Phases In As-cast Alloy Research Articles

Achieving ultra-high strength using densely ultra-fine LPSO phase

• An optimized FSP processing generates densely ultrafine LPSO blocks with an average width of ∼200 nm and an average length of ∼1 μm. • There is an ultrahigh strength up to approximately 800 MPa under the uniaxial compression of micropillars. • The strengthening effect by ultrafine LPSO blocks reaches approximately 630 MPa. Long period stacking ordered (LPSO) structures are an effective strengthening phase in Mg alloys. However, the coarse LPSO phases in as-cast alloys are very difficult to refine, and they dramatically reduce the strengthening effect. In the present study, friction stir processing (FSP) was employed to refine the structure of an Mg-12.8Y-4.7 Zn (wt%) alloy with a very high content of coarse LPSO phases. An optimized FSP regime refined the coarse LPSO phases into densely ultrafine blocks with an average width of ∼200 nm and an average length of ∼1 μm. An ultrahigh yield strength of ∼800 MPa was achieved under compression of the FSP region. Theoretical calculations indicated that the strengthening by the densely ultrafine LPSO phases was up to approximately 630 MPa.

Microstructural evolution and mechanical performance of cast Mg-3Nd-0.2Zn-0.5Zr alloy with Y additions

This work investigated the effects of different Y additions (0, 1.5, 3.0 and 4.5 wt.%) on the microstructural evolution and mechanical performance of cast Mg-3Nd-0.2Zn-0.5Zr alloy. The results show that as the Y content increases, the key secondary phases in as-cast alloys change from the Mg12Nd type to the Mg24Y5 type. Meanwhile, the number density of Zn-Zr particles in the grains of as-quenched alloys gradually decreases. HAADF-STEM observations of peak-aged samples reveal that element Y is greatly enriched in the globular β′ precipitates, leading to a significantly increased volume fraction and promoted precipitation kinetics of β′ precipitates, resulting in enhanced strength of the alloy. Tensile tests reveal that, with the addition of 4.5 wt.% Y, the yield strength of the base alloy is substantially increased by 88 and 61 MPa after being aged at 200 and 225 °C under peak-aged conditions, respectively.

In vitro corrosion properties of HTHEed Mg-Zn-Y-Nd alloy microtubes for stent applications: Influence of second phase particles and crystal orientation

Magnesium (Mg) alloys are promising materials for cardiovascular stent applications due to their good biocompatibility and biodegradability. However, in vitro and in vivo corrosion tests reveal that Mg alloy stents suffer from a rapid corrosion rate and severe localized corrosion, which is limiting their widespread application. To solve the problem of uneven degradation of stents, a HTHE (long-time and high-temperature heat treatment, large-reduction-ratio hot extrusion) process is used to manufacture Mg-Zn-Y-Nd alloy microtubes in this study. The heat treatment is to dissolve alloying elements and reduce the size of SPPs, and the hot extrusion is to acquire fine-grained and strongly textured microtubes. The microstructural characterization shows that coarse second phases in as-cast alloy are refined and uniformly distributed in matrix of microtubes. After hot extrusion, microtubes show strong texture with basal plain oriented parallel to the longitudinal section (LS). The corrosion testing indicates that severe localized corrosion occurs on the cross section (CS) while localized corrosion is alleviated on the LS. Based on the different corrosion properties of the LS and CS, HTHEed microtubes are promising for solving the problems of rapid corrosion rate and severe localized corrosion of Mg alloy stents.

Microstructures and Phase Formation of Refractory MoNbTaVZr High Entropy Alloy

Microstructures, component distribution and mechanical properties of a refractory high entropy alloy MoNbTaVZr were investigated by scanning electron microscope, X-ray diffractometer and universal testing machine, and the formation rule of second phases in as-cast alloy was discussed by the results of first principle calculations. The results show that as-cast MoNbTaVZr alloy is composed of two solid solutions with body centered cubic structure, together with Mo2Zr and an unknown phase. The chemical compositions of dendritic regions are relatively uniform, while Zr and V elements are rich in interdendritic regions. There is a short yielding phase when as-cast alloy was operated by the compression. According to calculations, the C15 Mo2Zr Laves phase with the strongest formation ability and highest structural stability would be formed preferentially in MoNbTaVZr refractory high entropy alloy.

