Microstructure Of As-cast Alloy Research Articles

Effects of multi-scale particles on the microstructure and mechanical properties of Mg-9Gd composites

In this paper, the multi-scale particles (Mg12La, Al2Gd, and TiB2) were successfully introduced to Mg-9Gd alloy in casting method. With the introduction of multi-scale particles, the microstructure of as-cast alloys transforms from dendrites into equiaxed crystals. Compared to single-sized submicron-Mg12La particles and bimodal micron-Al2Gd + nano-TiB2 particles, the multi-scale particles (Mg12La, Al2Gd, and TiB2 particles) show the best grain refining effect. The average grain size reduced from 299-734 μm in Mg-9Gd alloy to 44.3–61.4 μm in 6.1 wt% (TiB2-Al2Gd)/Mg-9Gd-0.5La composite. The YS (211 MPa) and UTS (263 MPa) of the 6.1 wt% (TiB2-Al2Gd)/Mg-9Gd-0.5La composite were 42.6 % and 15.9 % higher than those of the Mg-9Gd alloy (148 MPa and 227 MPa), respectively. The excellent strength of the extruded composite is attributed to the synthetic strengthening effects of Mg12La, Al2Gd, and TiB2 particles.

Experimental Investigation of Phase Equilibria of the Ho-Ir-O Ternary System at 1073 K.

An experimental study of the phase equilibria of the Ho-rich part of the Ho-Ir-O ternary system at 1073 K by means of x-ray diffraction, differential scanning calorimetry, and scanning electron microscopy has been carried out. Ho-hcp and four binary compounds, namely Ho3Ir, Ho5Ir2, Ho5Ir3, and Ho2O3, were identified in the Ho-Ir-O model alloys after long-term annealing (350-1220 h). No solubility of iridium in Ho2O3 oxide and Ho-hcp was observed. No ternary phase was found. Based on the experimental results, an isothermal section of the Ho-rich part of the Ho-Ir-O system at 1073 K was constructed. In addition, the microstructure of as-cast alloys was studied. An irregular eutectic consisting of faceted Ho-phase in Ho3Ir phase was observed in the alloys with Ho-hcp + Ho3Ir + Ho2O3 phase composition, and the temperature of the eutectic reaction Ho-hcp + Ho3Ir ↔ liquid was determined.

A novel method towards improving the hydrogen storage properties of hypoeutectic Mg-Ni alloy via ultrasonic treatment

Ultrasonic treatment has great contributions on modifying the morphology, dimension and distribution of constituent phases during solidification, which serve as dominate factors influencing the hydrogen storage performance of Mg-based alloys. In this research, ultrasonic treatment is utilized as a novel method to enhance the de-/hydriding properties of Mg-2Ni (at.%) alloy. Due to ultrasonic treatment, the microstructure of as-cast alloy is significantly refined and homogenized. Ascribing to the increased eutectic boundaries and shortened distance inside α-Mg for hydrogen atoms diffusion, the hydrogen uptake capacities and isothermal de-/hydriding rates improve effectively, especially at lower temperature. The peak desorption temperature reduces from 392.99 °C to 345.56 °C, and the dehydriding activation energy decreases from 101.93 kJ mol−1 to 88.65 kJ mol−1. Weakened hysteresis of plateau pressures and slightly optimized thermodynamics are determined from the pressure-composition isotherms. Owing to the refined primary Mg, a larger amount of hydrogen with the higher hydriding proportion is absorbed in the first stage when hydrides nucleate in eutectic region and grow on primary Mg periphery subsequently before MgH2 colonies impinging, resulting in the enhancement of hydrogenation rates and capacities.

