Influence of high-intensity ultrasonic treatment on the phase morphology of a Mg-9.0wt.%Al binary alloy

The effect of ultrasonic treatment on the β-phase (Mg₁₇Al₁₂) morphology of an Mg-9.0wt.%Al alloy was studied. The result shows that with high-intensity ultrasonic vibration employed during the solidifying of Mg-9.0wt.%Al, the β-phase in the entire cross section of the billet is significantly refined and also changed from continuous to discontinuous morphology. Spherical β-phase is formed during the solidification of the billet treated with high-intensity ultrasonic vibration.     

Nonstoichiometric precipitation in As-Cast Mg-xAl-lZn alloy

In the present study, the nonstoichiometric precipitation of the β-phase in as-cast Mg-xAl-lZn alloy was investigated from the viewpoint of solidification behavior. β-phase (Mg₁₇Al₁₂) has a nonstoichiometric composition of 28-41 wt.% Al and 6-12 wt.% Zn. Moreover, the chemical composition of the precipitate in Mg-xAl-lZn alloy locally varies depending on the Al content in the alloy and the solidification rate. However, it still maintains the same a-Mn structure as the β-phase (Mg₁₇Al₁₂) in Mg-xA1 alloy. Also, the slope of the calibration curve between the volume fraction of precipitate and intensity ratio (I_ppt/I_Mg) is 0.35343 and 0.31995 in Mg-xAl and Mg-xAl-lZn alloy, respectively.     

Al-Si7-Mg消失模振动凝固组织半固态热处理

利用消失模铸造振动凝固技术,得到Al-Si7-Mg合金的近等轴晶组织。在此基础上研究了580℃不同时间保温半固态等温热处理(SSIT)组织中α-Al晶体的形貌与尺寸变化。结果表明:合金580℃保温1h,得到α晶粒的等积圆直径在50~150μm的分布比例达85%,α晶粒圆度均值1.41。通过凝固动力学和热力学分析,得出半固态热处理过程中,低熔点的三元共晶和二元共晶相首先熔化,并随着溶质元素扩散速度的增加,液相不断增多,α晶体形状大小不断改变;当液相率达到平衡之后,α晶粒在界面张力的作用下,不断圆整球化,合并长大。半固态热处理α枝晶演变形式明显不同于等轴晶演变过程。     

Refinement and strengthening mechanism of Mg−Zn−Cu−Zr−Ca alloy solidified under extremely high pressure

Mg?Zn?Cu?Zr?Ca samples were solidified under high pressures of 2–6 GPa. Scanning electron microscopy and electron backscatter diffraction were used to study the distribution of Ca in the microstructure and its effect on the solidification structure. The mechanical properties of the samples were investigated through compression tests. The results show that Ca is mostly dissolved in the matrix and the Mg₂Ca phase is formed under high pressure, but it is mainly segregated among dendrites under atmospheric pressure. The Mg₂Ca particles are effective heterogeneous nuclei of α-Mg crystals, which significantly increases the number of crystal nuclei and refines the solidification structure of the alloy, with the grain size reduced to 22 μm at 6 GPa. As no Ca segregating among the dendrites exists, more Zn is dissolved in the matrix. Consequently, the intergranular second phase changes from MgZn with a higher Zn/Mg ratio to Mg₇Zn₃ with a lower Zn/Mg ratio. The volume fraction of the intergranular second phase also increases to 22%. Owing to the combined strengthening of grain refinement, solid solution, and dispersion, the compression strength of the Mg–Zn–Cu–Zr–Ca alloy solidified under 6 GPa is up to 520 MPa.     

Effect of Y addition on microstructure and mechanical properties of Mg–Zn–Mn alloy

The effects of Y on the microstructure and mechanical properties of Mg–6Zn–1Mn alloy were investigated. The results show that the addition of Y has significant effect on the phase composition, microstructure and mechanical properties of Mg–6Zn–1Mn alloy. Varied phases compositions, including Mg₇Zn₃, I-phase (Mg₃YZn₆), W-phase (Mg₃Y₂Zn₃) and X-phase (Mg₁₂YZn), are obtained by adjusting the Zn to Y mass ratio. Mn element exists as the fine Mn particles, which are well distributed in the alloy. Thermal analysis and microstructure observation reveal that the phase stability follows the trend of X>W>I>Mg₇Zn₃. In addition, Y can improve the mechanical properties of Mg–Zn–Mn alloy significantly, and the alloy with Y content of 6.09% has the best mechanical properties. The high strength is mainly due to the strengthening by the grain size refinement, dispersion strengthening by fine Mn particles, and introduction of the Mg–Zn–Y ternary phases.     

