Formation Of I-phase Research Articles

Effect of I-phase formation on hydrogen embrittlement behaviour of as-cast Mg-8 wt%Li based alloys

By performing microstructural characterization, tensile testing and failure analysis, the effects of electrochemical H-charging on microstructure and mechanical behaviour of two as-cast Mg-8Li (in wt%) based alloys were investigated and compared. Due to the in-situ formation of I-phase, the failure elongation ratio for Mg-8Li-6Zn-1Y alloy was 31.2 % and its hydrogen embrittlement (HE) susceptibility was about 1/4 that of Mg-8Li alloy after H-charging for 24 h. Moreover, the preferential location of H-induced pores transferred from two matrix phases to I-phase/matrix eutectic pockets due to the preferential H absorption of I-phase, resulting in the higher HE resistance of Mg-8Li-6Zn-1Y alloy.

Laser printed amorphous magnesium alloy: Microstructure, mechanical properties and degradation behavior

Amorphous magnesium (Mg) alloy, especially Mg–Zn–Ca series, has been receiving continuous attention in biomedical field, because of its favorable corrosion resistance, suitable modulus and biocompatibility. In this work, laser powder bed fusing (LPBF) with unique characteristics of rapid solidification and layer-by-layer fashion was used to fabricate bulk Mg–Zn–Ca and Mg–Zn–Ca–Y amorphous parts. It was revealed that the as-built parts contained amorphous structure, since the molten pool experienced an extremely high cooling rate that suppressed ordered arrangement of atoms. A few of nanocrystals (α-Mg and Ca2Mg5Zn13) were obtained due to repeated heat accumulation in heat-affected zones. More significantly, Y alloying altered the glass forming ability and stimulated in-situ formation of quasicrystal I-phase with high hardness and elastic modulus, which not only acted as reinforced particles, but also retarded the propagation of shear bond and thereby excited the emergence of secondary shear bands and vein patterns. Thus, Mg–Zn–Ca–Y part exhibited simultaneously improved yield strength and plasticity. Despite the micro galvanic corrosion induced by crystalline phases, it exhibited a moderate corrosion rate. Furthermore, in vitro cell tests proved its favorable biocompatibility. Our work highlighted the feasibility to prepare amorphous Mg alloy with controllable performance.

Quasicrystal-strengthened biomedical magnesium alloy fabricated by laser additive manufacturing

Magnesium (Mg) alloy, particular Mg-zinc (Zn) series alloy, owns great potential as implant materials, despite it shows uncontrollable corrosion rate and inadequate mechanical performance. Herein, alloying treatment with rare earth yttrium (Y) was adopted to improve the overall performance of Mg-Zn-Zr alloy. In detail, Mg-Zn-Y-Zr (ZW61) parts were fabricated using laser additive manufacturing. Results showed that Y addition had contributed to the formation of quasicrystal I-phase (Mg3Zn6Y), which could serve as weak cathode and alleviate the occurrence of micro-galvanic corrosion within Mg matrix. Meanwhile, Y addition also promoted the development of compact corrosion film, which further delayed the corrosion of Mg matrix. Thus, laser-processed ZW61 parts showed a considerably reduced corrosion rate of 0.67 mm/year. More importantly, the quasicrystal I-phase contributed to a more significant Orowan strengthening effect, as it exhibited a near-coherent interface with α-Mg. Besides, rare earth Y decreased the energy of stacking faults, and thereby activated numerous non-basal slip systems during deformation, as proved by TEM observation. Thus, as-built ZW61 part showed simultaneously improved strength and plasticity. Furthermore, in vitro cell testing proved its favorable biocompatibility. Our research exhibited that laser-processed ZW61 was a competitive candidate for orthopedics device application.