Experimental investigation and thermodynamic description of the Fe–Hf–Nb system

According to solidification microstructure of as-cast alloys and phase constituents of annealed samples, the liquidus surface projection and phase relationships at 1373 and 1273 K in the Fe–Hf–Nb system were constructed. A ternary phase τ with AlCuHf-structure was found, and the composition ranges of Nb in τ at 1373 and 1273 K were ∼ 20.5–30.8 at.% and ∼ 20.8–30.2 at.%, respectively. Moreover, the maximum solubilities of Nb in FeHf2 was determined to be ∼ 16.9 at.% and ∼ 16.0 at.% at 1373 and 1273 K, and the maximum solubilities of Hf in μ was measured to be ∼ 4.6 at% and ∼ 3.3 at.% at 1373 and 1273 K. Fe2Hf and Fe2Nb with the same C14 structure formed a homogeneous solid solution from Fe2Hf to Fe2Nb at 1373 and 1273 K. Seven primary solidification regions in the liquidus surface projection and two three-phase regions and seven two-phase regions in the isothermal sections at 1373 and 1273 K were experimentally determined. On the basis of experimental data, the thermodynamic parameters of individual phases in the Fe–Hf–Nb system were optimized by CALculation of PHAse Diagram (CALPHAD) method. Four solution phases (liquid, bcc, fcc, and hcp) were described as the substitutional solution. The intermetallic phases FeHf2, λ1, λ2, λ3 and τ were modeled as (Fe)1(Hf,Nb)2, (Fe,Hf,Nb)2(Fe,Hf,Nb)1, (Fe)2(Hf)1, (Fe)2(Hf,Nb)1, and (Fe,Hf,Nb)1(Fe,Hf,Nb)1 by a two-sublattice model, respectively. μ was described as (Fe,Hf,Nb)1(Hf,Nb)4(Fe,Hf,Nb)2(Fe,Hf,Nb)6 by a four-sublattice model. A set of self-consistent thermodynamic parameters of the Fe–Hf–Nb system was obtained.

Experimental investigation and thermodynamic description of the Co–Nb–Ti system

The solidification microstructures of as-cast alloys and the equilibria microstructure of annealed alloys at 1273 K in the Co–Nb–Ti system were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD), and the liquidus surface projection and partial isothermal section at 1273 K were constructed. Nine primary solidification regions, bcc(Nb,Ti), fcc(Co), Co3Ti, CoTi, CoTi2, λ1, λ2, λ3, and μ and five ternary invariant reactions, liq. → λ1 + λ2 + μ, liq. → CoTi + λ2 + μ, liq. → bcc(Nb,Ti) + CoTi + μ, liq. + fcc(Co) + λ3 → Co3Ti, and liq. + CoTi → bcc(Nb,Ti) + CoTi2 were experimentally determined or deduced in the liquidus surface projection. Three three-phase regions, CoTi + λ2 + μ, bcc(Nb,Ti) + CoTi + μ, and bcc(Nb,Ti) + CoTi + CoTi2 and six two-phase regions, Co3Ti + λ3, λ2 + μ, CoTi + λ2, CoTi + μ, bcc(Nb,Ti) + μ, and bcc(Nb,Ti) + CoTi at 1273 K were experimentally determined. The continuous compound λ2 with a C15 structure from the Co–Nb side to Co–Ti side was formed at 1273 K. The solubilities of Ti in μ and Nb in CoTi were determined to be ~11.1 at% and ~18.4 at% at 1273 K, respectively. And no new phases were found. According to the available experimental data from the present work and literatures, the Co–Nb–Ti system was optimized by CALPHAD (CALculatiuon of PHAse Diagram) method. Four solution phases (liquid, bcc, fcc, and hcp) were described using the substitutional solution model. A two-sublattice model (Co,Nb,Ti)2(Co,Nb,Ti)1 was used to described three compounds λ1, λ2, and λ3 according to their crystal structures and homogeneity ranges. Co7Nb2 and CoTi2 were treated as Co7(Nb,Ti)2 and Co(Nb,Ti)2 by a two-sublattice model, respectively. μ was treated as (Co,Nb,Ti)1(Nb,Ti)4(Co,Nb,Ti)2(Co,Nb,Ti)6 by a four-sublattice model. Co3Ti with L12 structure and CoTi with B2 structure were treated as the ordered phases of fcc and bcc solutions using the thermodynamic models (Co,Nb,Ti)0.75(Co,Nb,Ti)0.25Va1 and (Co,Nb,Ti,Va)0.5(Co,Nb,Ti,Va)0.5Va3, respectively. A set of self-consistent thermodynamic parameters of the Co–Nb–Ti system was obtained.

