Overcoming the Strength-Plasticity Trade-Off in a Multi-phase Equiatomic CrCuNiTiV 3d Transition Metal High-Entropy Alloy

The phase components, microstructures, and mechanical properties of a multi-phase equiatomic 3d transition metal high entropy alloy, CrCuNiTiV, were investigated. In the as-cast condition, the alloy was mainly composed of a body-centered cubic (BCC) dendritic phase and a face-centered cubic (FCC) interdendritic phase, with the BCC dendrite uniformly distributed in the FCC matrix. Through annealing, three phases decomposed from the FCC parent phase, and small particles and stripes were derived from the elongated stripes of the as-cast alloy. Outstanding plasticity was acquired with only a small sacrifice in the yield strength after annealing. Specifically, the plasticity increased from 12.7 pct in the as-cast condition to 23.4 pct in the annealed condition while high yield strengths of 965 and 877 MPa were retained in the as-cast and annealed alloys, respectively. Overcoming the strength-plasticity trade-off in the annealed CrCuNiTiV alloy was mainly achieved via the large volume fraction of FCC interdendritic phases, together with the precipitation of one BCC phase and two FCC phases.

     

Investigation of thermoelastic martensitic transformation in a CuZnAl alloy

Thermoelastic martensitic transformation in a Cu-29%Zn-3%Al alloy was investigated experimentally and theoretically. The phase structures and morphological changes occurring during transformation were studied using optical microscopy and high voltage TEM (1000–1200 kV), both equipped with combination heating and cooling stages. A “single crystal pure shear” experiment was designed to measure the relationship between M_s and shear stress, and from these data changes of enthalpy and entropy of the transformation were calculated through the Clausius-Clapeyron equation. The changes of enthalpy and entropy were also obtained by using calorimetric measurement. Both results were in good agreement. The kinetic behavior of the transformation in polycrystalline alloy as a function of applied stress was followed using electric resistance measurement. The slope of the transformation rate was constant over the range of 20–70% transformed, which corresponds physically to transformation occurring by plate growth in the unpartitioned parent phase or equivalently, the interphase boundary “moving freely.” A phenomenological theory was suggested to describe the constant slope portion of the transformation. Here, phase boundary motion is related to thermal hysteresis and quantitatively described the effect of applied stress on the transformation behavior.     

Processing of a new high entropy alloy: AlCrFeMoNiTi

This work, a new composition of high-entropy alloys (HEAs) was designed. The composition was carefully tailored with the aim to obtain a solid solution with a BCC crystalline structure to be an alternative binder in cermets. Thus, the composition of the HEA has been designed taking into account various criteria which has fulfilled a metallic binder of a Ti(C,N) cermet:(1) high hardness and oxidation resistance and (2) good wetting behaviour with Ti(C,N) particles because the processing of cermets is by LPS. The design of the alloy has been performed using theoretical calculations of physicochemical properties of the elements involved and the calculation of phase diagram by Thermocalc. The designed alloy has been processed by casting and powder metallurgy (PM) to study the influence of the processing route on the phases formed and on the resulting properties. The powders were produced by gas atomisation and then consolidated by hot pressing.

Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden.     

Development of a Novel Ni-Based Multi-principal Element Alloy Filler Metal,Using an Alternative Melting Point Depressant

Brazing is a crucial joining technology in industries where nickel-superalloy components must be joined. Nickel-based brazing filler metals are extensively employed, possessing excellent mechanical properties, corrosion resistance, and retained strength at elevated temperatures. To function as a filler metal, the alloy melting point must be reduced to below that of the materials being joined, but the addition of melting point depressants (MPDs) such as boron, silicon, and phosphorus can, however, lead to the formation of brittle intermetallics, potentially compromising the joint performance. In the present work, a novel multi-principal element brazing alloy (in the style of a high entropy alloy), utilizing Ge as an alternative MPD along with a reduced B addition, is investigated. The design process considered binary phase diagrams and predictions based on Thermo-Calc software and empirical thermodynamic parameters. The alloy was used to vacuum braze nickel-superalloy Inconel-718, and microstructural and mechanical investigations are reported. The maximum shear strength achieved was 297 MPa with a brazing temperature of 1100 °C and 60-minute hold time, with isothermal solidification completed. Shear strength was only slightly reduced with increased joint width. Assessments are made of the ability to accurately predict properties of multi-principle element alloys using Thermo-Calc software and empirical thermodynamic parameters.