Laser glazing of a quasicrystal-reinforced Al-Cu-Fe-Cr alloy: Implications for use in additive manufacturing

Quasicrystal-reinforced aluminum metal matrix composites exhibit combinations of properties that are attractive for metal additive manufacturing. It is challenging to evaluate such systems due to difficulties in modeling the solidification and microstructural evolution. Here, laser glazing in a powder bed fusion instrument was used as an efficient method to evaluate an Al85Cu6Fe3Cr6 alloy, and the resulting microstructures were characterized using electron microscopy. The arc-melted alloy contained only crystalline phases, but pre-glazing of the surface gave a refined microstructure comprising I-phase quasicrystals, Al2Cu and FCC Al. Individual laser tracks produced in this layer under different conditions exhibited an extremely fine and uniform phase mixture with over 70% by volume of I-phase in a FCC Al matrix with a thin film of Al2Cu at the interface, and no cracking or macrosegregation. The reasons for the formation of I-phase, rather than the D-phase that has been reported previously in this alloy, are discussed.

Inhibiting effect of I-phase formation on the plastic instability of the duplex structured Mg-8Li-6Zn-1.2Y (in wt.%) alloy

In this work, the tensile behaviors of Mg-8%Li and Mg-8%Li-6%Zn-1.2%Y alloys at ambient temperature were investigated and compared. It revealed that the plastic instability of Mg-8%Li alloy was quite remarkable and the variation range of serrated flowing stress can reach 5 MPa. For the Mg-8%Li-6%Zn-1.2%Y alloy, the formation of I-phase (Mg3Zn6Y) can simultaneously enhance the tensile strength and eliminate the plastic instability phenomenon, whilst its ductility was degraded. The in-situ tensile tests revealed that for the Mg-8%Li alloy, the severity and number of slip traces present in both α-Mg and β-Li matrix phases increased remarkably with the applied tensile strain. However, slip traces were quite fine in β-Li matrix phase and could be clearly observed when the applied tensile strain exceeded 18%. Due to the incompatibility of plastic deformation occurred in two matrix phases, the induced strain concentration at α-Mg/β-Li interfaces caused their subsequent cracking. For the Mg-8%-6%Zn-1.2%Y alloy, the I-phase distributed at α-Mg/β-Li interfaces suppressed the plastic deformation of α-Mg matrix phase and the tensile strain was dominated by the β-Li matrix phase, resulting in the disappearance of plastic instability. Moreover, the plastic strain would preferentially concentrate at I-phase/β-Li interfaces and subsequently induced the cracking of I-phase.

Effects of icosahedral phase on mechanical anisotropy of as-extruded Mg-14Li (in wt%) based alloys

Through investigating and comparing microstructure and crystallographic texture of as-extruded Mg-14Li and Mg-14Li-6Zn-1Y (in wt%) alloys, the differences in their mechanical anisotropy were investigated. It revealed that the formation of I-phase (Mg3Zn6Y, icosahedral structure) can effectively refine grain size. Moreover, compared with Mg-14Li alloy, the texture type of Mg-14Li-6Zn-1Y alloy changed slightly, but its texture intensity decreased remarkably. As a result, the stronger texture contributed to the “normal” mechanical anisotropy of Mg-14Li alloy with higher tensile strength and a lower elongation ratio along transverse direction (TD) than those along extrusion direction (ED). However, for Mg-14Li-6Zn-1Y alloy, the zonal distribution of I-phase particles along ED caused “abnormal” mechanical anisotropy, i.e. higher tensile strength and better plasticity along ED.

Effect of Cu and Mn content on solidification microstructure, T-phase formation and mechanical property of Al[sbnd]Cu[sbnd]Mn alloys

