Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy

Microstructure evolution in high-entropy alloy CoCrFeNiMn during plane-strain multipass rolling to a thickness strain of 80% at 293 and 77 K was studied. Deformation at both temperatures was found to be accompanied by twinning. At 77 K, twinning was more extensive in terms of the fraction of twinned grains and the length of the twinning stage thereby providing faster kinetics of the microstructure evolution. Micro-shear bands formed in the microstructure of the alloy at the late stages of rolling (at ε ≈ 80% at 293 K and ε ≈ 40% at 77 K). The ultimate tensile strength of specimens rolled at 77 K or 293 K was found to be 1500 or 1200 MPa, respectively while the strength in the initial homogenized condition was 440 MPa. The contribution of various mechanisms to the hardening of the alloy following rolling at 77 K and 293 K was analyzed quantitatively.     

Preparation of superfine-grained high entropy alloy by spark plasma sintering gas atomized powder

In this work, nanocrystalline CrMnFeCoNi HEAs were prepared by powder metallurgy. It was found mechanical milling can further refine the microstructures and morphologies of the gas-atomized powder, and increase the sintering ability. The HEAs sintered from the mechanically milled powder have much finer microstructures than that from the gas-atomized powder. The original morphology and defects in both the gas-atomized and the mechanically milled powders can be inherited to the bulk forms after the SPS. The SPSed HEAs have a tensile strength as high as 1000 MPa at room temperature and reasonable ductility. The strengthening mechanism can be attributed to the nanocrystalline microstructures, in which grain boundaries block the movement of dislocations. Powder metallurgy can be taken as a promising way for preparing HEAs with high mechanical properties.     

Microstructure and mechanical properties of beta TiAl alloys elaborated by spark plasma sintering

The microstructure evolution and room temperature mechanical property of beta containing Ti–44Al–3Nb–1Mo–1V–0.2Y alloy consolidated by spark plasma sintering was studied. Pre-alloyed powders were sintered for 2 min in the range 900–1250 °C under 100 MPa. It was found that duplex and lamellar microstructures were obtained depending on the SPS temperature. The duplex microstructure formed at 1150 °C and 1175 °C, and the lamellar structure was achieved above 1200 °C. However, coarsening of lamellar colonies occurred with further increasing of the sintering temperature. The specimen with fine lamellar colonies exhibited a relatively high compressive strength, whereas the one with duplex microstructure showed a superior final strain.     

The structural and kinetic characteristics of Mg1.9Al0.1Ni alloy synthesized by mechanical alloying

The structural and kinetic characteristics of the mechanically alloyed Mg_1.9Al_0.1Ni were investigated. It was found that Mg_1.9Al_0.1Ni can absorb/desorb about 3.55/3.44 mass% H at a high rate and it has a hexagonal crystal structure as Mg₂Ni. The hydriding/dehydriding (H/D) rates in the two-phase (–β) region of Mg_1.9Al_0.1Ni were measured and studied at temperatures ranging from 553 to 623 K under an approximately isobaric condition. The obtained data of H/D rates indicated that hydrogen diffusion was the rate-controlling step through the hydride phase. A new model was successfully used to calculate the kinetic experimental results. It can be seen that theoretical calculation agrees well with experimental data. The corresponding activation energies are 47 600 and 54 500 J/mol H₂ for H/D processes, respectively.     

放电等离子烧结制备生物Mg-Zn合金的组织及性能

以Mg粉和Zn粉为原料,采用高能球磨混粉和放电等离子烧结(SPS)的方法制备了Zn含量为0%,2%,4%,6%,8%(质量分数)的生物Mg-Zn合金,对其显微组织、力学性能和腐蚀性能进行了研究。结果表明:制备的Mg-Zn合金内部结构致密,组织分布均匀;显微硬度(HV)和抗压强度随Zn含量的增加而增加,当Zn含量为6%时达到最大值(690和379.5 MPa);模拟体液中的电化学腐蚀电位随Zn含量的增加而升高,腐蚀电流密度则降低,在6%时分别达到最大值和最小值。浸泡试验中,Zn含量为6%合金表现出最好的耐腐蚀性能,随Zn含量的增加,腐蚀形式由严重的点蚀和颗粒剥落转变为轻微的点蚀和颗粒内均匀的晶内腐蚀。     

The oxidation of nanocrystalline Ni3Al fabricated by mechanical alloying and spark plasma sintering

Nanocrystalline Ni₃Al was fabricated through mechanical alloying of elemental powders and spark plasma sintering. The nanocrystalline Ni₃Al has a nearly full density after being sintered at 1223 K for 10 min under a pressure of 65 MPa. Isothermal and cyclic oxidations of nanocrystalline Ni₃Al were tested at 1173–1373 K with intervals of 100 K. The results indicate that nanocrystalline Ni₃Al exhibits excellent isothermal and cyclical oxidation resistance. The oxide scales consist primarily of dense and continuous -Al₂O₃. The grain refinement is beneficial for improving the oxidation resistance of Ni₃Al by providing more nucleation centers for the Al₂O₃ formation, promoting the selective formation of Al₂O₃ and improving the adhesion of oxide scales to the matrix.     