Theoretical critical metastability temperature to interpret phase formation in a lamellar-like-structured high entropy alloy

Control over a lamellar-like-structured high entropy alloy (HEA) system is found to be possible by replacement of aluminium with boron into the FeCoNi(Bx Al1-x)0.1Si0.1 composition in the range of x = 0 to 1.0. The BCC/B2 microstructure of FeCoNi(Al0.1Si0.1) HEA is changed into a multiple phase system comprising of BCC/B2, FCC and Fe2B-type intermetallic phases. The microstructures of as-cast alloys were seen to be a lamellar-like structure comprised of nanostructured lamellae with alternating FCC and Fe2B phases. The concept of critical metastability temperature from nucleation theory is employed phenomenologically, and found to correlate with the presence of Fe2B in this alloy at different boron additions. With the substitution of boron, the stability of the disordered BCC solid solution is reduced, promoting the formation of secondary phases. We show a link between the interfacial energy of the phases present, the interlamellar spacing, and alloy metastability as a function of boron addition, the key relationship being that Fe2B phase formation correlates with a drastic reduction in the interfacial energy. These correlations bear further investigation and may be useful in the design of lamellar-like-structured multi-component systems.

Revealing complex microstructure formation mechanisms in the Al[formula omitted]Cr[formula omitted]Fe[formula omitted]Ni[formula omitted]Ti[formula omitted] compositionally complex alloy

The complex phase evolution sequence of a promising new high temperature structural alloy with composition Al20Cr20Fe35Ni20Ti5 (at.%) has been studied in situ using an electromagnetic levitation (EML) setup combined with time-resolved high energy X-ray diffraction (XRD), as well by scanning transmission electron microscopy (STEM) observations during in situ heating. These experiments were supplemented by both scanning electron microscopy (SEM) and high resolution STEM observations of the as-cast alloy microstructure, as well as by energy dispersive X-ray spectroscopy (EDS) composition measurements. Differential scanning calorimetry (DSC) was used to identify liquidus, solidus, A2 ↔ B2, and B2 ↔ L21 transition temperatures. The results obtained by these complementary techniques indicate that the alloy forms primary dendrites of A2-structured body-centered cubic (bcc) phase initially from the melt, followed by ordered bcc of B2-structure in interdendritic regions once compositional segregation has sufficiently enriched the remaining liquid. Ordered B2 precipitates form within the primary A2-structured dendrites and the interdendritic B2 regions undergo a continuous decomposition reaction into a maze-like arrangement of A2 and B2/L21 phases below the solidus. Below approximately 1380 K, the B2 domains in both dendritic and interdendritic regions likely undergo a continuous ordering transition to the L21 phase. The experimental observations of Al20Cr20Fe35Ni20Ti5 are found to be in good agreement with CALPHAD predictions but suggest that the solubility limits of the L21-Heusler phase in the database should be relaxed to include greater Fe solubility for the Ni sublattice and to consider deviations in order parameter from stoichiometric ordering.

Application of Color Metallography on As-cast Al-Mg-Si-Cu-Mn Alloy during Heat Treatment

Color etching was performed to characterize microstructure evolution and phase transformation on as-cast and homogenized Al-Mg-Si-Cu-Mn alloys. Initial intermetallic phases in microstructure of as-cast Al-Mg-Si-Cu-Mn alloy are mainly composed of primary Mg2Si, α-Al(FeMn)Si and quaternary Q phase. Post-etching microstructure of as-cast alloy presents a typical dendritic morphology with distinctive intermetallic distributions. Micro-segregation of solute elements are visualized as color difference within grains. Q phase is mainly located between the dendrite arms, while Mg2Si and α-Al(FeMn)Si are arranged along the grain boundaries. Heterogeneous nucleation sites provided by Al3Ti during solidification are also observed and identified at the core of grains. The distribution of alloying elements and intermetallic exhibited by color micrograph presents a good agreement with outcomes of EPMA. The color metallography of etched sample reveals the distribution of α-Al(MnCr)Si dispersoids and dispersoids free zone (DFZ), after the alloy is subjected to two-step homogenization. Micro-segregation of solute elements within solid solution is dramatically eliminated, associated with reduction of color difference. The dissolution of primary Mg2Si and Q phase during two-step homogenization are also directly detected. Therefore, color etching is an effective and reliable auxiliary approach to reveal microstructural evolution and phase transformation of as-cast Al-Mg-Si-Cu-Mn alloy during homogenization process.