Solidification microstructure of two AlCuMn alloys (Alloy-1: Al-1.6Cu-0.3Mn, Alloy-2: Al-2.2Cu-0.8Mn) and their precipitation behavior during solutionizing and aging treatments were investigated by optical microscopy, X-ray diffraction, Scanning and Transmission Electron Microscopies. The influence of Cu/Mn content and ratio on mechanical properties was also discussed in this study. Results reveal that Cu/Mn content and ratio have considerable influence on the solidification microstructure of these alloys. In Alloy-1 with low Cu/Mn content and ratio, only small amount of θ-CuAl2 and T-phase (Al20Cu2Mn3) are formed within the interdendritic regions and grains boundaries during solidification. But in Alloy-2 with high Cu/Mn content and ratio, a great amount of bone-like and round multiphase structure is observed in interdendritic regions, which consists of α-Al, θ-CuAl2 and Al13Cu4Mn3 phases. The Al13Cu4Mn3 phase in multiphase structure is demonstrated to be an icosahedral quasicrystalline phase (I-phase). The formation of I-phase in a conventional casting of AlCuMn alloy is related to the stabilizing effect of Fe. Cu/Mn content and ratio also have considerable influence on precipitation behavior of AlCuMn alloy during solutionizing and aging treatment. In Alloy-2, two morphologies of T-phase are formed: large particulate one is transformed from the instability decomposition of I-phase at high temperature and the fine rod-like is precipitated from supersaturated Al solid solution. And further aging does not lead to new precipitates. At each test temperature, both the YS and UTS of Alloy-2 are higher than that of Alloy-1 which is related to the formation of a great amount of T-phase particles during solution treatment.

Effect of Icosahedral Phase on Crystallographic Texture and Mechanical Anisotropy of Mg–4%Li Based Alloys

Through investigating and comparing the microstructure and mechanical properties of the as-extruded Mg alloys Mg–4%Li and Mg–4%Li–6%Zn–1.2%Y (in wt%), it demonstrates that although the formation of I-phase (Mg3Zn6Y, icosahedral structure) could weaken the crystallographic texture and improve the mechanical strength, the mechanical anisotropy in terms of strength remains in Mg–4%Li–6%Zn–1.2%Y alloy. Failure analysis indicates that for the Mg–4%Li alloy, the fracture surfaces of the tensile samples tested along transverse direction (TD) contain a large number of plastic dimples, whereas the fracture surface exhibits quasi-cleavage characteristic when tensile samples were tested along extrusion direction (ED). For the Mg–4%Li–6%Zn–1.2%Y alloy, typical ductile fracture surfaces can be observed in both “TD” and “ED” samples. Moreover, due to the zonal distribution of broken I-phase particles, the fracture surface of “TD” samples is characterized by the typical “woody fracture”.

Effect of icosahedral phase on the crystallographic texture and mechanical anisotropy of duplex structured Mg–Li alloys

Abstract Through comparing the microstructure and mechanical properties of Mg–6%Li and Mg–6%Li–6%Zn–1.2%Y alloys, the difference in their mechanical anisotropy have been deeply investigated. Due to the formation of I-phase (Mg 3 Zn 6 Y, icosohedral structure), the volume fraction of β-Li phases formed in Mg–6%Li–6%Zn–1.2%Y alloy was two times higher than that in Mg–6%Li alloy even these two alloys having the same Li content. Texture analysis demonstrated that I-phase particle stimulated nucleation of recrystallization (PSN) and the formation of higher volume fraction of β-Li phases can remarkably weaken the basal texture of α-Mg phases, resulting in a very weak mechanical anisotropy. On the contrary, the basal texture of the α-Mg phases in Mg–6%Li alloy was quite strong and obvious mechanical anisotropy between differently oriented samples can be observed.

Effect of icosahedral phase on the thermal stability and ageing response of a duplex structured Mg–Li alloy

The formation of I-phase (Mg3Zn6Y, icosahedral structure) can suppress the lithium diffusion from β-Li (Mg solid solution in Li) to α-Mg (Li solid solution in Mg) and then remarkably improve the thermal stability of β-Li. Although the lithium diffusion is suppressed by I-phase pockets, quick precipitation and growth of MgLiZn particles can lead to the age-softening response of β-Li. However, a normal age-hardening response of α-Mg can be maintained. Thus, the macro hardness of the Mg–6%Li–6%Zn–1.2%Y alloy depends on the competition between the age-softening of β-Li and age-hardening of α-Mg.