Enhancing the Corrosion Resistance Performance of Mg-1.8Zn-1.74Gd-0.5Y-0.4Zr Biomaterial via Solution Treatment Process.

Microstructure and corrosion behavior of the solution-treated Mg-1.8Zn-1.74Gd-0.5Y-0.4Zr (wt%) alloy were studied. The results of microstructure indicated that the second phases of as-cast alloy was mainly comprised of Mg12Zn(Gd,Y) phase, Mg3Zn3(Gd,Y)2 phase and (Mg,Zn)3(Gd,Y) phase. After solution treatment process, the second phase gradually dissolved into the matrix, and the grain size increased. The effect of microgalvanic corrosion between α-Mg matrix and second phase was also improved. At the range of 470~510 °C solution treatment temperature, the corrosion resistance of the samples increases at first and then decreases slightly at 510 °C. All the solution-treated Mg-Zn-Gd-Y-Zr samples exhibit better corrosion resistance in comparison with as-cast sample. The existence form of the remaining phase affects the morphology of the corroded surface that relatively complete dissolution with homogeneous microstructure makes the sample more effective to obtain uniform corrosion form. The optimum temperature for solution treatment is 490 °C, which shows a much better corrosion resistance and uniform corrosion form after soaking for a long time.

Effect of Al content on the microstructure and mechanical properties of Mg-5wt%Sn-xAl as-cast alloys

Microstructure and mechanical properties of Mg-5wt.% Sn-xAl as-cast alloys were studied, for the Al content ranged from 0.5 to 3.0wt.%. It was found that Al element was mainly distributed in Mg17Al12 phase in as-cast alloys, and its increase rose the Mg17Al12 phase share. There were no ternary phases in these alloys. Only two forms of Mg2Sn and Mg17Al12 phases were detected, namely (i) Mg17Al12 phase attached to the end of Mg2Sn phase and (ii) Mg2Sn phase enclosed by Mg17Al12 phases. The grain sizes of these alloys got refined with the growing content of Al element, and 1.5wt% Al content corresponded to the grain size of 33.5um. As for the mechanical properties, the ultimate tensile and yield strengths of the alloys were improved at larger contents of Al. The Mg-5wt.%Sn-3.0wt% alloy had the highest ultimate tensile and yield strengths of 212 and 167MPa, respectively, while Mg-5wt.%Sn-1.5wt%Al alloy had the best elongation of 16.5%.

Microstructure Evolution and Mechanical Properties of Mg-7Gd-3Y-1Nd-2Zn-0.5Zr Alloy during Two-Step Homogenization Heat Treatment

This paper proposes a two-step homogenization heat treatment to dissolve the eutectic structure and long period stacking ordered phase (LPSO) formed during solidification into the α-Mg matrix. The microstructure evolution and mechanical properties of Mg–7Gd–3Y–1Nd–2Zn–0.5Zr alloy during the two-step homogenization heat treatment have been investigated systemically. The results reveal that as-cast alloy is composed mainly of α-Mg, (Mg,Zn)3RE, eutectic phase, stacking fault, block-like LPSO phase and square-shaped compounds rich in RE. The HRTEM results suggest that the block-like long period stacking ordered phase in as-cast alloy is 14H-type rather than 18R structure, and the stacking sequences of the 14H-LPSO phase are ABABACBCBCBCAB. After the first step homogenization of 520°C for 48 h, the eutectic structure has dissolved into the matrix, whereas the 14H-LPSO phase remains in the alloy. To further dissolve the LPSO phase into matrix, the second step homogenization of 540°C for 24h was adopted. After the second-step of homogenization, the residual 14H-LPSO phase has dissolved into the matrix totally. The as-homogenized alloy is composed mainly of α-Mg and square-shaped compounds rich in RE. The tensile tests at room temperature (RT) exhibit that the ultimate tensile strength (UTS), yield strength (YS) and elongation of as-cast alloy are 172 MPa, 128MPa and 2.8%, whereas the UTS, YS and elongation of as-homogenized alloy are 253 MPa, 185 MPa and 8.4, respectively.

