Interdendritic Phase Research Articles

Microstructure and Mechanical Properties of Equiatomic CrMnCoNiCu High Entropy Alloy

Microstructure and mechanical properties of equiatomic CrMnCoNiCu alloy in which Fe was substituted by Cu from Cantor alloy was studied. The separation of solid solution phase into two solid solutions (Cr-Co rich and Cu-rich phases) were observed in CrMnCoNiCu. The coarsening and widening of interdendritic Cu-rich phase after homogenization was observed and supported by the increase of XRD peak height from Cu-rich phase compared to that from Cr-Co rich phase after homogenization. The increase of the peak from Cu-rich phase can be attributed to the thermodynamic stability of Cu due to positive mixing enthalpy of adding Cu. The stress-strain curves of CrMnCoNiCu alloy exhibited the reasonably high strength and excellent deformability for the cast alloy. The yield stress of CrMnCoNiCu was observed to be 390MPa and it could be deformed without crack formation up to the true strain 0.85 to reach the flow stress as high as 662Mpa.

Formation of crystalline phase in the glass matrix of Zr-Co-Al glass-matrix composites and its effect on their mechanical properties

The microstructural evolution and mechanical properties of Zr-Co-Al alloys, with compositions of (Zr50Co50)x (Zr56Co26Al18)1-x (x = 1/6, 2/6, 3/6, 4/6, 5/6, 1) and Zr54Co35Al11, (referred to as Z1, Z2, Z3, Z4, Z5, Z6, and Z4.5), were investigated. Alloys Z1-Z3 consisted of crystalline phases, while alloys Z4 and Z4.5 consisted of crystalline phase particles (~3 vol% and ~35 vol%, respectively) embedded within the glassy matrix. Alloys Z5 and Z6 consisted of a monolithic glass phase. The crystalline phase of alloys Z1-Z4.5 consisted of primary B2-ZrCo dendrite and an interdendritic B2-ZrCo/Zr6CoAl2 eutectic phase. The B2-ZrCo dendritic phase exhibited a high work-hardening rate, which originated from the deformation-induced B2-to-B33 martensitic transformation. However, when the brittle interdendritic B2-ZrCo/Zr6CoAl2 eutectic phase fraction increased, the work-hardening rate significantly decreased. The ductility of the glass-matrix composites was significantly impaired by the presence of the interdendritic eutectic phase in the crystalline phase. The results indicate that the design of the crystalline particle microstructure is important with regard to enhancing the plasticity of glass-matrix composites.

Microstructural Characterization and Phase Separation Sequences During Solidification of Ni3Al-Based Superalloy

As-cast microstructure of a designed polycrystalline Ni3Al-based superalloy is characterized using optical microscope, scanning electron microscope, transmission electron microscope equipped with selected area diffraction system, and the intermetallic phase transformations involved during solidification process are determined based on thermal analysis measurements. The as-cast microstructure is mainly composed of 80.63 vol% dendritic and 19.37 vol% interdendritic phases, and the dendrite is identified as quasi-cuboidal $$\gamma_{\text{I}}^{\prime }$$ phase connected by γ-channels where ultrafine $$\gamma_{\text{II}}^{\prime }$$ particles are distributed, and the interdendritic phases are determined as γ′–γ eutectic structure consisting of γ E phase with dotted quasi-spherical $$\gamma_{\text{E}}^{\prime }$$ particles. During solidification, the dendrite firstly nucleates from liquid melt near 1348 °C; subsequently, the residual liquidoid is transformed into interdendritic phases around 1326 °C. Afterward, γ′ phase will precipitate from dendritic γ-matrix with two-stage characteristics, resulting in the distinct precipitation of $$\gamma_{\text{I}}^{\prime }$$ and $$\gamma_{\text{II}}^{\prime }$$ phases when approaching to 1190 and 1043 °C, respectively. The corresponding transformations involved during the solidification process can be translated as:

Modeling phase transformation kinetics during homogenization of aluminum alloy 7050