Effect of Ag content on thermal stability and crystallization behavior of Zr–Cu–Ni–Al–Ag bulk metallic glass

Thermal stability and crystallization behavior of Zr70−xCu12.5Ni10Al7.5Agx (x=0–16) bulk metallic glasses (BMGs) have been investigated. It is revealed that glass transition temperature (Tg), crystallization temperature (Tx) shift towards higher temperature with the increase of Ag content. The supercooled liquid region ΔTx increases from 72K for the Ag-free BMG to 110K for the BMG with x=16. Two or three stages of crystallization process with primary precipitation of icosahedral phase (I-phase) occurs in the BMGs with x=0–10. However, single stage of crystallization mode with the formation of Zr2Cu and/or Zr2Ni crystalline phase takes place in the BMGs with x=12–16. The formation of I-phase involves inward diffusion of Zr and outward diffusion of Cu and Ni. Partial substitution of Zr with Ag stabilizes the supercooled liquid by suppressing the redistribution of constitutional elements to form I-phase or crystalline phase, leading to the enhancement of thermal stability of Zr-based BMGs.

Microstructure and tensile properties of as-extruded Mg–Li–Zn–Gd alloys reinforced with icosahedral quasicrystal phase

Mg–Li–Zn–Gd alloys reinforced with icosahedral quasicrystal phase (I-phase) were prepared by casting and deformed by hot extrusion. The microstructure and tensile properties of as-extruded Mg–Li–Zn–Gd alloys were investigated. It is found that I-phase formed in as-cast Mg–Li–Zn–Gd alloys and was broken into small particles during extrusion. After hot extrusion, both α-Mg and β-Li phases were elongated along the extrusion direction. Dynamic recrystallization occurred in as-extruded Mg–Li–Zn–Gd alloys. Compared with Mg–9Li (L9) alloy, the improvement in mechanical properties especially the tensile strength of the as-extruded Mg–Li–Zn–Gd alloys was mainly ascribed to the formation of I-phase and their volume fraction. Moreover, the finely recrystallized grain structure can also attribute to the strength enhancement of the alloy.

Microstructure, electromagnetic shielding effectiveness and mechanical properties of Mg–Zn–Y–Zr alloys

The microstructure, electromagnetic interference (EMI) shielding effectiveness (SE) and mechanical properties of Mg–Zn–Y–Zr alloys with 0–3.91wt.% Y were investigated systematically in this work. The results indicated that addition of Y brought about the formation of I-phase (Mg3Zn6Y) and W-phase (Mg3Zn3Y2) and refined grains in as-cast state. After hot extrusion, there was more and more broken particles dispersing in matrix when Y content ranged from 0 to 3.19wt.%. With increasing Y content, EMI SE was enhanced significantly in extruded state. The alloy with 1.9wt.% Y exhibited the optimal EMI shielding capacity with the SE value of 79–118dB. It was found that good mechanical properties could be achieved by adding very low Y content. The extruded alloy with 0.5wt.% Y presented higher yield strength (268MPa), ultimate tensile strength (334MPa) and good elongation (δ=12.3%) compared with other extruded alloys. A subsequent aging treatment on the extruded alloy with 1.29wt.% Y exhibiting outstanding comprehensive EMI SE and mechanical properties resulted in precipitation of W, β1′ and β2′ phases, which led to further improvement in EMI SE. The peak-aged sample showed the superior mechanical properties. Based on the microstructure observation, the changes of EMI shielding capacity and mechanical properties have been discussed.

Effect of quasicrystalline phase on improving the corrosion resistance of a duplex structured Mg–Li alloy

The in situ formation of I-phase improves the corrosion resistance of the duplex structured Mg–6% Li alloy. Corrosion attack on the surfaces of Mg–6% Li alloy is very inhomogeneous and severe pits and filiform corrosion clearly occur on the surfaces after immersion in 0.1 M NaCl solution. For the I-phase-containing Mg–6% Li–6% Zn–1.2% Y alloy, the corrosion attack is homogeneous and no severe pits and filiform corrosion can be observed.

