. In the present work, through detailed investigations of deformation electron density, we show that the electron structures of 10H, 14H, 18R and 24R LPSO structures in Mg originate from those of deformation stacking faults in Mg, and their formation energies can be scaled with respect to formation energy and the number of layers of deformation stacking faults, while the electron structure and formation energy of the 6H LPSO structure are between those of deformation and growth stacking faults. The simulated images of high resolution transmission electron microscopy compare well with experimental observed ones. The understanding of LPSO structures in Mg enables future quantitative investigations of effects of alloying elements on properties of LPSO structures and Mg alloys.

Microstructure and Mechanical Properties of a Mg94Y4Ni2 Alloy with Long Period Stacking Ordered Structure

Microstructure and mechanical properties of the Mg94Y4Ni2 alloy (at.%) during homogenization, extrusion, and aging processes were systematically investigated using x-ray diffractometer, optical microscopy, scanning electron microscopy, transmission electron microscopy, and electronic universal testing machine. The results showed that the morphology evolution of 18R LPSO structure during annealing in Mg94Y4Ni2 alloy was different from that of Mg-Y-Zn systems. The 18R-type Mg12YNi phase was thermal stable and was not transformed into 14H structure when annealed at 773 K. After solution treatment at 773 K for 10 h and aging at 498 K for 24 h (T6 treatment) of the extruded alloy, a great amount of fine β′ phases were precipitated dispersedly in the matrix. The tensile tests showed that the extruded Mg94Y4Ni2 alloy after T6 treatment exhibited good tensile properties with ultimate tensile strength of 453 MPa and elongation to failure of 2.4% at room temperature. Thus, a high-strength Mg94Y4Ni2 alloy, which is strengthened by the coexisted LPSO phases and β′ precipitates, can be prepared via simple hot extrusion and T6 treatment.

Mg–Zn–Y alloys with long-period stacking ordered structure: In vitro assessments of biodegradation behavior

Using Dulbecco's modified eagle medium (DMEM) with 10% fetal bovine serum (FBS) as simulated body fluid, degradation behavior of Mg100−3x(Zn1Y2)x (1≤x≤3) alloy series with long period stacking order (LPSO) structures was investigated. As indicated, with increasing the volume fraction of LPSO phase, degradation rate of the alloys is accelerated. Further refining the grain size by microalloying with zirconium and warm extrusion has a significant effect to mitigate the degradation rate of the Mg97Zn1Y2 alloy. Time-dependent behavior during degradation of the magnesium alloys can be described using an exponential decay function of WR=exp(a+bt+ct2), where WR is normalized residual mass/volume of the alloy. A parameter named as degradation half-life period (t0.5) is suggested to quantitatively assess the degradation rate. For the localized-corrosion controlled alloys, the t0.5 parameter physically scales with electrochemical response ΔE which is a range between corrosion potential (Ecorr) and pitting potential (Ept). In comparison with conventional engineering magnesium alloys such as the AZ31, WE43, ZK60 and ZX60 alloys, extruded Mg96.83Zn1Y2Zr0.17 alloy with LPSO structure exhibits a good combination of high mechanical strength, lower biodegradation rate and good biocompatibility.

Formation and characterization of microstructure of as-cast Mg–6Gd–4Y–xZn–0.5Zr (x = 0.3, 0.5 and 0.7 wt.%) alloys

Abstract Mg–6Gd–4Y–xZn–0.5Zr (x = 0.3, 0.5 and 0.7 wt.%) alloys were prepared via conventional ingot metallurgy (I/M) in this study. The as-cast microstructures of these alloys were established by X-ray diffraction (XRD) analyses, optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. Lamellar stacking order (SF) and 14H-type long period stacking order (LPSO) structure within α-Mg matrix are formed in the three as-cast alloys. The eutectic secondary phase is (Mg,Zn)24(Gd,Y)5 for the alloy containing 0.3 wt.% Zn, while, it is (Mg,Zn)3(Gd,Y) for the alloys containing 0.5 wt.% Zn and 0.7 wt.% Zn. Moreover, X phase-(Mg,Zn)12(Gd,Y) is formed in the latter two as-cast alloys.

The predominant role of Zn6Y9 cluster in the long period stacking order structures of Mg–Zn–Y alloys: a first-principles study

The strengthening mechanisms underlying the long period stacking order (LPSO) phases in Mg–Zn–Y alloys have largely remained unsolved due to unclear understanding of the local arrangements of Zn and Y atoms in the LPSO phases. The local arrangements of Zn and Y atoms in the LPSO structures are theoretically refined with first-principles method in this study. The calculations clearly demonstrate that Zn and Y atoms prefer clustering in the form of Zn6Y9 rather than arranging in random or ordered arrangements, or in the form of Zn6Y8 cluster, as proposed in previous experiments. The Zn6Y9 cluster, which leads to the formation of ABCA-type building block of the LPSO structures, can be particularly regarded as the ideal stoichiometric component of the LPSO structures. Various non-stoichiometric ZnmYn(Mg) clusters derived from the Zn6Y9 cluster can also easily exist in the LPSO structures. The non-stoichiometric ZnmYn(Mg) clusters may correspond to the arrangements of Zn and Y atoms in the LPSO phases with different Zn/Y ratios. This study indicates that the novel structures of the LPSO phases consisting of ABCA-type building blocks stem from the formation of the Zn6Y9 cluster and its derivative clusters, ZnmYn(Mg).

Forgeability and Flow Stress of Mg-Zn-Y Alloys with Long Period Stacking Ordered Structure at Elevated Temperatures

To develop a forging process for high strength Mg-Zn-Y alloys with a long period stacking ordered (LPSO) structure, the forgeability and flow stress of Mg97Zn1Y2 (at%) alloys were investigated with the upsettability test at an initial strain rate of 0.55 s � 1 . As-cast and extruded Mg97Zn1Y2 alloys can be successfully forged without fracture up to 0.5 and 1.0 respectively of average equivalent strain at a forging temperature of 773 K. To remove the influence of the temperature change during upsetting from the experimentally obtained flow stress curve, the isothermal flow stress curve was estimated by combining experimental results with finite element analysis. Furthermore, the mechanical properties of the forged Mg-Zn-Y alloys were measured, and the influence of microstructure on the forging properties was discussed.