Role of precipitation in strength and ductility of FeCo-2V-Cr alloy with low coercivity

A good combination of high tensile strength as well as large elongation and low coercivity is achieved by optimal precipitation for FeCo-2V-Cr alloy. The precipitate is characterized by bcc structure with plate-like contrast and is disordered phase due to its enrichment of V and Cr in contrast to the partial ordering observed in matrix phase. The Cr addition is significant in such precipitation that leads to the improvement of both tensile strength and ductility without deteriorating soft magnetic property.     

The high-entropy alloys with high hardness and soft magnetic property prepared by mechanical alloying and high-pressure sintering

The equiatomic multiprincipal CoCrFeCuNi and CoCrFeMnNi high-entropy alloys (HEAs) were consolidated via high pressure sintering (HPS) from the powders prepared by the mechanical alloying method (MA). The structures of the MA'ed CoCrFeCuNi and CoCrFeMnNi powders consisted of a face-centered-cubic (FCC) phase and a minority body-centered cubic (BCC) phase. After being consolidated by HPS at 5 GPa, the structure of both HEAs transformed to a single FCC phase. The grain sizes of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were about 100 nm. The alloys keep the FCC structure until the pressure reaches 31 GPa. The hardness of the HPS'ed CoCrFeCuNi and CoCrFeMnNi HEAs were 494 Hv and 587 Hv, respectively, much higher than their counterparts prepared by casting. Both alloys show typical paramagnetism, however, possessing different saturated magnetization. The mechanisms responsible for the observed influence of Cu and Mn on mechanical behavior and magnetic property of the HEAs are discussed in detail.     

Alloying behavior and characterization of (CoCrFeNiMn)90M10 (M=Al,Hf) high-entropy materials fabricated by mechanical alloying

The alloying behavior and microstructures of the (CoCrFeNiMn)₉₀M₁₀ (M=Al, Hf) high-entropy alloy (HEA) powders fabricated by mechanical alloying were studied. The CoCrFeNiMn)₉₀Al₁₀ powders have duplex solid-solution structures. In contrast, nanocrystalline HfNi₃ anchoring in amorphous structures is found in the (CoCrFeNiMn)₉₀Hf₁₀ powders. The (CoCrFeNiMn)₉₀Al₁₀ powders show better ferromagnetic behaviors, mainly explained by the facilitated motion of the magnetic domain induced by the coherent interface between duplex phases. Combined with our previous work, the rules of forming solid-solution and amorphous phase in as-milled HEA powders are preliminarily proposed. It is found that, compared with the as-cast HEA reported previously, the variation range of mixing enthalpy with atomic size difference of the solid-solution formed in as-milled HEA powders is broader. Moreover, the variation ranges between mixing enthalpy and entropy with atomic size difference of the amorphous phase in HEA powder become wider than those of high-entropy bulk metallic glass.     

双步球磨和放电等离子烧结TiAl基合金的显微组织和力学性能(英文)

采用双步球磨法和放电等离子烧结技术制备细晶Ti-45Al-2Cr-2Nb-1B-0.5Ta-0.225Y(摩尔分数,%)合金,并研究烧结温度、显微组织和力学性能之间的关系。结果表明:双步球磨粉末的颗粒形状较规则,其颗粒尺寸为20～40μm,主要由TiAl和Ti3Al相组成。放电等离子烧结后的块体由主相TiAl、少量的Ti3Al相及Ti2Al和TiB2相组成。当烧结温度为900°C时,烧结块体获得的主要组织是等轴晶组织,等轴晶粒尺寸大多数在100~200nm的范围内,合金的压缩断裂强度为2769MPa,压缩率为11.69%,抗弯强度为781MPa;当烧结温度为1000°C时,等轴晶粒明显长大,TiB2相明显增多,合金的压缩断裂强度为2669MPa,压缩率为17.76%,抗弯强度为652MPa。随着烧结温度的升高,合金的维氏硬度由658降低到616。压缩断口形貌分析表明,合金的断裂方式为沿晶断裂。     