Experimental and calculated phases in two as-cast and annealed Mg–Zn–Y alloys

Abstract The CALPHAD (Calculation of Phase Diagram) method was used to select ternary alloys from Mg–Zn–Y system, aimed at determining the role of precipitates in the microstructure and texture evolution of Mg during and after deformation. The selected alloys are Mg–6Zn–1.2Y and Mg–5Zn–2Y. The constituent phases in the as-cast Mg–6Zn–1.2Y alloy are α-Mg solid solution phase and I (Mg3YZn6) intermetallic phase. The as-cast Mg–5Zn–2Y alloy is composed of α-Mg, I and W (Mg3Y2Zn3) phases. The intermetallics in the two alloys form by eutectic reaction, which in Mg–5Zn–2Y alloy results in initially W-phase formation and ultimately I-phase formation during solidification. After heat treatment, the Mg–6Zn–1.2Y and Mg–5Zn–2Y alloys contain nearly the same amount of ternary intermetallics (I and W phases, respectively) in equilibrium with α-Mg solid solution phase. The main solute in α-Mg phase is Zn with the same amount in the two alloys. The type and quantity of the phases obtained experimentally disagree with the results obtained from the thermodynamic database. One important discrepancy is that, in Mg–6Zn–1.2Y alloy, the I phase is not stable at the temperature of 430 °C, and that the W phase is the stable phase at this temperature. The differences in the experimental and calculated data indicate that the Mg–Zn–Y system requires to be reassessed with more experimental data.

Effect of Zn/Er weight ratio on phase formation and mechanical properties of as-cast Mg–Zn–Er alloys

The results of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and microstructure observations have shown that various Zn/Er weight ratio leads to different phase compositions in the investigated alloys. The composition range for the formation of icosahedral quasicrystalline (I-phase) and face-centered cubic phase (W-phase) in Mg-rich corner is obtained. The Zn/Er weight ratio adjusting from10 to 6 results in the formation of I-phase exclusively as main secondary phase in the as-cast Mg–Zn–Er alloys. However, when the ratio is less than 0.8, it is mainly contributed to the precipitation of W-phase. When the ratio is in the range from 1 to 4, I-phase coexisting with W-phase is inclined to be formed. The results of tensile tests suggest that the as-cast Mg–5Zn–5.0Er alloy with I-phase and W-phase possesses the highest ultimate strength (UTS). It is concluded that UTS and elongation rate (ER) of as-cast Mg–Zn–Er alloys with I-phase as the main secondary phase are excellent. The UTS of the alloys with coexistence of I-phase and W-phase show the tendency of increasing firstly then decreasing with the increasing volume fraction of W-phase. The UTS declines when W-phase completely replaces I-phase in the alloy.

Microstructures and mechanical properties of conventionally solidified Al 63Cu 25Fe 12 alloy

In this study, the microstructures and mechanical properties of conventionally solidified Al 63Cu 25Fe 12 alloy after different heat-treatments were investigated. The microstructures of the as-cast and subsequently heat-treated samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). The XRD results showed the presence of quasicrystalline icosahedral phase (i-phase) together with crystalline phases corresponding to β-AlFe(Cu) solid solution phase (β-phase) and τ-AlCu(Fe) solid solution phase (τ-phase). The SEM investigations clearly showed the formation of i-phase with pentagonal dodecahedra structure. However, the i-phase together with β-phase was also observed in the heat-treated samples and the peak intensity of the β-phase decreased with increasing heat-treatment temperature. From the DTA curves, the melting point of i-phase was determined as 890 °C for this alloy composition. Mechanical properties of the as-cast and subsequently heat-treated samples were measured by a Vickers indenter. Results showed that the microhardness ( HV) and the elastic modulus ( E) of the as-cast sample were around 598 kg fmm −2 (5.86 GPa) and 104 GPa, respectively. In addition, the characteristic of material plasticity ( δ H ) value was calculated to be 0.54.