Microstructural evolution and the changing kinetics of the phase transformations during homogenization of Al-Zn-Cu-Mg-Zr alloys are studied. The phase transformations kinetics during the process is modeled using a novel approach, coupled with Thermo-Calc™. A model for evolution of interdendritic phases which predicts transformation of the as-cast η to S phase and subsequent dissolution of the S phase is coupled with a model for precipitation of dispersoids of Al3Zr dispersoids across the grains. The η to S phase transformation is predicted to be interface reaction rate controlled while all the other phase transformations are diffusion controlled. Based on the numerical study, which is verified through SEM, EDS, XRD and DSC measurements, a three step homogenization process for aluminum alloy 7050 (Al-6.2Zn-2.3Cu-2.35Mg-0.13Zr) is proposed. Homogenization at 420°C for 10h, followed by steps at 470°C for 4h and 480°C for 15h, precipitates numerous nanosized Al3Zr dispersoids and minimizes the volume of the η and S phases. This microstructure is attractive because it is easy to extrude and does not readily recrystallize. The model provides a cost effective tool for optimizing the heat treatment schedules for 7050 and similar 7XXX alloys.

Effects of annealing on the microstructural evolution and phase transition in an AlCrCuFeNi2 high-entropy alloy

An AlCrCuFeNi2 high entropy alloy (HEA) was prepared by arc melting, followed by annealing at different temperatures. The elemental distributions, phase formation, morphology and microstructural evolution of the HEA in both the as-cast state and annealed state were investigated. The results indicate that the HEA undergoes elements segregation, precipitation and spinodal decomposition. The as-cast alloy consists of Cr-Fe-Ni rich FCC dendritic (DR) phase and BCC interdendritic (ID) phase. Spherical Fe-Cr rich BCC precipitates were found to disperse in Al-Ni rich B2 (ordered BCC) matrix. After heat treatments, the distribution of elements clearly changes, along with changes of the constituent phase and morphology. After annealing at 600°C, the DR region remains Cr-Fe-Ni rich phase, while some spherical precipitates transform into the needle-like structure within the ID region. The L12 (ordered FCC) nanorod-shaped phase ((Ni,Cu)3Al) and plate-like Al-Ni rich phase form within the DR region when annealing up to 900°C. The L12 phase almost dissolves in the FCC matrix due to the order-disorder transition and an obvious coarsening of the Fe-Cr rich phase occurs after annealing at 1100°C. The segregation of Cu atoms at the interface between DR and ID regions is found at the as-cast state, while a uniform distribution of Cu atoms in DR and ID regions was observed after annealing at 1100°C.

Impact of heat treatment on mechanical behaviour of Inconel 718 processed with tailored microstructure by selective laser melting

Additive manufacturing (AM) technologies are known to allow the production of parts with an extreme degree of complexity, enabling design and functional part optimization. However, the resulting microstructures and mechanical properties of AM materials are not well understood due to unique and complex thermal cycles observed during processing. This study aims to adjust the microstructure of Inconel 718 specimens produced by selective laser melting (SLM). The microstructural design was achieved through process parameters manipulation and post-process heat treatment. The effects of heat treatment on microstructure, process induced defects, deformation behaviour and failure mechanisms were studied. Directional columnar grained microstructure accompanied by interdendritic Laves phases and carbide particles was observed in as-processed material. Hot isostatic pressing (HIP) improved mechanical properties, which was attributed to dissolution of undesirable Laves and δ-phase as well as pore closure. All investigated samples maintained their intended tailored microstructural build up with distinct differences in mechanical properties. The results presented in this study show the capability of the SLM process to produce parts with mechanical properties better than conventional Inconel material. The microstructural design demonstrated here can be exploited in AM fabrication of complex components requiring challenging high-temperature mechanical performance.