Phase transformation behavior of Al9(Mn,Ni)2 eutectic phase during heat treatment at 600 °C in Al-4Ni-2Mn alloy

The morphology evolution and phase transformation of Al₉(Mn,Ni)₂ eutectic phase in an Al-4Ni-2Mn alloy during heat treatment at 600 °C were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results show that nearly all of the eutectic fibers change into prolate ellipsoid and spherical particles in the process of heat treatment, and Ostwald ripening phenomenon occurs in the eutectic region with the increase of the heat treatment time. Besides, a phase transformation from Al₉(Mn,Ni)₂ to O-phase is confirmed. The morphologies of the transformed particles indicate that the O-phase preferentially nucleates on the specific crystal plane of the Al₉(Mn,Ni)₂ eutectic phase and grows in a certain direction. During the phase transformation, the (010)[001] slip system in O-phase is activated, and the resultant slip traces appear on the surface of some O-phase particles.     

Microstructure transition from stable to metastable eutectic growth in Ni–25%Al alloy

The microstructural evolution during directional solidification of the Ni–25%Al (mole fraction) alloy was investigated in the range of growth velocity from 10 to 100 μm/s under a given thermal gradient of 10 K/mm. The solidification microstructures reveal a transition from γ‘–β equilibrium eutectic to γ–β metastable eutectic plus β dendrites. A mixed microstructure of γ‘–β and γ–β eutectics produced at a growth velocity of 25 μm/s illustrates that the transition occurs during the competitive growth between γ and γ' phases. The growth temperature for each phase was considered to understand the microstructure selection during solidification. The experimental results show that a phase or a microstructure solidifying with the highest temperature under a given growth condition is preferentially selected upon solidification. In addition, both stable eutectic and metastable eutectic are shown to coexist and simultaneously grow in the velocity range between 25 and 60 μm/s due to their similar growth temperatures.     

Solidification of Be-free Ni-based dental alloy

A novel, Nb- and Si-rich and Be-free Ni-based alloy was cast by two methods of investment casting and continuous casting to study the microstructure evolution during solidification and its mechanical properties. The solidification of the alloy started with the primary crystallization of FCC-γ, followed by a binary eutectic reaction, with the formation of a heterogeneous constituent: FCC-γ+G-phase, which replaced the low-melting eutectic (FCC-γ+NiBe) in the Be-bearing alloys. AlNi₆Si₃ and γ′ formed during the terminal stages of solidification by investment casting, while the formation of AlNi₆Si₃ was suppressed by continuous casting. The Scheil solidification model agreed very well with the experimental results.     

Method for determining reaction rate of mild steel containers during melting of magnesium-aluminium alloys and effect of aluminium content on directionally solidified microstructures

A magnesium alloy of eutectic composition (33 wt-%Al) was directionally solidified in mild steel tubes at two growth rates, 32 and 580 μm s^?1 in a temperature gradient between 10 and 20 K mm^?1. After directional solidification, the composition of each specimen varied dramatically, from 32%Al in the region that had remained solid to 18%Al (32 fim s^?1 specimen) and 13%Al (580 (μm s^?1 specimen) at the plane that had been quenched from the eutectic temperature. As the aluminium content decreased, the microstructure contained an increasing volume fraction of primary magnesium dendrites and the eutectic morphology gradually changed from lamellar to partially divorced. The reduction in aluminium content was caused by the growth of an Al-Fe phase ahead of the Mg-Al growth front. Most of the growth of the Al-Fe phase occurred during the remelting period before directional solidification. The thickness of the Al-Fe phase increased with increased temperature and time of contact with the molten Mg-Al alloy. IJCMR/455     

Microstructure evolution of Al0.6CoCrFeNi high entropy alloy powder prepared by high pressure gas atomization

The influence of cooling rate on the microstructure of Al_0.6CoCrFeNi high entropy alloy (HEA) powders was investigated. The spherical HEA powders (D₅₀≈78.65 μm) were prepared by high pressure gas atomization. The different cooling rates were achieved by adjusting the powder diameter. Based on the solidification model, the relationship between the cooling rate and the powder diameter was developed. The FCC phase gradually disappears as particle size decreases. Further analysis reveals that the phase structure gradually changes from FCC+BCC dual-phase to a single BCC phase with the increase of the cooling rate. The microstructure evolves from planar crystal to equiaxed grain with the cooling rate increasing from 3.19×10⁴ to 1.11×10⁶ K/s.     