Quasicrystalline phase formation during heat treatment in mechanically alloyed Al65Cu20Fe15 alloy

Quasicrystalline phase formation during heat treatment in the mechanically alloyed Al65Cu20Fe15 powders was studied by X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The mechanical alloying was performed at speed of 300 rev min–1 for times up to 70 h. It was found that mechanical alloying of the Al65Cu20Fe15 powders did not result in the quasicrystalline (QC) icosahedral phase (i-phase) formation. The long time milling resulted in the formation of a cubic Al (Cu,Fe) solid solution phase (β-phase). The cubic Al (Cu,Fe) solid solution identified as β-phase was observed to be present as one of the major phases in the Al65Cu20Fe15 alloy. The formation of the quasicrystalline icosahedral phase (i-phase) was only observed for short time milled powders after additional annealing at temperature above 500°C. The present investigation also showed that a tetragonal Al2Cu phase (θ-phase) forms with short time milling. The tetragonal Al7Cu2Fe1 phase (w-phase) was observed after heat treatment of the short time milled and unmilled powders. The present investigation indicated that an effective process to prepare the quasicrystalline materials was using a combination of short time milling and subsequent annealing.

Tensile and creep behaviors of Mg–5Zn–2.5Er alloy improved by icosahedral quasicrystal

The tensile and tensile creep properties of the as-cast Mg–5Zn–2.5Er (in wt.%) alloy strengthened by icosahedral quasicrystalline phase (I-phase) were studied. The Mg–5Zn–2.5Er alloy had better tensile properties both at room temperature and 175°C than AE42 and Mg–5Zn alloys. Compared to AE42 alloy, the as-cast Mg–5Zn–2.5Er alloy had smaller steady-state creep rate and total creep strain under the creep conditions of 175°C and 70MPa for 100h. The better creep resistance of the Mg–5Zn–2.5Er was mainly due to the formation of I-phase with high-thermal stability distributed along grain boundary. Moreover, microstructure analysis indicated that some fine precipitates in the α-Mg matrix can pin the dislocation effectively, improving the creep resistance of the alloy.

Influence of Higher Zn/Y Ratio on the Microstructure and Mechanical Properties of Mg-Zn-Y-Zr Alloys

This work mainly investigated the microstructure and mechanical properties of Mg-Zn-Y-Zr alloys with Zn/Y ratios of 5 and 10. An X-ray diffraction (XRD) analysis indicated that the two alloys were mainly composed of an icosahedral phase (I-phase) and α-Mg matrix. For the alloy with a Zn/Y ratio of 10, however, the diffraction peaks of the I-phase were stronger. Microstructure observation showed that the I-phase preferentially existed in the form of I-phase/α-Mg matrix interdendritic eutectic pockets at grain boundaries. Moreover, when the Zn/Y ratio was increased 2 times, the volume fraction of the I-phase in the α-Mg matrix increased 1.5 times and a tiny Mg7Zn3 phase formed. Energy-dispersive spectroscopy (EDS) mapping and electron probe microanalysis (EPMA) results suggested that the chemical composition of the I-phase was not a constant value. Computer-aided cooling curve analysis (CA-CCA) indicated that, for the alloy with a Zn/Y ratio of 5, formation of the I-phase relied on the W-phase transformation and the eutectic reaction of the residual melt. However, the I-phase formation for the alloy with a Zn/Y ratio of 10 depended on the eutectic reaction of the melt. Tensile tests indicated that the mechanical properties of the two as-cast alloys were poor. After hot extrusion processing, the mechanical properties of the alloy with a Zn/Y ratio of 10 were noticeably increased. The ultimate tensile strength (UTS) and elongation to failure reached 320 MPa and 13 pct, respectively.