Effect of grain size and volume fraction of eutectic structure on mechanical properties and corrosion behavior of as-cast Zn–Mg binary alloys

In this work, the effects of Mg content on the microstructure evolution, mechanical properties, and corrosion behavior of as-cast Zn-xMg (x = 0.1, 0.3, 0.5, 1.0, 1.5, and 3.0, wt.%) binary alloys were investigated. The microstructure of as-cast alloys is composed of α-Zn matrix and a ternary eutectic phase (α-Zn + Mg2Zn11+MgZn2). With the increase of Mg content, the average grain size of the alloys decreases from 104 μm to 11 μm while the volume fraction of eutectic structure increases from 3% to 84%, resulting in the improvement of tensile strength. In vitro immersion tests demonstrate that the corrosion rate decreases to a minimum value of 0.086 mm y−1 as Mg content increases from 0.1 to 0.3, followed by an increasing trend with the further increase of Mg content. The variation of corrosion rate with Mg content originates from the competition between the positive effect of grain refinement and the negative effective of eutectic volume fraction increment. Zn-0.3 Mg alloy has the optimal corrosion resistance because of the dominant role of grain refinement induced corrosion resistance enhancement. In addition, the corrosion mechanism varies from localized pitting to a more uniform corrosion as Mg content rises.

Microstructure and mechanical properties of as-cast and directionally solidified large size Ti–45Al–8Nb alloys by electromagnetic confinement

Microstructure, fracture toughness and high-temperature tensile property of as-cast and directionally solidified Ti–45Al–8Nb alloys were investigated. The microstructure of as-cast alloy consists of equiaxed grains, in which there are a large number of reticular B2 phases and massive γ phases. Compared with as-cast alloy, the transverse grain boundaries of the directionally solidified alloy were eliminated, and the lamellar orientation was controlled. The volume fraction of α2 phase increased from 11.6% in as-cast alloy to 19.3% in directionally solidified alloy and the volume fraction of γ phase changed little. From as-cast condition to directional solidification, the morphology of B2 phase changed from reticular shape to elongated shape, and the volume fraction decreased from 10.2% to 1.4%. The lamellar spacing of the alloy decreased from 1500 nm to 1020 nm and it becomes more uniform. The room temperature fracture toughness increases from 21.1 MPa m1/2 in as-cast alloy to 29.5 MPa m1/2 in directionally solidified alloy. The high-temperature tensile strength and the elongation at 850 °C of the alloy increased from 376.6 ± 16 MPa and 4.18 ± 0.9% in as-cast alloy to 592.5 ± 29.5 MPa and 11.6 ± 1.5% in directionally solidified alloy. Correspondingly, the fracture mode changed from intergranular fracture in as-cast alloy to translamellar fracture in directionally solidified alloy. In addition, the toughening mechanism and strengthening mechanism were discussed.

Microstructure evolution, mechanical properties, and corrosion behavior of novel Zr–Ti–V alloys

Effects of different V contents on microstructure and properties of Zr–Ti alloys are systematically investigated in this study. Microstructure of as-cast alloys is characterized by optical microscope, X-ray diffractometry, scanning electron microscopy, electron probe microanalysis, and transmission electron microscopy. Mechanical properties of alloys are analyzed using compression and microhardness tests and their corrosion properties are examined using electrochemical tests. α lath gradually decreases and relative content of β phase increases as the V content increases. A stable β alloy can be obtained when the V content is 8 at%. α lath precipitation at β grain boundary reduces plasticity of alloys, and as-cast alloys demonstrate satisfactory plastic deformation when the V content is 6–10 at%. The addition of V element reduces the corrosion resistance of Zr–Ti binary alloys, while Zr–Ti–8V alloy exhibits improved mechanical properties and corrosion resistance.