Significantly improved corrosion resistance of Mg-15Gd-2Zn-0.39Zr alloys: Effect of heat-treatment

The effects of heat-treatment on corrosion behavior of Mg-15Gd-2Zn-0.39 Zr alloys were investigated through microstructure characterization, corrosion tests, and scanning Kelvin probe force microscope (SKPFM) analysis. In long-term corrosion experiments, the corrosion rates of Mg-Gd-Zn-Zr alloys were mainly determined by the effects of micro-galvanic corrosion. During heat-treatment, the β-(Mg,Zn)3Gd eutectic phase in as-cast alloys transformed into a long-period stacking ordered (LPSO) phase, coupled with the precipitation of small precipitates. As heat-treatment proceeded, the local potential and the volume fraction of the LPSO phases reduced gradually compared with the eutectic phase, which resulted in a remarkable decrease of the micro-galvanic effect between the second phase and Mg matrix. As a result, the corrosion resistance of heat-treated alloys improved significantly.

Effect of Nd on Subunits Structure and Electrochemical Properties of Super-Stacking PuNi3-Type La–Mg–Ni-Based Alloys

The super-stacking PuNi3-type La0.67-xNdxMg0.33Ni3.0 (x = 0, 0.12) alloys have been prepared using induction melting followed by annealing treatment method. The alloys are composed of PuNi3-type main phase and CaCu5-, MgCu4Sn-, Ce5Co19- and Gd2Co7-type minor phases in as-cast alloy. As annealing temperature increases from 850 to 950°C, the minor phases gradually transform to the PuNi3-type phase via peritectic reactions, forming PuNi3-type single-phase La0.67Mg0.33Ni3.0 alloy. Partial substitution of Nd for La has enhanced the phase stability of Gd2Co7-type phase, resulting in the existence of Gd2Co7-type phase as a secondary phase with the PuNi3-type main phase in the La0.55Nd0.12Mg0.33Ni3.0 alloy after annealing treatment. It is found that Nd mainly replace La in [AB5] slabs in the PuNi3-type structure, which increases the volume ratio of [A2B4] slabs in the cell and decreases the volume change of [A2B4] slabs during hydrogenation/dehydrogenation process, thus contributing to the improvement in cycling stability at the 100th cycle of the alloy, from 76.3% to 80.3%. Moreover, the cell volume contraction due to Nd substitution heightens the discharge plateau pressure and potential of the alloy electrode, and helps to the enhancement in high rate dischargeability (HRD) from 55.7% to 68.1% at a 1200 mA g−1 discharge current density.

Microstructural Evolution of Aluminum Alloy 2618 During Homogenization and Its Kinetic Analysis

AbstractThe microstructure evolution of aluminum alloy 2618 and its homogenization kinetics were investigated by optical microscope, scanning electron microscope, energy dispersive spectroscopy and differential scanning calorimeter. The results show that the main constituent phases in as-cast alloy 2618 are α–Al, Al9FeNi phase and non-equilibrium binary θ (Al2Cu) phase instead of typical S (Al2CuMg) phase. The θ phase was dissolved into α–Al gradually and the continuous dendritic-network structure was broken with the increase of homogenization temperature and time. DSC analysis shows that the overburnt and liquidus temperatures of as-cast alloy 2618 are 506.4 °C and 638.0 °C, respectively. After the alloy was homogenized at optimized temperature 500 °C for 16 h, the θ phase was completely dissolved in matrix. The size and morphology of Al9FeNi phase had little change, while the liquidus temperature shifted to 641 °C. The calculated homogenization time for alloy 2618 at 500 °C is 15 h, which is in accordance with that obtained in homogenization experiments.

Intermetallic phase evolution of 5059 aluminum alloy during homogenization

Intermetallic phase evolution of 5059 aluminum alloy during homogenization was investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), differential scanning calorimetry (DSC) and X-ray diffraction analysis (XRD). The results show that severe dendritic segregation exists in as-cast alloy. The dissolvable intermetallic phases in as-cast alloy consist of Zn- and Cu-rich non-equilibrium β (Al3Mg2) phase, Fe-rich eutectic Al6Mn phase and equilibrium Mg2Si phase. During the homogenization, Zn- and Cu-rich non-equilibrium β (Al3Mg2) phase, Fe-rich eutectic Al6Mn phase and equilibrium Mg2Si gradually dissolve into matrix. Fine dispersed β(Al3Mg2) particles and rod-shaped Al6Mn particles form in the Al matrix after homogenization. The proper homogenization processing is at 450 °C for 24 h, which is consistent with the results of homogenizing kinetic analysis.

