Huan Liu

Microstructures and Mechanical Properties of Mg-2Y-xZn (x=1, 2, 3 at%) Alloys

Abstract The microstructures and the mechanical properties of the Mg-2Y- x Zn ( x = 1, 2, 3 at%) alloys in as-cast, as-annealed and as-extruded conditions have been investigated. The results show that the 18R-LPSO phases are distributed in a continuous network mainly along the grain boundaries in the alloy with 1 at% Zn. With increasing of Zn content, the 18R-LPSO phase and W-phase coexist in Mg-2Y-2Zn alloy but only W-phase remains in Mg-2Y-3Zn alloy. The 14H-lamellas are precipitated from the block-shaped 18R-LPSO phases into the matrix, and W-phases are broken into particles during homogenizing annealing. After extrusion, the LPSO phases and W-particles are rearranged along the direction of extrusion. The tensile tests show that the as-extruded Mg-2Y-1Zn alloy exhibits the supreme UTS of 320 MPa, with an elongation of 11.2% at room temperature.

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 773xa0K. After solution treatment at 773xa0K for 10xa0h and aging at 498xa0K for 24xa0h (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 453xa0MPa 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.

Microstructure, Martensite Transition and Mechanical Properties Investigations of Polycrystalline Co-Ni-Al Alloys with Er Doping

Using a multi-technique approach, we explore the effect of Er doping on the mechanical properties and phase transition temperature of polycrystalline Co-Ni-Al alloy. The un-doped alloy exhibits poor mechanical properties and a very low phase transition temperature. Therefore, the alloy could not obtain the apparent magnetic-field-induced strain. We show that the microstructure is typical of a multi-phase structure at room temperature. Within the grain boundary, a γ phase exists and is shown to continuously grow surrounding the matrix as the Er is being doped. This results in the appearance of Co2Er in the γ phase when Er rises above 0.5xa0at.%. The phase transformation temperature clearly increases with doping and reaches room temperature when doping is at 1xa0at.% Er. The yield stress and ductility of the alloy increased remarkably at first and then slightly decreased with further doping. The sample exhibits an interesting shape memory effect that is enhanced by Er doping or thermo-mechanical cycles.

Microstructure, Magnetism and Magnetic Field Induced-Strain in Er-Doped Co-Ni-Al Polycrystalline Alloy

A large magnetic field-induced strain (MFIS) was discovered in single-crystal alloys, whereas it is proven difficult for such apparent strain values to be obtained in polycrystalline alloys. In order for an apparent strain discovery to occur, the polycrystalline Co-Ni-Al system was doped by 0–1xa0at.% of Er and the effects of doping on microstructure, magnetism and MFIS were studied via scanning electron microscopy, x-ray diffraction, transmission electron microscopy and vibrating sample magnetometer in the present work. The microstructure of the alloy was a dual-phase microstructure, including the matrix and the γ phase. Following the Er doping, the γ phase was continuously coarsened, forming a network of precipitates surrounding the grains. Also, a Co-Er-rich intermetallic compound was formed in the Co-rich γ phase when the Er content exceeded 0.1xa0at.%. The martensitic transformation temperature has a decreasing tendency during the Er being doped from 0xa0at.% to 1xa0at.% and the martensitic structure of the sample is of the L10 type, forming twin grains in the (111) twinning plane. On the contrary, the magnetic properties were improved by Er doping, especially saturation magnetization and magneto-crystalline anisotropy constantly increased to 60.45xa0emu/g and 3.13xa0×xa0106xa0erg/cm3 when the Er content reached 1xa0at.%, respectively. Also, the strain recovery ratio (Rs) of Co-Ni-Al-Er alloys can be enhanced by thermo-mechanical cycles and Er doping. At 5% of the total strain, the Rs value exceeded 83% following thermo-mechanical cycles when the Er doping was 1xa0at.%. The strain in the applied magnetic field was increased by Er doping and an excess of 140xa0ppm of MFIS was obtained in the polycrystalline Co-Ni-Al-Er alloys.

Precipitation behavior of 14H LPSO structure in single 18R phase Mg–Y–Zn alloy during annealing at 773 K

Abstract The microstructural evolution of a 18R single phase (S18) alloy during annealing at 773 K for 100 h was investigated in order to reveal the formation mechanism of 14H phase. The results showed that the as-cast S18 alloy was composed of 18R phase (its volume fraction exceeds 93%), W particles and α –Mg phase. The 18R phase in S18 alloy was thermally stable and was not transformed into 14H long period stacking ordered (LPSO) phase during annealing. However, 14H lamellas formed within tiny α –Mg slices, and their average size and volume fraction increased with prolonging annealing time. Moreover, the 14H phase is nucleated within α –Mg independently on the basis of basal stacking faults (SFs). The broadening growth of 14H lamellas is an interface-controlled process which involves ledges on basal planes, while the lengthening growth is a diffusion-controlled process and is associated with diffusion of solute atoms. The formation mechanism of 14H phase in this alloy could be explained as α –Mg→ α –Mg+14H.