Microstructural evolution after thermomechanical processing in an equiatomic,single-phase CoCrFeMnNi high-entropy alloy with special focus on twin boundaries

The FCC-structured equiatomic CoCrFeMnNi high-entropy alloy was produced by arc melting and drop casting. After homogenization, the drop-cast ingots were cold rolled to sheets with six different final thicknesses (thickness reductions of 21, 41, 61, 84, 92 and 96%). Samples were cut from the rolled sheets and annealed for 1 h at temperatures between 400 and 1000 °C. The recrystallization temperature was then determined as a function of cold work by means of scanning electron microscopy and electron backscatter diffraction measurements. Additionally, Vickers indentation was performed on these samples. It was found that the microhardness first tends to increase slightly upon annealing below the recrystallization temperature but then drops steeply for higher annealing temperatures due to the onset of recrystallization. To study grain growth kinetics, samples that underwent 96% cold rolling were first recrystallized for 1 h at 800 °C, which is the lowest temperature at which complete recrystallization occurs, and then annealed at temperatures between 800 and 1150 °C for various times. The grain growth exponent was determined to be approximately n = 3, and the activation energy Q = 325 kJ/mol, both of which agree well with published values for this alloy. EBSD measurements were made in the as-recrystallized and grain growth samples to analyze the annealing twins. The density of annealing twins in the grain growth samples was found to depend only on grain size, i.e., it was independent of annealing temperature and time. No such correlation could be found for the as-recrystallized samples. These observations are discussed in the framework of existing theories for the formation of annealing twins.     

Structural and Mössbauer studies on mechanical milled SmCo5/α-Fe nanocomposite magnetic powders

The SmCo₅/α-Fe nanocomposite powders were prepared by high energy ball milling and the inter-diffusion reaction between the SmCo₅ and α-Fe magnetic phases were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM) and ⁵⁷Fe Mössbauer spectroscopy. While structural and magnetic measurements could reveal only the presence of SmCo₅ and α-Fe phases, Mössbauer studies could clearly specify the extent of alloying between Fe and Co atoms in terms of evolution of α-Fe(Co) phase as a function of milling time. It has been found that the fractional volume of α-Fe(Co) solid solution tends to increase at the expense of the initial α-Fe phase upon progressive milling.     

Enhanced mechanical properties of HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys

Although refractory high-entropy alloys have exceptional strength at high temperatures, they are often brittle at room temperature. One exception is the HfNbTaTiZr alloy, which has a plasticity of over 50% at room temperature. However, the strength of HfNbTaTiZr at high temperature is insufficient. In this study, the composition of HfNbTaTiZr is modified with an aim to improve its strength at high temperature, while retaining reasonable toughness at room temperature. Two new alloys with simple BCC structure, HfMoTaTiZr and HfMoNbTaTiZr, were designed and synthesized. The results show that the yield strengths of the new alloys are apparently higher than that of HfNbTaTiZr. Moreover, a fracture strain of 12% is successfully retained in the HfMoNbTaTiZr alloy at room temperature.     

Polycrystalline elastic moduli of a high-entropy alloy at cryogenic temperatures

CrMnCoFeNi is a FCC high-entropy alloy (HEA) that exhibits strong temperature dependence of strength at low homologous temperatures in sharp contrast to pure FCC metals like Ni that show weak temperature dependence. To understand this behavior, elastic constants were determined as a function of temperature. From 300 K down to 55 K, the shear modulus (G) of the HEA changes by only 8%, increasing from 80 to 86 GPa. This temperature dependence is weaker than that of FCC Ni, whose G increases by 12% (81–91 GPa). Therefore, the uncharacteristic temperature-dependence of the strength of the HEA is not due to the temperature dependence of its shear modulus.     

Nanocrystalline Al3Ni2 alloy with high hardness produced by mechanical alloying and high-pressure hot-pressing consolidation

An elemental powder mixture corresponding to Al₃Ni₂ phase stoichiometry was subjected to mechanical alloying. A metastable nanocrystalline AlNi intermetallic phase with the mean crystallite size of 12 nm was formed upon milling. Heating of the synthesised powder in a calorimeter up to 720 °C caused phase transformation into an equilibrium Al₃Ni₂ intermetallic phase with the mean crystallite size of 41 nm. The product of mechanical alloying was consolidated at 1000 °C under the pressure of 5 GPa and 7.7 GPa. During consolidation, a phase transformation analogous with the one observed in the course of heating in the calorimeter took place. Both bulk materials have nanocrystalline structure with mean crystallite size of 67 nm and 58 nm, the smaller one in the sample consolidated under the higher pressure. The hardness of the produced Al₃Ni₂ intermetallic is 8.81 GPa (898 HV1) and 8.72 GPa (887 HV1), while the specific yield strength, estimated using the Tabor relation, is 624 kNm/kg and 617 kNm/kg for the sample hot-pressed under 5 GPa and 7.7 GPa respectively. On the basis of the obtained results, we can assume that the quality of consolidation with preserving a nanocrystalline structure is satisfactory and the hardness as well as the estimated specific yield strength of the produced materials are relatively high.     