Effect of microstructural evolution in thermal exposure on mechanical properties of HP40Nb alloy

The phase transformation of HP40Nb alloy at various temperatures and times was investigated and the microstructural evolution in aging process was characterised. Additionally, the mechanical tests were implemented for aged specimens. Experimental results indicate the microstructure of as-cast alloy consists of the austenite matrix with skeletal-shaped primary carbides. NbC carbide changes into G phase from 700 °C to 990 °C, and the nose temperature of transformation ranges from 900 °C to 950 °C. M7C3 carbide tends to transform into M23C6 carbide at higher temperature. Meanwhile, numerous secondary carbides are precipitated in the initial aging stage and dissolves gradually in further process. The interdendritic precipitates coalesce and coarsen continuously in aging treatment. The tensile strengths and the micro-hardness increase slightly followed by the continuous reduction in exposure, while the elongation and impact toughness decrease continuously. The microstructural evolution during aging process is consistent with the variation of mechanical properties.

Austenitic transformation and influence of prior ferrite formation on martensite transformation characteristics of Al-added P91 ferritic-martensitic steel

In the present study austenitic transformation and effect of prior ferrite formation on martensite transformation characteristics of 0.48 wt% Al-added P91 steel was investigated by thermal analysis using differential scanning calorimetry and related microstructure characterization. Starting microstructure of as-cast alloy was dual phase consisting of α′-martensite+δ-ferrite. Austenitization heat treatments were carried out in the temperature range of 1273-1673 K at constant heating/cooling rates and hold time. γ-austenite formation from the initial dual phase structure occurred in two stages with increasing temperature: α-ferrite (martensite) → γ-austenite transformation at 1170 K followed by δ-ferrite and Al enriched fibrous α-ferrite → γ formation that begins at 1210 K. Isothermal characteristics of γ-austenite transformations were studied by Dictra® simulations. Elemental segregations were found to be the reason for two stage γ-austenite formation with temperature. These transformations involving α-ferrite, δ-ferrite and γ-austenite and extent of these transformations at various temperatures affected the martensite formation characteristics. Formation of α′-martensite was favored after heat treatment at 1373 K and 1473 K. Dissolution of ferrite and corresponding changes in the stability of γ-austenite at 1273 K and 1373 K, interfacial effects at 1473 K, grain size and new δ-ferrite formation effects at 1573 K and 1673 K dominated martensite transformation of Al-added P91 steel.

The microstructure and creep properties of as-cast Mg-Sn-Si-(Al) magnesium alloys

Magnesium alloys containing rare earth metals exhibit good creep resistance up to 300 °C and good tensile properties at ambient temperature. The high cost of rare earth has led to studies regarding the creep resistance of Mg alloys with cheap alloying elements (Sn, Ca, Si) that could be substituted for Mg-RE alloys. In this paper, the influence of Si and Al on microstructure and mechanical properties of Mg-7Sn alloy was investigated using optical (LM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), tensile tests and creep tests at 200–250 °C. Microstructure of as-cast alloys consists of α-Mg matrix and intermetallic compounds at the interdendritic regions. Heat treatment consisting of solid solution treatment and ageing increases the tensile properties at ambient temperature due to the precipitation of the fine Mg2Sn phase. The creep resistance of aged Mg-7Sn alloy is poor. The addition of Si and Al to Mg-7Sn alloy has resulted in improving the creep resistance due to the refinement of Mg2Sn phase and the appearance of Mg2Si phase at the grain boundaries. The Mg-7Sn-1Si alloy exhibits better creep resistance at 200 °C than Mg-7Sn-5Si and Mg-7Sn-5Si-2Al alloys. The Mg-7Sn alloys with 5% Si have better creep properties at 250 °C in comparison to Mg-7Sn-1Si alloy.

