Structural and magnetic properties of Ni2CoSi: First-principles calculation and experimental realization

BCC Heusler phase Ni₂CoSi has been predicted to be a promising candidate to realize magnetic field induced martensitic transformation. We tried to prepare Ni₂CoSi single phase using different methods. Single phase Ni₂CoSi cannot be synthesized by arc-melting and annealing. Then we used mechanical alloying method to synthesize Ni₂CoSi. But a FCC phase rather than BCC was obtained after ball-milling. The lattice constant of FCC Ni₂CoSi is 3.52 Å and the Curie temperature is around 900 K. The saturation magnetization at 5 K is 2.44μ_B/f.u. This FCC phase is stable and no transition is observed when heating to 1173 K. The electronic structure and phase stability of the FCC and BCC Heusler phase have been investigated by first-principles calculations. The FCC Ni₂CoSi has lower total energy compared with BCC, agreeing with the experimental observation. But the calculated total moment is much smaller than the M_s at 5 K. This difference is related to the atomic disorder and was discussed by KKR-CPA calculation.     

Microstructure and Mechanical Behavior of FeNiCoCr and FeNiCoCrMn High-Entropy Alloys Fabricated by Powder Metallurgy

The present work reports a systematic investigation on phase evolution,microstructure and microstructure-property relationship of two typical face-centered cubic(FCC) structured high-entropy alloys(HEAs),FeNiCoCr and FeNiCoCrMn,prepared via mechanical alloying(MA) followed by spark plasma sintering(SPS).Following 50 h of MA,the two HEAs consisted of a mixture of FCC and body-centered cubic phases.Following SPS,the bulk FeNiCoCr alloy showed a primary FCC phase with a small amount of Cr_(23)C_6 and Cr_2 O_3 contaminants,while the bulk FeNiCoCrMn alloy was composed of a primary FCC phase with some(Cr,Mn)_(23)C_6 and MnCr_2 O_4 contaminants.The average grain size of the primary FCC phase in the bulk FeNiCoCr alloy was ～416 nm,while that of the primary FCC phase in the bulk FeNiCoCrMn alloy was ～547 nm.The yield strength,compressive strength and strain-to-failure of the bulk FeNiCoCr alloy are 1525 MPa,1987 MPa and 24.4%,respectively,whereas those of the bulk FeNiCoCrMn alloy are 1329 MPa,1761 MPa and 21.9%,respectively.It suggests that the bulk FeNiCoCrMn exhibited lower strength and plasticity in comparison with the bulk FeNiCoCr alloy.Clearly,the smaller grain size of the primary FCC phase in the FeNiCoCr alloy is mainly responsible for the better mechanical performance.     

Solid state and liquid phase sintering of mechanically activated W–20 wt.% Cu powder mixture

W–20 wt.% Cu powder mixture was mechanically alloyed by high-energy ball milling for various times and the effect of mechanical alloying (MA) on the sintering response of the composite compacts was investigated. The densification, microstructure, hardness and electrical conductivity after solid phase sintering (SPS) and liquid phase sintering (LPS) were examined. It was shown that the microstructure of mechanically alloyed powder profoundly influence the sintering response, i.e. a meaningful relationship between the sintering kinetics and the milling time was observed. It is suggested that MA disintegrates the W–W particle networks and increases the contribution of solid phase sintering (SPS) of nanostructured Cu and W particles on the densification. Higher hardness and conductivity were achieved by prolonged MA and SPS, indicating a lower W–W contiguity of the milled powders compared with the conventionally prepared W–Cu composite. On the other hand, depression of the melting temperature of copper up to 145 °C was noticed by affording a prolonged MA. The lower melting temperature and finer distribution of the Cu particles in the W matrix enhanced the densification during LPS and improved the homogeneity and properties of the final product.     

Microstructure and mechanical properties of the TiZrNbMoTa refractory high-entropy alloy produced by mechanical alloying and spark plasma sintering

The equiatomic refractory high entropy alloy (HEA) TiZrNbMoTa was investigated. The alloyed powders with face-centered cubic (FCC) structured solid solution phase were prepared by mechanical alloying (MA) and then sintered by spark plasma sintering (SPS) at 1300, 1400, 1500, and 1600 °C. The microstructure and mechanical properties of the bulk alloy were investigated. The body-centered cubic (BCC) structured solid solution phase and the ZrO₂ phase precipitated from the FCC structured solid solution phase during cool-down from sintering. The highest compression fracture strength (3759 MPa) and fracture strain (12.1%) were obtained in the refractory HEA sintered at 1400 °C. The grain boundary strengthening, precipitation strengthening, solid solution strengthening, transformation-induced plastic (TRIP) effect, and toughening effect of the ZrO₂ phase are the important factors for the high strength and ducitily of the refractory HEA prepared in this study.     