Effects of Si on Structure and Properties of Mg-Sn Alloy

Magnesium alloys are becoming ever more prevalent in automotive and aerospace industries as energy conservation and performance demands increase. However, it is limited on further application in the automotive field for its low strength compared with steel and aluminum alloys. Effects of Si addition on microstructure and mechanical properties of as-cast Mg-5%Sn magnesium alloy were also studied using SEM, XRD and tensile testing. The results show that the phases of as-cast alloy are composed of α-Mg, Mg2Sn; Mg2Si. The addition of Si suppresses the precipitation of the Mg2Sn, Product Chinese character shape phases of Mg2Si. With the increase of Si content, the effect becomes more apparent. When Si content is up to 1.25wt%, the grain refining effect reached best level. The average alloy grain size reduced from 90μm to 30μm, the tensile strength is the highest. The average tensile strength increased from 128 MPa to 156 MPa. When the Si added at 1.25wt% or less, the hardness and strength were improved with the increase of Si. When 1.25% Si was added, grain did not become further refinement. While Si content was continue to be increase, the tensile strength decreased.

Experimental study and thermodynamic assessment of the Cu–Fe–Ti system

The Cu–Fe–Ti ternary system has been systematically studied through experimental investigation and thermodynamic modeling. Combining diffusion multiple technique with selected equilibrated alloys, equilibrium phase relations in the Cu–Fe–Ti system at 800°C have been established by means of SEM/EDS (Scanning Electron Microscopy/Energy Dispersive Spectrum) and EPMA (Electron Probe Micro-Analysis). DSC (Differential Scanning Calorimetry) analysis has been also employed to measure phase transformation temperatures in order to construct various vertical sections of the Cu–Fe–Ti system. Liquidus projection of the ternary system has been determined by identifying primary crystallization phases in as-cast alloys. Based on all available experimental information from literatures and this work, the Cu–Fe–Ti ternary system was modeled with the aid of the CALPHAD (CALculation of PHAse Diagram) method. Comprehensive comparisons made between experimental and calculated results shed light on reliability of the current thermodynamic description.

Intermetallic phase evolution of 7050 aluminum alloy during homogenization

Intermetallic phase evolution of 7050 aluminum alloy during homogenization was investigated in detail by optical microscopy, scanning electron microscopy, energy dispersive spectrometry, differential scanning calorimetry, electron probe micro-analysis and X-ray diffraction methods. The results show that severe dendritic segregation exists in as-cast alloy. The dissolvable intermetallic phases in as-cast alloy consist of equilibrium η (MgZn2) phase, Cu and Mg enriched non-equilibrium aluminides and Cu enriched non-equilibrium aluminides. During homogenization, Cu and Mg enriched non-equilibrium aluminides, Cu enriched non-equilibrium aluminides and η (MgZn2) phase gradually dissolve into matrix. Equilibrium S (Al2CuMg) intermetallic phase nucleates and grows along the grain boundaries, and it disappears completely after multi-step homogenization. The proper homogenization processing is 400 °C × 10 h + step470 °C × 24 h + step485 °C × 4 h, which is consistent with the results of homogenizing kinetic analysis.

Effect of AlN on Microstructure and Mechanical Properties of Mg-Al-Zn Alloy

Effects of AlN addition on the microstructure and mechanical properties of as-cast Mg-Al-Zn magnesium alloy were investigated using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and tensile testing. Five different samples were made with different amounts of AlN(0wt%, 0.12wt%, 0.30wt%, 0.48wt%, 0. 60wt%). The results show that the phases of as-cast alloy are composed of α-Mg,β-Mg17Al12. The addition of AlN suppressed the precipitation of the β-phase. And, with the increase of AlN content, the microstructure of β-phase was changed from the reticulum to fine grains. When AlN content was up to 0.48wt% in the alloy, the β-phase became most uniform distribution. After adding 0.3wt% AlN to Al-Mg-Zn alloy, the average alloy grain size reduced from 102μm to 35μm ,the tensile strength of alloy was the highest. The average tensile strength increased from 139MPa to 169.91MPa, the hardness increased from 77.7HB to 98.4HB, but the elongation changes indistinctively. However, when more amount of AlN was added, the average alloy grain size did not reduce sequentially and increased to 50μm by adding 0.6wt% AlN and the β-phase became a little more. Keywords: Al-Mg-Zn alloy; AlN; β-Mg17Al12; Tensile strength

Effects of Gadolinium on Aging Behavior and Mechanical Properties of Mg-5Y-3Nd-Zr Alloy