Prospects of Making Nanoporous Ruthenium from Transition Metal-Ru Alloys

Dealloying of Ag-Au alloys is known to produce Au-rich, bicontinous, nanoporous layers [1-2]. The high functionality of nanoporous gold, demonstrated in a variety of applications [3-4], invites us to study the fabrication of other nanoporous metals of interest by exploiting corrosion processes. Recent studies have shown how the use of ruthenium, which is the cheapest Pt-group metal, for catalysis could be expanded from the classical example of CO oxidation [5], to other important reactions such as hydrogen evolution [6]. Although several porous/non-porous Ru-containing systems have been studied before [7-8], there remains a knowledge-gap with regards to the design and optimization of Ru-based catalysts.As a first step towards the development of nanoporous ruthenium by metal corrosion, this work shall focus on the design of binary transition metal-Ru systems. High resolution APT (atom probe tomography) studies of Ru-lean alloys (Fe-Ru and Ni-Ru) will assess the bulk microstructure of as-cast alloys and how it is affected by processing conditions [9]. Furthermore, pathways to inducing Ru-surface enrichment in different thermochemical environments (oxidative/inert/reductive) will be investigated by combined APT and XPS (X-ray photoelectron spectroscopy) [10]. The results of this study will provide guidance to making low-cost, high surface area nanoporous ruthenium catalysts by dealloying in acidic media.

Microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy

The microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy were investigated using OM, SEM, EDS, TEM and tensile tests. The microstructure of as-cast alloy, including the well-aligned lamellar structures, the radial emanating lamellar structure and mesh-like structure, consists of NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. From the longitudinal section of directionally solidified alloy, when the withdrawal rate is 6 μm/s, the solid-liquid interface morphology is planar, and the well-aligned lamellar structure is observed. The three-dimensional morphologies of well-aligned lamellar structures are well characterized by deep etching. With increasing the withdrawal rate to 15 μm/s, the solid-liquid interface morphology change from planar to cellular, and the microstructure changes from planar eutectic to cellular eutectic, even the dendritic eutectic at local region occurs at 60 and 120 μm/s. From the transverse section, the well-aligned lamellar structures occur in the cell interior, and the interlamellar spacing decreases gradually with increasing the withdrawal rates. The TEM result indicates that NiAl-rich phase has an orientation relationship with CoCrFeNi-rich phase of [1¯11] NiAl-rich//[1¯22]CoCrFeNi-rich and (211)NiAl-rich//(022¯)CoCrFeNi-rich. The nano-precipitates occur in NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. The directionally solidified alloy possesses a good combination of strength and ductility at 60 μm/s. In particular, using the directional solidification technology is a good way to improve the ductility of the alloy (∼37%). The strengthening and plasticizing mechanisms are discussed by analyzing the fracture and side surfaces.

Investigation of mechanical properties and microstructural behavior of 7050 aluminium alloy by multi directional forging technique

Multi directional forging (MDF) is a process of severe plastic deformation used to produce fine grained bulk materials. In the present study, 7050 Aluminium alloy is used due to its wider applications in the field of aerospace industry particularly for making blade stringers. During experimentation, As-cast alloy samples were subjected to solutionizing heat treatment at 400 °C for 20 h to remove segregation effects and undergone multidirectional forging at ambient warmth for different equivalent strains. The mechanical properties such as Vickers’s hardness and compression tests were done on processed and unprocessed samples. The microstructural characterization was done using optical microscopy. It was observed that initial compression strength was increased from 505 MPa to 586 MPa and hardness was increased from 141 Vickers hardness number (VHN) to 182 Vickers hardness number (VHN) due to refinement of grains from 60 µm to 10 µm. It was found that solutionizing heat treatment reduces the defects like porosity which was developed during casting process. In the present work, more emphasis is given on the improvement of mechanical properties and microstructure of as-cast alloys that are multi-directionally forged.