     

Characterization of Oxide Dispersed AlCoCrFe High Entropy Alloy Synthesized by Mechanical Alloying and Spark Plasma Sintering

The present study deals with phase evolution of oxide dispersed AlCoCrFe high entropy alloy during mechanical alloying and spark plasma sintering. Mechanical alloying of AlCoCrFe resulted in a single BCC phase. However, ordering of BCC phase with evolution of chromium carbide and sigma phase were observed after spark plasma sintering. High hardness of 1,050 ± 20 HV₁ and 1,070 ± 20 HV₁ was observed for AlCoCrFe high entropy alloy without and with oxide dispersion, respectively. Significant contribution from solid solution strengthening effect in high entropy alloys appears to have overwhelmed the effect of oxide dispersion on hardness.     

Inverse Magnetocaloric Effect Related to the Magnetostructural Phase Transition in Quaternary Ni–Co–Mn–Al Alloy

The inverse magnetocaloric effect of Ni–Co–Mn–Al quaternary alloy with the relatively low material cost is achieved firstly in a theoretical study (V. Sokolovskiy et al.: J. Appl. Phys., 2020, vol. 127, p. 163901). To investigate and prove this study, the exact composition of \(\hbox{Ni}_{{40}}\hbox{Co}_{{10}}\hbox{Mn}_{{36}}\hbox{Al}_{{14}}\) alloy is selected and and explored by the combination of X-ray diffraction, scanning electron microscopy, resistivity, and magnetic studies. The quaternary alloy reveals that the main phase is associated with a martensitic L1₀ phase structure with some austenitic B2 phase in the vicinity of room temperature. The results show that the alloy maintains both Austenite and Martensite phases and has a grand scale change in magnetization of approximately 95 emu \(\hbox{g}^{-1}\) around the Martensitic phase transition (in the range of 20 K) that exhibits a first-order magnetic transition from ferromagnetic to non-ferromagnetic state. The alloy reveals the inverse magnetic entropy change of about 12 and 8 J \(\hbox{kg}^{-1}\,\hbox{K}^{-1}\) and the relative cooling power of 125 and 76 J kg⁻¹ under only 15 and 10 kOe, respectively. Likewise, the MR value of 11.5 pct obtains in the external magnetic field source of 10 kOe in the heating direction. The experimental results support the referenced theoretical study and make this material prominent in future magnetocaloric and magnetoresistivity studies.

     

选区激光熔化CoCrFeNi-X高熵合金的研究进展

CoCrFeNi高熵合金因其单一稳定的面心立方固溶体结构,具有优异的塑性变形能力和较高的屈服强度,已成为众多追求高韧性制件研究的热门体系之一。同时选区激光熔化技术因其成形尺寸灵活和超快加热冷却速率,具备传统制备方式不可比拟的优势。通过梳理近些年选区激光熔化技术成功制备出的CoCrFeNiX高熵合金体系,首先针对8种不同合金体系的相结构和组织形貌,分析了组织结构对力学性能的影响;其次针对3种采用不同工艺参数制备的CoCrFeNi-X高熵合金成形件,分析制备工艺对成形密度及力学性能的影响;最后就合金成分设计对CoCrFeNi-Alx、CoCrFeNi-Mn两种主流合金体系做了详细研究现状分析。期望对采用选区激光熔化技术制备CoCrFeNi-X体系高熵合金的实验研究和工业应用提供一定的理论指导。     

Local Composition Migration Induced Microstructural Evolution and Mechanical Properties of Non-equiatomic Fe40Cr25Ni15 Al15Co5 Medium-Entropy Alloy

A newly designed composition of non-equiatomic Fe₄₀Cr₂₅Ni₁₅Al₁₅Co₅ medium-entropy alloy (MEA) was produced by induction melting (IM). The as-cast alloy was found to consist of a two-phase microstructure of BCC (2.87 ± 0.01 Å) and ordered B2 (2.88 ± 0.02 Å) type phases. The structures of these phases were confirmed through X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. It was observed that the Ni-Al-enriched ordered B2 phase of cuboidal shapes (~ 100 to 200 nm) is homogeneously distributed in Fe-Cr-rich BCC matrix with a cube-on-cube orientation relationship. The formation of the columnar dendrites (width 50 to 100 μm) was identified through optical microscopy (OM). The structural and microstructural stability of the alloy was investigated by heat-treating the alloy through different schedules. Heat-treated samples at different temperatures (< 1273 K) exhibit a similar type of two-phase microstructure with columnar dendrites. However, compositional rearrangement takes place during long time exposure to develop polymorphically related phases. The alloy was observed to possess a high compressive yield strength and hardness, i.e., ~ 1047 MPa and 391 ± 9 HV, respectively, at room temperature. Heat-treated samples at 600 °C and 900 °C (873 K and 1173 K) showed an increase in yield strength and ultimate strength with a significant increase in plasticity due to the increase in volume fraction of B2 phase and softening of the BCC matrix phase. The thermal stability and high strength of this alloy may open new avenues for high-temperature applications.