Stability of remelting and solidification interfaces of triple-phase region during peritectic reaction at lower speed

Peritectic reaction was studied by directional solidification of Cu-Ge alloys. A larger triple junction region of peritectic reaction was used to analyze the interface stability of the triple junction region during peritectic reaction. Under different growth conditions and compositions, different growth morphologies of triple junction region are presented. For the hypoperitectic Cu-13.5%Ge alloy, as the pulling velocity (v) increases from 2 to 5 μm/s, the morphological instability of the peritectic phase occurs during the peritectic reaction and the remelting interface of the primary phase is relatively stable. However, for the hyperperitectic Cu-15.6%Ge alloy with v=5 μm/s, the nonplanar remelting interface near the trijunction is presented. The morphological stabilities of the solidifying peritectic phase and the remelting primary phase are analyzed in terms of the constitutional undercooling criterion.     

Effect of mixed rare earth oxides and CaCO<Subscript>3</Subscript> modification on the microstructure of an <Emphasis Type="Italic">in-situ</Emphasis> Mg<Subscript>2</Subscript>Si/Al-Si composite

The effects of mixed rare earth oxides and CaCO₃ on the microstructure of an in-situ Mg₂Si/Al-Si hypereutectic alloy composite were investigated by optical microscope, scanning electron microscope, and energy dispersive spectrum analysis. The results showed that the morphology of the primary Mg₂Si phase particles changed from irregular or crosses to polygonal shape, their sizes decreased from 75 μm to about 25 μm, and the compound of both the oxide and CaCO₃ was better than either the single mixed rare earth oxides or CaCO₃.     

Mechanical properties of Al–Cu–Fe quasicrystalline and crystalline phases: An analogy

The mechanical properties of the ω-Al₇Cu₂Fe crystalline phase have been investigated over a large temperature range (650–1000 K). Despite of its antinomic structure with the icosahedral Al–Cu–Fe quasicrystalline phase, i.e. periodic vs non-periodic, its mechanical properties are very similar to those of the quasicrystalline phase, which strongly suggest similar deformation mechanisms. Consequently, as for the quasicrystalline structure, we propose that dislocation climb might control the plastic deformation of the ω-phase. However, in the present case, the specificities of the quasicrystalline structure cannot be invoked to justify the predominance of dislocation climb, which questions the role of quasiperiodicity on dislocation mobility. We suggest that this deformation mode certainly results from specific non-planar extensions of the dislocation core.     

δ-phase precipitation regularity of cold-rolled fine-grained GH4169 alloy plate and its effect on mechanical properties

Cold rolling and heat-treatment were used for the grain refinement of GH4169 superalloy plate. The effects of cold rolling reduction ratio and heat-treatment time on the precipitated δ phase, and the effects of δ-phase content and morphology on the mechanical properties of the GH4169 alloy plates, are studied. The results demonstrate that cold- rolling can promote the precipitation of the δ phase and its transformation from the δ-Ni₃Nb phase to the δ-NbNi₄ phase. The comprehensive properties of the alloy are better when the heat treatment time is 1 h, with 132 MPa increase in the tensile strength and only 2.9% decrease in the elongation relative to those of the original material. The mechanical properties of the alloy are shown to change greatly with the change in the δ-phase morphology.     

Solidification behaviour and the effects of homogenisation on the structure of an Al-Cu-Mg-Ag-Sc alloy

The solidification behaviour and structural evolution during homogenisation annealing of a 0.17 mass% Sc and 0.1 mass% Ge modified Al-Cu-Mg-Ag alloy was examined. The formation of the primary Mg₂Ge phase and the Sc-enriched θ-phase (Al₂Cu) was found to occur under solidification; no evidence for the formation of the ternary W-phase (Al_8−xCu_4+xSc) was observed. Approximately 70% of the overall Sc content is fixed in the supersaturated solid solution. However, only two thirds of this Sc is consumed by the formation of a dispersion of nanoscale Al₃Sc particles within the Al matrix under homogenisation annealing. A third of the Sc is consumed by the formation of the W-phase because of Sc diffusion into the primary θ-phase, which leads to its transformation into the W-phase. Finally, the W-phase consumes ∼50% of the overall Sc content. No effects of homogenisation annealing on the volume fraction of Mg₂Ge were observed. The optimal chemical composition of alloys in the Al-Cu-Mg-Ag-Sc system is discussed.     