Effects of Nb additions on the microstructure and mechanical property of CoCrFeNi high-entropy alloys

A series of five-component CoCrFeNiNb_x high entropy alloys (HEAs) were synthesized to investigate alloying effects of the large atom Nb on the structure and tensile properties. Microstructures of these alloys were examined using scanning electron microscopy and the phase evolution was characterized and compared using the ΔH_mix–δ and ΔX criteria. It was found that the microstructure changes from the initial single face-centered cubic (FCC) to duplex FCC plus hexagonal close-packed (HCP) structure with additions of Nb. The current alloy system exhibits a hypoeutectic structure and the volume fraction of the Nb-enriched Laves phase with the HCP structure increases with increasing the Nb content, which is mainly responsible for the increment in the yield and fracture strength. Particularly, the Nb_0.155 alloy containing a 9.3% Nb-enriched Laves phase exhibits the most promising mechanical properties with the yield strength and plastic strain as high as 321 MPa and 21.3%, respectively. The ΔH_mix–δ criteria well describe the phase selection for the thermally treated alloys, while the physical parameter ΔX fails to predict the appearance of the Nb-enriched Laves phase in this alloy system.     

Affordable FeCrNiMnCu high entropy alloys with excellent comprehensive tensile properties

Fe_0.4Cr_0.4NiMn_xCu (0 ≤ x ≤ 1.4) high entropy alloys (HEAs) were prepared by copper-mold casting. The phase selection, microstructure, tensile properties and fracture morphologies were investigated. The microstructure with dual FCC phases was formed in the as-cast HEAs with x ≤ 1, and BCC phase was crystallized from the central FCC dendrites of HEAs with x = 1.2 and 1.4. In homogenized Fe_0.4Cr_0.4NiMnCu HEA, needle-like shaped BCC phase was formed resulting in a slight enhancement of yield strength. Compositional heterogeneity was detected in both FCC and BCC dendrites. These HEAs exhibit excellent comprehensive tensile properties, e.g. the yield strength, ultimate strength and elongation of the HEA with x = 1 reaches 439 MPa, 884 MPa and 23.4%, respectively. High density of dislocations in FCC matrix was formed after tensile deformation. FCC type of fine polyhedra, which is mainly composed of Cr, Mn and O, is formed in dendrites. In this work, the phase selection and strengthening mechanism were evaluated based on atomic size factor. It was found that two criteria can be employed to predict the phase regions of current alloys. The solid solution strengthening for this HEA system is the most important among the four kinds of strengthening mechanisms.     

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.     

Microstructure and transformation of Al-containing nanostructured 316L stainless steel coatings processed using spark plasma sintering

Three austenitic stainless steel alloys containing 0, 2 and 6 wt.% Al were prepared by cryomilling and spark plasma sintering. It was shown that aluminum influences the strain-induced phase transformation that occurs during milling. The milled powders consisted of finely dispersed particles with the powder particle size distribution increasing with aluminum concentration. Consolidation of the SS0Al (stainless steel containing 0 wt.% Al) powder via the spark plasma sintering (SPS) process onto a solid stainless steel substrate yields an equiaxed structure due to the original particle morphology resulting from cryomilling. The SS2Al and SS6Al SPS consolidated powder coatings exhibit a lamellar structure due to the increased aspect ratio of the particles. The degree to which the BCC structure induced during cryomilling of all three powder systems reverted to FCC was dependent upon the Al content. The SPS process was found to minimally influence the FCC recovery compared to conventional powder consolidation heat treatments. The energy supplied by the SPS process was insufficient to overcome the activation energy governing the rearrangement of dislocations required to complete the FCC recovery. The microhardness of the coatings processing using SPS was found to be highly dependent on the Al content by controlling the ratio of the BCC/FCC crystals in the formed coating.     