     

Microstructure-Based Estimation of Strength and Ductility Distributions for $$\alpha +\beta $$ Titanium Alloys

Titanium alloys are processed to develop a wide range of microstructure configurations and therefore material properties. While these properties are typically measured experimentally, a framework for property prediction could greatly enhance alloy design and manufacturing. Here a microstructure-sensitive framework is presented for the prediction of strength and ductility as well as estimates of the bounds in variability for these properties. The framework explicitly considers distributions of microstructure via new approaches for instantiation of structure in synthetic samples. The parametric evaluation strategy, including the finite element simulation package FEpX, is used to create and test virtual polycrystalline samples to evaluate the variability bounds of mechanical properties in Ti-6Al-4V. Critical parameters for the property evaluation framework are provided by measurements of single crystal properties and advanced characterization of microstructure and slip system strengths in 2D and 3D. Property distributions for yield strength and ductility are presented, along with the validation and verification steps undertaken. Comparisons between strain localization and slip activity in virtual samples and in experimental grain-scale strain measurements are also discussed.

     

Microstructures and properties of CoCrCuFeNiMox high-entropy alloys fabricated by mechanical alloying and spark plasma sintering

CoCrCuFeNiMox (x values in molar ratio, x?=?0, 0.2, 0.4 and 0.8) high-entropy alloys were prepared by mechanical alloying and spark plasma sintering method. The effects of Mo addition on microstructure and mechanical properties were investigated. The X-ray diffraction (XRD) result showed that the addition of Mo into CoCrCuFeNi high-entropy alloy (HEA) changed the original phase constitution from FCC to FCC?+?σ?+?μ and the peak intensity of (1 1 1) shifted to the left and decreased steadily. The field emission scanning electron microscope confirmed that the Cu-rich second FCC phase disappeared and the σ phase with a tetragonal structure expanded as the Mo content was increased. Additionally, a high density of dimple-like features were seen in CoCrCuFeNi HEA while typical quasi-cleavage facets could be observed from the fracture surfaces of the HEAs with the addition of Mo. The Mo0.8 alloy showed a good wear resistant and appropriate strength with fracture strain 22.70%, fraction coefficient 0.65, hardness 530?HV and compressive strength 1448?MPa.

Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden.     

Cellular Automaton Study of Hydrogen Porosity Evolution Coupled with Dendrite Growth During Solidification in the Molten Pool of Al-Cu Alloys

Welding porosity defects significantly reduce the mechanical properties of welded joints. In this paper, the hydrogen porosity evolution coupled with dendrite growth during solidification in the molten pool of Al-4.0 wt pct Cu alloy was modeled and simulated. Three phases, including a liquid phase, a solid phase, and a gas phase, were considered in this model. The growth of dendrites and hydrogen gas pores was reproduced using a cellular automaton (CA) approach. The diffusion of solute and hydrogen was calculated using the finite difference method (FDM). Columnar and equiaxed dendrite growth with porosity evolution were simulated. Competitive growth between different dendrites and porosities was observed. Dendrite morphology was influenced by porosity formation near dendrites. After solidification, when the porosities were surrounded by dendrites, they could not escape from the liquid, and they made pores that existed in the welded joints. With the increase in the cooling rate, the average diameter of porosities decreased, and the average number of porosities increased. The average diameter of porosities and the number of porosities in the simulation results had the same trend as the experimental results.

     

Understanding Ceramic Particle-Stimulated Heterogeneous Recrystallization in a Medium Entropy Alloy

The present work investigates the recrystallization behavior of a medium entropy alloy composite containing large Cr₂N ceramic particles. Heterogeneous recrystallization of the alloy matrix is observed at different annealing temperatures. The regions containing the ceramic particles demonstrate noticeably higher recrystallization kinetics as compared to other areas without ceramic particles, which is ascribed to the particle stimulated nucleation facilitated by the stored dislocation energy.