Microstructures and mechanical properties of conventionally solidified Al63Cu25Fe12 alloy

In this study, the microstructures and mechanical properties of conventionally solidified Al₆₃Cu₂₅Fe₁₂ 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.     

Al-Cu-Cr quasicrystalline coatings prepared by low power plasma spraying

Al-Cu-Cr quasicrystalline coatings were prepared by low power plasma spraying with axially-fed powder systerm. The Al₆₅Cu₂₀Cr₁₅ powders were deposited on AISI 1045 steel substrate at the power ranged from 4.0 to 6.0 kW. The effects of H₂/Ar flow ratio on the phase composition, microstructure and microhardness properties of the as-sprayed coatings were investigated. The XRD results showed that the original powders and as-sprayed coatings contained a predominant icosahedral quasicrystalline phase I-Al₆₅Cu₂₄Cr₁₁ and three minor crystalline phases, including a body-centered cubic α-Al₆₉Cu₁₈Cr₁₃, a monoclinic θ-Al₁₃Cr₂ (i.e. Al₈₃Cu₄Cr₁₃) and a hexagonal ε-Al₂Cu₃. A qualitative analysis on the XRD patterns indicated that the volume fraction of any crystalline phase (α, ε or θ) in the coatings increased, while the quasicrystalline I-phase decreased with increasing hydrogen content in the plasma gas. As H₂/Ar flow ratio increased from 4.8% to 18.8%, the coating hardness increased whilst its porosity decreased, and they reached the maximum (4.98 Gpa) and the minimum (8%) respectively. However, with increasing H₂ content further, the hardness of the coating slightly decreased and its porosity increased because of the excessive vaporization of aluminum at higher plasma energy.     

In situ observation of solidification behavior in undercooled Pd–Cu–Ni–P alloy by using a confocal scanning laser microscope

In the undercooled melt of Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy, the solidification behavior including the nucleation and growth of crystals at the micrometer level has been observed in situ by use of a confocal scanning laser microscope combined with an infrared image furnace. The Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy specimens were cooled from the liquid state to various undercooled states under a helium gas flow. Images of solidification progress were obtained by the charge-coupled device image sensor of the confocal scanning laser microscope. Depending on the degree of undercooling, the morphology of the solidification front changed among various types: faceted front, columnar dendritic front, cellular grain and equiaxed grain, etc. The velocities of the solid–liquid interface were measured to be 10⁻⁵–10−7 m/s, which are at least two orders of magnitude higher than the theoretical crystal growth rates. Combining the morphologies observed in the three undercooling regimes and their solidification behaviors, we conclude that phase separation takes place in the undercooled molten Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy. The continuous-cooling–transformation (CCT) diagram was derived from the experimental time–temperature-transformation diagram constructed from solidification onset times under various isothermal annealing conditions. The CCT diagram suggests that the critical cooling rate for glassy solidification is about 1.8 K/s, which is in agreement with previous calorimetric findings.     

Microstructural evolution during partial remelting of AM60B magnesium alloy refined by MgCO3

AM60B magnesium alloy was refined by MgCO₃ and its microstructural evolution was investigated during partial remelting. The results indicate that MgCO₃ is an effective grain refiner for AM60B alloy and can decrease the grain size from 329 μm of the unrefined alloy to 69 μm. A semisolid microstructure with small and spheroidal primary particles can be obtained after being partially remelted. The microstructure evolution can be divided into four steps: the initial rapid coarsening, structure separation, spheroidization and final coarsening. Correspondingly, these four steps result from the phase transformations of β→α, α+β→L and α→L, α→L and two reverse reactions of α→L and L→α, respectively. One spheroidal primary particle in the semisolid microstructure usually originates one dendrite in the as-cast microstructure. The variation of primary particle size with holding time does not obey the LSW law, D_t³?D₀³=Kt, after the semisolid system is in its solid-liquid equilibrium state. Longer heating duration makes the primary particles more globular, but it makes their size larger at the same time.     

Icosahedral quasicrystalline phase in an as-cast Mg-Zn-Er alloy

The microstructure of an as-cast Mg-Zn-Er alloy was investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). The results indicate that two different second phases, one with eutectoid-lamellar morphology and the other with granular shape, distribute in the α-Mg matrix. The coexistence of the face-centered icosahedral quasicrystalline phase (I-phase) and W-phase with the face-centered cubic structure is found in the as-cast alloy. The coexistence of I-phase and W-phasc in the Mg-Zn-Er alloy is because the W-phase is the primary phase and the I-phase forms by peritectic reaction during solidification.