Hot Workability of the as-Cast 21Cr Economical Duplex Stainless Steel Through Processing Map and Microstructural Studies Using Different Instability Criteria

To develop a fundamental understanding of the flow behavior and optimal hot workability parameters of this material, the hot workability and deformation mechanisms of the as-cast 21Cr EDSS were studied using processing map technology combined with microstructure analysis and isothermal hot compression over the temperature range of 1000–1150xa0°C and strain rate range of 0.01–10xa0s−1. The processing maps and constitutive equation of peak stress were developed based on Prasad’s and Murty’s criteria. The results show that the processing maps exhibit a stable domain at 1000–1150xa0°C and 0.01–1xa0s−1. The instability domain is exhibited at high strain rates (≥1xa0s−1). This implies that Murty’s criterion can predict the unstable domain with high reliability. The detailed deformation mechanisms are also studied by microstructure observation, showing that the flow localization and microcracking are responsible for the flow instability.

Microstructure and mechanical properties of Mg94Zn2Y4 extruded alloy with long-period stacking ordered structure

Abstract The microstructure and mechanical properties of Mg 94 Zn 2 Y 4 extruded alloy containing long-period stacking ordered structures were systematically investigated by SEM and TEM analyses. The results show that the 18R-LPSO structure and α -Mg phase are observed in cast Mg 94 Zn 2 Y 4 alloy. After extrusion, the LPSO structures are delaminated and Mg-slices with width of 50–200 nm are generated. By ageing at 498 K for 36 h, the ageing peak is attained and β ′ phase is precipitated. Due to this novel precipitation, the microhardness of α -Mg matrix increases apparently from HV108.9 to HV129.7. While the microhardness for LPSO structure is stabilized at about HV145. TEM observations and SAED patterns indicate that the β ′ phase has unique orientation relationships between α -Mg and LPSO structures, the direction in the close-packed planes of β ′ precipitates perpendicular to that of α -Mg and LPSO structures. The ultimate tensile strength for the peak-aged alloy achieves 410.7 MPa and the significant strength originates from the coexistence of β ′ precipitates and 18R-LPSO structures.

Microstructure and corrosion resistance of yellow MAO coatings

ABSTRACT Yellow ceramic coatings were prepared on ultra-fine grained (UFG) AZ91 magnesium alloy using micro-arc oxidation (MAO) process in the alkaline-silicate electrolyte with different KMnO4 addition. Microstructure and phase composition were investigated by SEM, EDS (energy-dispersive spectrometer) and XRD (X-ray diffraction). With 2u2005gL−1 KMnO4 addition, the extreme sparking discharge was inhibited and the coating exhibited more compact and uniform morphology. However, adding 3u2005gL−1 KMnO4 in electrolyte extremely woke the discharge intensity and caused deleterious influence such as micro-crack. The generation of complex oxide Mg6MnO8 phase resulted in the yellow colour of the coatings. Electrochemical measurements including potentiodynamic polarisation and electrochemical impedance spectroscopy tests were carried out in 3.5% NaCl solution. The UFG alloy with the MAO coating produced with 2u2005gL−1 KMnO4 addition exhibited superior corrosion resistance, which was owing to the more compact and uniform microstructural features.

Mechanical and Biological Properties of a Biodegradable Mg-Zn-Ca Porous Alloy: Mechanical And Biosecurity Of Porous Mg Alloy

As promising alternative to current metallic biomaterials, the porous Mg scaffold with a 3‐D open‐pore framework has drawn much attention in recent years due to its suitable biodegradation, biocompatibility, and mechanical properties for human bones. This experiments aim is to study the mechanical properties, biosafety, and osteogenesis of porous Mg‐Zn alloy.

Rebuilding the Strain Hardening at a Large Strain in Twinned Au Nanowires

Metallic nanowires usually exhibit ultrahigh strength but low tensile ductility, owing to their limited strain hardening capability. Here, our larger scale molecular dynamics simulations demonstrated that we could rebuild the highly desirable strain hardening behavior at a large strain (0.21 to 0.31) in twinned Au nanowires by changing twin orientation, which strongly contrasts with the strain hardening at the incipient plastic deformation in low stacking-fault energy metals nanowires. Because of this strain hardening, an improved ductility is achieved. With the change of twin orientation, a competing effect between partial dislocation propagation and twin migration is observed in nanowires with slant twin boundaries. When twin migration gains the upper hand, the strain hardening occurs. Otherwise, the strain softening occurs. As the twin orientation increases from 0° to 90°, the dominating deformation mechanism shifts from slip-twin boundary interaction to dislocation slip, twin migration, and slip transmission in sequence. Our work could not only deepen our understanding of the mechanical behavior and deformation mechanism of twinned Au nanowires, but also provide new insights into enhancing the strength and ductility of nanowires by engineering the nanoscale twins.