Liquid-phase separation in undercooled CoCrCuFeNi high entropy alloy

Liquid-phase separation in a CoCrCuFeNi high entropy alloy is evidenced by supercooling method. The CoCrCuFeNi high entropy alloy is supercooled by glass fluxing method with a sample mass about 100 g which allows tensile test at different undercoolings. The microstructure evolution of CoCrCuFeNi high entropy alloy solidified at different undercoolings are investigated. The variation of interdendritic Cu-rich phase and the hardness of the dendrites are described in detail and liquid-phase separation is confirmed to have taken place when undercooling is equal or greater than 100 K. The plasticity and elastic modulus during tensile tests are strongly affected by compositional change upon liquid-phase separation. The samples solidified after liquid-phase separation exhibit much higher plasticity and lower elastic modulus.

Evolution of LPSO Phases and Their Effect on Dynamic Recrystallization in a Mg-Gd-Y-Zn-Zr Alloy

The effect of long-period stacking ordered (LPSO) phases on the dynamic recrystallization (DRX) of the compressed Mg-6.9Gd-3.2Y-1.5Zn-0.5Zr (wt pct) alloy was investigated. LPSO phases of various morphologies were obtained by homogenization at 783 K (510 °C) for various durations. During homogenization, intragranular thin-platelet LPSO phases form near the α-Mg grain boundaries first, then penetrate through the grain interior, and finally coagulate. During compression at 753 K (480 °C), intragranular thin-platelet LPSO phases reduce the DRX ratio by hindering the migration of α-Mg grain boundaries. Intragranular-coagulated LPSO phases stimulate DRX near the α-Mg grain boundaries, while faintly affecting the DRX behavior in the interior of α-Mg grains. However, interdendritic phases enhance the nucleation of DRX-ed grains via the particle-stimulated nucleation mechanism. Among all the compressed alloys, the alloy rich in thin-platelet LPSO phases exhibits the optimal mechanical properties.

The effect of nickel on shaping the structure of Al-Cu-Mn alloys

This study investigated the effect of nickel on shaping the structure of aluminum alloys of the Al-Cu-Mn type in the “as-cast” condition and after heat treatment according to the T6 procedure. The aluminum alloys of type Al-5%Cu-1%Mn, containing nickel in a range of up to 1.9%, were taken into consideration in this work. Experiments were carried out for thin-walled thickness casting (g = 5 mm) and for reference casting with a wall thickness of g = 35 mm. Metallographic investigations of both the macro- and micro-structure were conducted to estimate the secondary dendrite arm spacing (SDAS), average diameter (dav) of the primary α (Al) grains, and surface fraction of the interdendritic phases (f). Moreover, the degree of dissolution of these interdendritic phases during the solution treatment process was determined. An SEM-EDS analysis was conducted, from which it follows that the addition of nickel at the level of 0.5% changes the un-dissolved particles from a needle-like β-Fe shape to blocky and coagulated. Higher additions of nickel starting from 0.88%) give rise to as many as four phases with higher copper content, the deficit of which results in the smaller strengthening effect of α (Al) dendrites.

Influence of Strontium Addition on Microstructure and Mechanical Properties of an Al–10Si–5Cu Alloy

The microstructure and mechanical properties of Al–10Si–5Cu cast alloys with micro-addition of alloying elements (V, Cr and Ni) were studied before and after strontium addition. Samples were examined using the X-Ray diffraction, the optical microscope, the scanning electron microscope and the energy dispersive spectrometer. The results indicated that the α-Al matrix, eutectic Si phase and Al2Cu phase were the main constituent phases of Al–10Si–5Cu alloys before or after strontium addition. Strontium addition affected the refining of the α-Al grains and transforming the configuration of interdendritic phases. The un-modified alloy showed a brittle nature because of existing brittle and aggregated AlSiMnFe phases. Contributing to the alteration of microstructure in strontium modified alloy, the strength and elongation of the alloy were improved. In addition, the fracture mechanism and crack propagation process were investigated in both the alloys.