     

Transient inclusion formation and evolution in silicon killed steels

Abstract

Transient inclusion formation in low carbon silicon killed steel as a result of ferroalloy additions of ferro-silicon (Fe75Si) and ferro-manganese (FeMn) (for de-oxidation and alloying) was studied in the laboratory using a novel experimental technique. Inclusion chemistries and morphologies were analysed using automated scanning electron microscopy (ASPEX). Formation of these inclusions was studied using thermodynamic software (FactSage) and a mechanism was proposed. Effect of solute concentration gradients during alloy dissolution and mixing was observed. Inclusions which formed initially, at low concentrations of ferroalloy, were liquid phase with a high concentration of FeO. Depending on the ferroalloy addition, initial inclusions become richer in SiO₂ or MnO. Inclusions also underwent further phase transformations on cooling.     

Deformation Behavior of Powder Metallurgy Connecting Rod Preform During Hot Forging Based on Hot Compression and Finite Element Method Simulation

Powder-forged connecting rod with a complex geometry shape always has a problem with nonuniform density distribution. Moreover, the physical property of preform plays a critical role for optimizing the connecting rod quality. The flow behavior of a Fe-3Cu-0.5C (wt pct) alloy with a relative density of 0.8 manufactured by powder metallurgy (P/M, Fe-Cu-C) was studied using isothermal compression tests. The material constitutive equation, power dissipation (η) maps, and hot processing maps of the P/M Fe-Cu-C alloy were established. Then, the hot forging process of the connecting rod preforms was simulated using the material constitutive model based on finite element method simulation. The calculated results agree well with the experimental ones. The results show that the flow stress increases with decreasing temperature and increasing strain rate. The activation energy of the P/M Fe-Cu-C alloy with a relative density of 0.8 is 188.42 kJ/mol. The optimum temperature at the strain of 0.4 for good hot workability of sintered Fe-Cu-C alloy ranges from 1333 K to 1380 K (1060 °C to 1107 °C). The relative density of the hot-forged connecting rod at the central part changed significantly compared with that at the big end and that at the small end. These present theoretical and experimental investigations can provide a methodology for accurately predicting the densification behavior of the P/M connecting rod preform during hot forging, and they help to optimize the processing parameters.

     

Processing and characterisation of nano crystalline AlCoCrCuFeTix high-entropy alloy

In this work multi-component equiatomic and non-equiatomic AlCoCrCuFeTi_x hexanary high-entropy alloys (HEA) was synthesised through mechanical alloying. The prepared powder was sintered via spark plasma sintering. Influence of alloying element variation in the multi-component system was studied in terms of phase formation and crystal structure by using Thermo-Calc and X-ray diffraction characterization technique (XRD). Particle morphology and chemical analysis studies were carried out through scanning electron microscopy along with Electron Dispersive X-ray Spectroscopy. The crystal structure and nano crystallinity of the hexanary system were recognised using transmission electron microscope (TEM and Selected Area Electron Diffraction [SAED]) while the formation of a solid solution was also studied and discussed. From the XRD and TEM characterisation of 20?h in, milled powders and sintered samples, it was confirmed that the developed HEA system forms a single solid solution BCC phase. The sintered alloy exhibits 97% relative density and an average hardness of 590?VHN.

Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden.     

添加钨对USC141镍基合金力学性能的影响简

 参照USC 141镍基合金的化学成分，进行了热力学模拟计算。计算结果表明：钨能提高M23C6和M6C的? 湮露龋⒋俳谩湎嗪蚆6C的析出。在USC 141化学成分基础上，添加钨元素取代部分钼元素，设计了一种试验型镍基合金(HR100)。通过室、高温力学性能测试、持久试验和析出相分析，对其力学性能和析出相进行了研究。试验结果表明：经相同热处理后，试验型镍基合金HR100中的γ′相和M6C的析出明显多于USC 141，使其室温综合力学性能、高温强度以及持久寿命均明显优于USC 141，但同时大量析出的M6C会降低其高温和持久塑性。通过调整钨和钼的含量来调节M6C相的析出量，可以作为一种提高USC 141室、高温综合性能的方法。     

Effect of Ni on Microstructure and Mechanical Properties of CrMnFeCoNi High Entropy Alloy