Mg-5Y-3Nd-Zr-xGd (x=0 and 4 wt.%) alloys were prepared by metal mould casting, and aging behavior, mechanical properties and fracture morphology were investigated. The result shows that after T6 treatment, the massive eutectic phase in as-cast alloys dissolved and finer Mg5RE phase precipitated dispersively in the matrix, and mechanical properties were improved simultaneously. Mg-5Y-3Nd-4Gd-Zr alloy exhibits good age hardening behavior and the peak hardness is about 20% higher than the Gd-free alloy. Gd addition can significantly improve mechanical properties of the alloy especially at the elevated temperature. The ultimate tensile strength and yield strength of Mg-5Y-3Nd-4Gd-Zr alloy at 250°C, with the value of 276 and 168 MPa, respectively, are over 20% higher than those of the Gd-free alloy. It is mainly attributed to the increase and homogeneous distribution of the fine heat-resistant Mg5RE precipitate in the matrix.

Formation and stability of icosahedral phase in Al65Ga5Pd17Mn13 alloy

The formation and characterization are described of a quaternary (pseudo-ternary) icosahedral quasicrystal in Al65Ga5Pd17Mn13 alloy. X-ray diffraction and transmission electron microscopy observations confirmed the formation of icosahedral, B2 type and ξ′ crystalline (orthorhombic structure with unit cell a = 23.5 Å, b = 16.6 Å and c = 12.4 Å) phases in as-cast alloy. The icosahedral phase is formed after annealing at 800°C for 60 h. This is the first report of the formation of an icosahedral phase in an Al–Ga–Pd–Mn quaternary alloy by present technique. The energy dispersive X-ray analysis investigations suggest the presence of Ga (∼5 at.%) in the alloy. Icosahedral Al–Ga–Pd–Mn provides a new opportunity to investigate various characteristics, including surface characteristics. Attempts are made to discuss the micromechanisms for the formation of the quasicrystalline phase in Al–Ga–Pd–Mn alloys.

Research on microstructure and alloy phases of AM50 magnesium alloy

Abstract Comprehensive researches on the microstructure and alloy phases of AM50 magnesium alloy of different status, varying from casting to solid-solution treated and aged ones, were carried out in the present paper. Special attention was drawn to Al–Mn phases and their effects on electrochemical properties of the alloy. The results indicated that Al–Mn phases were the major alloy phases in as-cast, solid-solution treated and aged AM50 alloys. They were quite heat-resistant and almost kept unchanged in terms of morphology, distribution and amount. The majority of Mg 17 Al 12 phase in as-cast alloy could be reduced when the temperature of solid-solution treatment was up to 410 °C and it precipitated as reinforcing phase during aging treatment. According to the microstructure and the corresponding microhardness analysis, it was believed that the strengthening mechanism of AM50 might not be limited to precipitation reinforcement; some other factors such as substructure and re-distribution of alloy elements might play critical roles too. Electrochemical experiments further showed that Al–Mn phases were detrimental to the corrosion resistance of the alloy in question, especially when Al-rich α-Mg and β-Mg 17 Al 12 phases were eliminated.

The effect of chromium additions on the mechanical properties of Ni-25Al-27.5Fe-based intermetallic compounds

To further improve room temperature ductility of alloys based on Ni-25Al-27.5Fe, alloys containing additional chromium were prepared by arc-melting and drop-casting into ingots. The mechanical properties of those alloys were characterized by means of hardness testing and tensile testing. The microstructural evolution of these alloys was examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results reveal that two phases, γ′ and β′ phases, exist in these casting alloys. The dendritic β′ phase mixed with the interdendritic of the β′ and γ′ phases in as-cast alloys. After annealing at 1000 °C for 4 h, the ordered γ′ phase transformed to a disordered γ phase, and a large number of γ phase particles were precipitated from the β′ phase. These precipitates have almost the same chemical composition and lattice constant as the interdendritic γ phase. The γ phase contained higher iron and chromium concentrations when compared with the β′ phase. The chemical compositions of the β′ phases in both dendrite and interdendrite appear to be almost the same. Increasing chromium content in this alloy system results in increasing the hardness and elongation, but decreasing the yield strength; for example an alloy with 1 at.% chromium had 1230 MPa of ultimate strength, 484 MPa of yield strength and 11.3% of elongation.