Towards understanding the effects of magnesium addition on microstructural and thermal properties of NiAlCr alloys in as-cast and heat treated states

The effects of magnesium addition on the microstructure, hardness, chemical composition and thermal expansion coefficient of NiAlCr alloys in as-cast and heat-treated states were studied. NiAlCr alloys containing 1, 2.5 and 3.8 at.% Mg were prepared via atmosphere-controlled induction melting following vacuum arc melting. According to the XRD results, as-cast alloys were mainly composed of martensite, β-NiAl and γ′-Ni3Al. High amounts of Mg in alloys resulted in a slight increase in CTE. Increasing Mg also changed the morphology of the interdendritic Ni3Al phase from continuous network to discontinuous structure, resulting in Ni2Mg precipitates with hardness of 1280 HV0.002. In the furnace cooling procedure following the annealing procedure at 1050 °C for 96 h, martensite completely transformed into β-NiAl with α-Cr precipitates. The single phase Ni3Al transformed into Al0.9Ni4.22-Ni3Al lamellar structure and Ni2Mg phase disappeared. The presence of Mg in solid solution did not influence CTE of the alloys.

Mechanical Properties of Magnesium Alloys Produced by the Heated Mold Continuous Casting Process

Abstract Investigation of the tensile and fatigue properties of cast magnesium alloys, created by the heated mold continuous casting process (HMC), was conducted. The mechanical properties of the Mg-HMC alloys were overall higher than those for the Mg alloys, made by the conventional gravity casting process (GC), and especially excellent mechanical properties were obtained for the Mg97Y2Zn1-HMC alloy. This was because of the fine-grained structure composed of the α-Mg phases with the interdendritic LPSO phase. Such mechanical properties were similar levels to those for conventional cast aluminum alloy (Al84.7Si10.5Cu2.5Fe1.3Zn1 alloys: ADC12), made by the GC process. Moreover, the tensile properties (σUTS and εf) and fatigue properties of the Mg97Y2Zn1-HMC alloy were about 1.5 times higher than that for the commercial Mg90Al9Zn1-GC alloy (AZ91). The high correlation rate between tensile properties and fatigue strength (endurance limit: σl) was obtained. With newly proposed etching technique, the residual stress in the Mg97Y2Zn1 alloy could be revealed, and it appeared that the high internal stress was severely accumulated in and around the long-period stacking-order phases (LPSO). This was made during the solidification process due to the different shrinkage rate between α-Mg and LPSO. In this etching technique, micro-cracks were observed on the sample surface, and amount of micro-cracks (density) could be a parameter to determine the severity of the internal stress, i.e., a large amount to micro-cracks is caused by the high internal stress.

A Comparative Study of Microstructures and Properties of Two Types of Nickel-Base Alloy Covered Electrodes

Two types of nickel-base alloy covered electrodes, ENiCrMo-6 and ENiCrFe-9, were investigated and compared among their deposited metal compositions, microstructures, strengths, and cryogenic impact values. They all exhibited dendritic microstructures which were composed of dendritic fcc nickel-base solid solution, interdendritic phases, and grain boundary carbides. The molybdenum in the deposited metals tended to migrate and aggregate toward the edges of the dendrite arms during solidification. The niobium preferred to form oxide and/or carbide and aggregate in the interdendritic regions. The grain boundaries were filled with the continuous carbides and oxides. The differences in the tensile mechanical properties of the deposited metals of the two types of electrodes were relatively minor. The impact values of ENiCrMo-6 at −196 °C were above 80 J; while that of ENiCrFe-9 were in the range of 54-66 J. The relatively high level of carbon and sulfur and more grain boundary precipitates should be responsible for the lower cryogenic impact value of the ENiCrFe-9 covered electrode.