In-situ precipitation of Al2O3 and κ-Fe3AlC0.5 in iron aluminides through spark plasma sintering: Microstructures and mechanical properties

Iron aluminides are ordered intermetallic alloys which offer good resistance to corrosion and sulfidation. At the same time, their Achilles' heel is low ductility at room temperature and sometimes they have poor mechanical properties. By means of mechanical alloying and spark plasma sintering (MA–SPS) it is possible to obtain bulk nanostructured iron aluminides which show high hardness and high yield stress.

In this work we present the production of nanostructured powders and their consolidation through spark plasma sintering. The inevitable use of methanol as processing control agent (PCA) leads to a supersaturation in carbon and oxygen of the milled powder and a consequent in-situ precipitation of carbides and oxides during SPS. The presence of carbides, oxides and a nanostructured matrix leads to high mechanical properties with hardness 5.20 ± 0.05 GPa and a yield stress of 1305 MPa.     

放电等离子烧结Ti-43Al-9V合金显微组织和力学性能(英文)

采用机械合金化和放电等离子烧结工艺制备细晶Ti-43Al-9V合金,研究不同烧结温度与显微组织和力学性能之间的关系。结果表明:机械球磨后粉末形状规则,尺寸在5～30μm之间,烧结所得块体材料主要由γ-TiAl、α2-Ti3Al和少量B2相组成。烧结温度为1150°C时,获得的等轴晶粒尺寸为300nm～1μm。烧结温度升高到1250°C时,等轴晶粒的尺寸明显增大,显微硬度从HV592降低到HV535,抗弯强度从605降低到219MPa,压缩断裂强度从2601降低到1905MPa,压缩率从28.95%降低到12.09%。     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

Microstructure and mechanical properties of FeCoCrNiMn high-entropy alloy produced by mechanical alloying and vacuum hot pressing sintering

FeCoCrNiMn high-entropy alloys were produced by mechanical alloying (MA) and vacuum hot pressing sintering (VHPS). Results showed that the nano-crystalline alloy powders were obtained by MA and the corresponding phase structures were composed of FCC matrices and low amounts of BCC and amorphous phases. After VHPS, the BCC phases almost disappeared, simultaneously with the precipitation of σ phases and M₂₃C₆ carbides. An increase of sintering temperature resulted in grain growth of the precipitated phases. As the sintering temperature was increased from 700 to 1000 °C, the strain-to-failure of the alloys rose from 4.4% to 38.2%, whereas the yield strength decreased from 1682 to 774 MPa. The bulk FeCoCrNiMn HEAs, consolidated by VHPS at 800 °C and 900 °C for 1 h, showed relatively good combination of strength and ductility.     

Corrosion behavior and surface film characterization of TaNbHfZrTi high entropy alloy in aggressive nitric acid medium

Corrosion behavior of TaNbHfZrTi high-entropy alloy (HEA) was investigated in nitric and fluorinated nitric acid at ambient (27 °C) and boiling (120 °C) conditions. The alloy passivated spontaneously during potentiodynamic polarization in 11.5 M HNO₃ at ambient condition. The corrosion rate was negligible in boiling 11.5 M HNO₃, exposed for 240 h. Scanning electron microscopic (SEM) studies did not show any significant corrosion attack. The high corrosion resistance of TaNbHfZrTi HEA was attributed to its single phase bcc structure. X-ray photoelectron spectroscopic (XPS) analysis revealed that the protective passive film formed in boiling nitric acid was predominantly composed of Ta₂O₅, in contrast to the presence of ZrO₂ and HfO₂ in air-formed native film. Potentiodynamic polarization studies indicated a pseudo-passivation behavior of the HEA in 11.5 M HNO₃ + 0.05 M NaF at ambient condition. In boiling fluorinated nitric acid, SEM images of TaNbHfZrTi HEA displayed a severely corroded morphology indicating the instability of the metal-oxides of the alloying elements. XPS investigations confirmed the presence of ZrF₄, ZrOF₂ and HfF₄ along with un-protective oxides of Ta, Nb and Ti on the film, resulting in decreased corrosion resistance of TaNbHfZrTi HEA in fluorinated nitric acid.