The effect of Ni content on microstructure and mechanical properties of the CrMnFeCoNi high entropy alloy (HEA) has been studied. The Ni content varied from 0 to 20 at% in the composition (CrMnFeMn)_100?xNi_x, where x?=?0, 2.5, 5, 10, 15, and 20 at%. The alloys were synthesized by vacuum arc melting and the microstructure as well as hardness of the as-cast alloys were studied. Alloys with low Ni content (x?≤?2.5%) consists of a two-phase microstructure of dendritic and inter-dendritic regions with fcc (matrix) and tetragonal (sigma) crystal structure, respectively. When the Ni content is 5 at%, two-phase structure with fcc (matrix) and bcc (secondary phase) is observed, with the addition of Mn-rich inclusions that are present in the entire matrix. Alloys with higher Ni content (x?≥?10, at%) exhibit a single phase of fcc structure. Hardness of the HEAs decreases from 320 to 120 Hv with increase in Ni content, and the high hardness of these alloys with low Ni content is due to the mixture of both fcc and hard tetragonal (sigma) phases.

     

Microstructure evolution and densification behaviour of powder metallurgy Al–Cu–Mg–Si alloy

ABSTRACT

An Al–Cu–Mg–Si alloy was prepared by conventional press-sintering powder metallurgy using elemental Al powder. The phase transformation process of Al–Mg, Al–Si alloy and Cu during the sintering process was investigated in details. It was found that a series of phase transitions take place in the alloy to disrupt the oxide film of Al particle and enhance the densification process. The relative density of the sintered samples reached 98%. A new Al–Mg–Cu–O compound was found at the grain boundaries except the MgAl₂O₄ phase, it is speculated that the disruption of the oxide film was also associated with the other alloy compositions except for Mg. Furthermore, no detectable AlN compound was found at the grain boundary region although sintering with flowing nitrogen atmosphere, which is benefit from the high density of the green compact and the excellent wettability between the liquid phase and the aluminium.     

Electrochemical Characterization of a Novel Mg70Al18Zn6Ca4Y2 Low Entropy Alloy in Different Aqueous Environments

In the present investigation, design and development of a novel Mg-based multicomponent low entropy alloy (Mg LEA-Mg₇₀Al₁₈Zn₆Ca₄Y₂) was carried out using Disintegrated Melt Deposition (DMD) technique. Various electrochemical techniques such as potentiodynamic polarization test (PDP) and electrochemical impedance spectroscopy (EIS) are used to investigate the electrochemical behaviour of the present Mg-LEA alloy at different molar concentrations in acidic (HCl), neutral (NaCl) and alkaline (NaOH, at different pH levels) solutions. The results show that, this alloy easily gets corroded with the increase of Cl⁻ ion concentration due to the breakdown of the Mg(OH)₂ passive layer in both acidic and neutral solutions. However, in the case of alkaline solution, the corrosion resistance of the alloy increases due to the formation of a stable Mg(OH)₂ layer along with AlMg₂Zn and Al₂Y phases, which is more stable than αMg. The ranking of Mg-LEA alloy’s corrosion rate is as HCl> NaCl> NaOH. The impedance measurements have correlated well with polarizations results and the data obtained according to the equivalent circuit provide insights between electrode and electrolyte interface. Through SEM analysis, pitting corrosion was observed in Mg-LEA alloy in acidic and neutral solutions and their chemical compositions were obtained using energy-dispersive X-ray spectroscopy (EDS).

     

Microstructural evolution of austenite under conditions simulating thin slab casting and hot direct rolling

Abstract

The hot direct rolling (HDR) of thin slabs introduces some new microstructural phenomena with respect to conventional hot rolling of steels. This paper aims to investigate the microstructural changes of as cast austenite under these conditions. Current laboratory techniques for HDR simulation require a freshly cast slab for every experiment and a perfect link between casting and hot deformation. The present work adopted a new approach; the C–Mn steel is substituted by austenitic Fe–30Ni alloy, Conventional reheating before rolling replaces the direct link. The experimental ingot casting of Fe–30Ni alloy resulted in a solidification structure in good agreement with that of thin slabs of C–Mn steels. From metallographic observations, a mixed softening process and a strong grain refinement and homogenisation characterise the microstructural changes during HDR simulation. The microstructural behaviour and the grain refinement measured for the Fe–30Ni alloy is closely comparable with that predicted for C–Mn steels for the same conditions. The steel substitution appears to constitute a suitable and advantageous experimental approach for HDR simulation.