An Experimental Study of Thermophysical Properties for Quinary High-Entropy NiFeCoCrCu/Al Alloys**Supported by the National Natural Science Foundation of China under Grant Nos 51571163, 51371150, 51271150 and 51327901

Two quinary high-entropy alloys (HEAs) with equiatomic concentrations formed by doping either Cu or Al elements into the quaternary NiFeCoCr alloy are produced by arc melting and spray casting techniques. Their entropy of fusion, thermal expansion coefficient and thermal diffusivity are experimentally investigated with differential scanning calorimetry, dilatometry and laser flash methods. The NiFeCoCrCu HEAs contain a face-centered cubic high-entropy phase plus a minor interdendritic (Cu) phase and display a lower entropy of fusion and the Vickers hardness. The NiFeCoCrAl HEAs consist of two body-centered cubic high-entropy phases with coarse dendritic structures and show higher entropy of fusion and the Vickers hardness. Both the thermal expansion coefficient and the thermal diffusivity of the former Cu-doped alloy are significantly larger than those of the latter Al-doped alloy. Although the temperature dependence of thermal diffusivity is similar for both HEAs, it is peculiar that the thermal expansion curve of the NiFeCoCrAl alloy exhibits an inflexion at temperatures of 860–912 K.

Experimental investigation on microstructure and mechanical properties of gravity-die-cast magnesium alloys

The influence of various alloying elements (i.e., Sn, Al, Si, Mm, Sc) with different quantities on the microstructure and mechanical properties of gravity die cast magnesium alloys are investigated. Experimental evidences show that the secondary dendrite arm spacing is increasing with the increase in Sn content from 5% to 8% in Magnesium alloy. Misch Metal (Mm) addition in Mg-5%Sn causes grain refinement (∼30%) and the shape of interdendritic second phase particles changes from rod/irregular geometry to feathery shape. In Mg-8%Sn-3%Al-1%Si alloy, α-Mg, Mg2Sn, Mg17Al12 and Mg2Si phases are identified. Addition of Al-Sc master alloy refines the grain size (∼12%) of Mg-8%Sn alloy. Solution heat treatment at 520 °C for 48–72 h leads to complete dissolution of Mg2Sn intermetallic for Mg-Sn alloys. During ageing of Mg-Sn alloys at 200 °C, discontinuous precipitation of Mg2Sn occurs at grain boundaries and their triple junctions at the initial stage and later they become coarse in nature. Tensile properties (i.e., YS, UTS and %EL) are improved after 100 h of aging for all alloys used compared with the cast alloys of same composition. Cast Mg-8%Sn-3%Al-1%Si alloy exhibits superior tensile properties as compared to cast Mg-5%Sn, Mg-8%Sn,Mg-5%Sn-2%Mm,Mg-8%Sn-3%(Al-Sc) alloys.

Synthesis and microstructural characterization of a novel multicomponent equiatomic ZrNbAlTiV high entropy alloy

A high entropy alloy containing ZrNbAlTiV in equiatomic ratio was produced by suction casting technique. Cross-sectional microstructure showed equiaxed morphology at the peripheral and dendrite morphology at the center region of the sample. Electron microscopic analysis revealed that the single phase and the dendrite phase have ordered bcc structure and the inter-dendrite phase has hexagonal structure. Compositions of the single phase and dendrites were same as that of the alloy composition, whereas, edges of dendrite and inter-dendrite regions show preferential partitioning of Nb and Zr respectively. These results have been rationalized in terms of cooling rates and interactions among elements.

A Comparative Study on Permanent Mold Cast and Powder Thixoforming 6061 Aluminum Alloy and Sicp/6061Al Composite: Microstructures and Mechanical Properties.

Microstructural and mechanical characterization of 10 vol% SiC particles (SiCp) reinforced 6061 Al-based composite fabricated by powder thixoforming (PTF) was investigated in comparison with the PTF and permanent mold cast (PMC) 6061 monolithic alloys. The results reveal that the microstructure of the PMC alloy consists of coarse and equiaxed α dendrites and interdendritic net-like eutectic phases. However, the microstructure of the PTF composite, similar to that of the PTF alloy, consists of near-spheroidal primary particles and intergranular secondarily solidified structures except SiCp, which are distributed in the secondarily solidified structures. The eutectics amount in the PTF materials is distinctly lower than that in the PMC alloy, and the microstructures of the former materials are quite compact while that of the latter alloy is porous. Therefore, the PTF alloy shows better tensile properties than the PMC alloy. Owing to the existence of the SiC reinforcing particles, the PTF composite attains an ultimate tensile strength and yield strength of 230 MPa and 128 MPa, representing an enhancement of 27.8% and 29.3% than those (180 MPa and 99 MPa) of the PTF alloy. A modified model based on three strengthening mechanisms was proposed to calculate the yield strength of the PTF composite. The obtained theoretical results were quite consistent with the experimental data.