Effect of C Addition on as-Cast Microstructures of High Nb Containing TiAl Alloys

Two high Nb-containing TiAl alloys, Ti46.6Al7.5Nb0.5Si0.2B (Alloy A) and Ti46.1Al7.4Nb5C0.5Si0.2B (Alloy B), were prepared by graphite mold casting. As-cast microstructures of the two alloys were characterized to clarify the effect of carbon addition. The results show that 5 at.% carbon addition can change the primary solidification phase from β phase to α phase. The as-cast microstructure of Alloy A consists of a fully α2 + γ lamellar structure and interdendritic eutectic silicide with a volume fraction of 2.3%. However, in Alloy B, the lamellar structure only forms in the dendritic stem and the massive γ is observed in the interdendritic regions. Two types of carbides, Ti2AlC and TiC, are produced in Alloy B. A large number of randomly distributed primary Ti2AlC particles with volume fraction of 14.9% are observed in both the dendritic and interdendritic regions. Irregularly shaped TiC remains inside of the large Ti2AlC particle, suggesting TiC carbides transformed to Ti2AlC during cooling. The addition of carbon also changes the morphology of the silicides from a eutectic structure to a blocky structure in the massive γ matrix or at the interface of the Ti2AlC and the γ matrix. High level of niobium greatly increases the solid solution limit of carbon, since C content in the matrix is much higher than the solid solubility of that in the TiAl binary system. The hardness of the matrix increases from 325 HV to 917 HV caused by the addition of carbon.

Effect of Zn addition on mechanical and corrosion properties of as-cast and as-extruded WE43 magnesium alloys

In this study, the microstructure, mechanical properties and corrosion resistance of as-cast and as-extruded WE43-xZn (x = 0, 0.5, 1.0 wt%) magnesium alloys were investigated. The microstructure of the as-cast alloys showed that the WE43-xZn alloys contained Mg41Nd5 and Mg24Y5 intermetallic compounds. Besides, LPSO (Long-period stacking ordered) type Zn-rich phases were also originated in the microstructure with increasing Zn content. Furthermore, LPSO structure was observed both in the α-Mg grain and at the grain boundaries in Zn-containing WE43 alloys. The Vickers hardness values increased by the raising Zn content in the as-cast, solution treated and as-extruded WE43-xZn alloys. It was observed that yield strength (YS) increased gradually by the addition of Zn content, but 0.5 wt% Zn addition to as-cast WE43 alloy resulted in much more improvements on the ultimate tensile strength (UTS) and elongation % (EL) values compared to as-cast WE43-1.0Zn alloy. It was seen that mechanical properties improved with increasing Zn content for the as-extruded alloys. The as-cast specimens showed that corrosion resistance improved by increasing Zn content due to the presence of LPSO structures, but in the extruded alloys it varied in relation to the presence and distribution of second phases and also the morfologies of LPSO structures.

Thermodynamic description and simulation of solidification microstructure in the Co-Ti system

The Co-Ti binary system has been investigated by means of experimental measurements and thermodynamic calculations. In order to provide new phase relationship in the Co-Ti system, 9 key alloys were prepared by arc melting under vacuum. All alloys were examined for phase and composition analysis by means of X-ray diffraction and electron probe microanalysis after annealing at 900 °C for 60 days and 1000 °C for 30 days, respectively. The invariant equilibrium among liquid, L12 and C36 Laves phases was experimentally determined to be an eutectic with the reaction temperature at 1154 °C. A revised Co-Ti phase diagram is presented according to the present experimental results and reliable literature data. A consistent set of thermodynamic parameters for the Co-Ti system was then obtained by means of CALPHAD (CALculation of PHAse Diagram) approach. The present thermodynamic parameters can describe the experimental results satisfactorily. Scheil solidification simulation was employed to analyse the experimental microstructure of as-cast alloys. The calculated mole fractions of the constitution phases in a few key as-cast alloys are in good agreement with the experimental data obtained by the image J software. The present work indicated that the comprehensive application of the thermodynamic calculation and decisive experiment is an efficient strategy to obtain the desired solidification microstructure.