The effect of waterjet beginning time on the microstructure and mechanical properties of A356 aluminum alloy during the ablation casting process

The effect of delay time (time interval between melt pouring and waterjet beginning) achieved by ablation process on the microstructure and mechanical properties of A356 aluminum alloy is investigated. By applying the process of ablation cooling 0.5min after melt pouring, eutectic silicon and iron intermetallic size were reduced extremely and the dendrite arm spacing was refined almost considerably. For 0.5min and 1min of delay times ablated samples, besides interdendritic phases size reduction, the morphology of eutectic silicon was converted from coarse needle-like particles to fine and fibrous morphology. Also gas and shrinkage porosities were minimized considerably without any melt treatment. The mechanical properties of 0.5min ablated samples were improved significantly and in comparison with the conventional casting samples in the sand mold, the ultimate tensile strength and elongation were increased from 161MPa to 264.5MPa and from 3% to 12% respectively. Fractography investigations showed a ductile fracture surface for ablated samples.

The role of metastable LPSO building block clusters in phase transformations of an Mg-Y-Zn alloy

We present a systematic atomic scale analysis of the structural evolution of long-period-stacking-ordered (LPSO) structures in the (i) α-Mg matrix and in the (ii) interdendritic LPSO phase of an Mg97Y2Zn1 (at. %) alloy annealed at 500 °C, using high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Various types of metastable LPSO building block clusters have been observed in both regions. The thermodynamic phase stabilities computed by density-functional-theory calculations explain the diversity of the LPSO structures which are distinguished by their different arrangements of the Y/Zn enriched LPSO building blocks that have a local fcc stacking sequence on the close packed planes. A direct evidence of the transformation from 18R to 14H is presented. This finding suggests that LPSO structures can change their separation distance — quantified by the number of α-Mg layers between them — at a low energy penalty by generating the necessary Shockley partial dislocation on a specific glide plane. Based on our results the most probable transformation sequence of LPSO precipitate plates in the α-Mg matrix is: single building block → various metastable LPSO building block clusters → 14H, and the most probable transformation sequence in the interdendritic LPSO phase is: 18R→ various metastable LPSO building block clusters → 14H. The thermodynamically most stable structures in both the α-Mg matrix and the interdendritic LPSO phase are a mixture of 14H and α-Mg.

Effects of heat treatment on microstructure and tensile properties of laser melting deposited AISI 431 martensitic stainless steel

As a martensitic stainless steel, AISI 431 steel is widely used as a key material in marine and steam turbine applications. In this research, a plate of AISI 431 stainless steel was first fabricated using laser melting deposition process, and the microstructure evolutions during the deposition process, the phase transformation under different heat treatment conditions and the influences of microstructures on tensile properties of the steel were investigated. The results show that the laser deposited AISI 431 steel had a fine well-aligned directional microstructure, which consists of dendritic ferrite phases, inter-dendritic ferrite phases and (Cr, Fe)23C6 carbides. Due to the low bonding force between ferrite and carbides, cracks can easily generate and propagate along the interface, resulting in unsatisfied mechanical properties. After solution treatments at temperatures from 1000 to 1100°C, the interlayer heat affected zones of the steel were eliminated, and the carbides gradually dissolved when the solution temperatures were raised. The amount of ferrite did not change until reaching 1100°C, and the steel had the optimal mechanical properties after heat treatment at 1050°C with a strength of 1283MPa.