烧结方式对CoCrNi中熵合金组织及力学性能的影响

采用机械合金化法（MA）球磨制备CoCrNi中熵合金原料粉末，结合放电等离子烧结（SPS）或高真空烧结制取CoCrNi中熵合金，研究了球磨时间以及退火对CoCrNi中熵合金原料粉末微观形貌、颗粒尺寸及相结构的影响规律，对不同烧结方式制备的合金块体进行微观结构及力学性能研究。结果表明：随着球磨时间的延长，各单质粉末颗粒尺寸不断减小并逐渐融合，在球磨25h后，原料粉末主要为FCC固溶体结构，还有少量的BCC相；在后续烧结过程中，少量BCC相发生相转变，组织中只有FCC相结构；退火烧结样品的弹性模量为6.57GPa，是真空烧结的1.55倍，屈服强度为279.28MPa，与真空烧结后样品的屈服强度相当，退火烧结的延伸率为35.97%，明显大于直接真空烧结；SPS烧结的块体合金表现出高达793.72MPa的屈服强度和61.08%的塑性应变，且维氏硬度达到399HV，与其它两种烧结方法相比，SPS在实现HEAs快速低温烧结方面更具潜力。     

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.     

Effect of CNT addition approach on the microstructure and properties of NiAl-CNT nanocomposites produced by mechanical alloying and spark plasma sintering

An approach was offered to synthesize NiAl-CNT intermetallic matrix nanocomposite by mechanical alloying (MA) to ensure the effective distribution of carbon nanotubes (CNTs) within the matrix. For this purpose 0.5 and 1 wt% of CNTs were added to the powder mixture before the completion of reaction between Ni and Al. The resultant powders were compared to the powders obtained from MA of ex situ synthesized NiAl intermetallic compound with CNTs. Bulk samples were fabricated by spark plasma sintering (SPS) that retained the integrity of CNTs in the matrix. Structural evolutions were investigated by X-ray diffractometery (XRD). Field emission scanning electron microscopy (FESEM) micrographs showed that the offered MA approach caused the CNTs to uniformly embed in the in situ synthesized NiAl matrix. Meanwhile better distribution of CNTs resulted in higher density of SPSed bulk nanocomposite as well as higher hardness up to 5.6 GPa compared to 5.41 of NiAl intermetallic obtained from the same MA time. Fracture toughness showed more than 6.4% increase with bridging and pull out of CNTs and deflection of cracks as toughening mechanisms.     

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.     

机械合金化制备N型Si80Ge20块体热电材料的微观结构与电性能的研究

本文采用球磨工艺合成了SiGe合金粉末,结合放电等离子烧结制备出了掺杂GaP与P的N型Si<,80>Ge<,20>块体合金.利用XRD、SEM等手段对球磨粉末与烧结体进行了表征,并对烧结体进行了电性能的测试.结果表明:Si、Ge粉末经球磨后可完全形成单相的SiGe合金;通过谢乐公式的计算,机械合金化的合金粉末的平均品粒...     

Correlation between microstructure and properties of fine grained Mo–Mo3Si–Mo5SiB2 alloys

Three phase α-Mo–Mo₃Si–Mo₅SiB₂ alloys of various compositions, namely Mo–6Si–5B, Mo–9Si–8B, Mo–10Si–10B and Mo–13Si–12B (at.%) were processed by a powder metallurgical (PM) route. Increasing the Si and B concentration in these Mo–Si–B alloys resulted in increasing volume fractions of the intermetallic phases Mo₃Si (A15) and Mo₅SiB₂ (T2) and the distribution of the three phases present in these alloys was dependent on the volume fractions of the individual phases. Above volume fractions of about fifty percent, bcc Mo solid solution (α-Mo) formed the matrix. Consequently, Mo–6Si–5B and Mo–9Si–8B alloys, which possessed a continuous α-Mo matrix provided increased fracture toughness at ambient temperatures. Additionally, a decreased BDTT of about 950 °C is caused by the homogeneous α-Mo matrix. In contrast, Mo–13Si–12B with 65 vol.% of the intermetallic phases that formed the matrix phase in this material had a BDTT value higher than 1100 °C, while the strength at elevated temperatures up to 1300 °C was significantly increased compared to alloys that have the α-Mo matrix. Alloy compositions with ≥50 vol.% of intermetallic phases (corresponding to alloys containing a minimum of 9 at.% Si and 8 at.% B) were oxidation resistant with minimal mass loss under cyclic conditions for 150 h at 1100 °C due to the formation of a dense borosilicate glass layer that protects the material surface.     

Low-temperature fabrication of high purity Ti3SiC2

In this study, we fabricated high purity Ti₃SiC₂ ceramic by mechanical alloying (MA) and spark plasma sintering (SPS), and investigated the effect of trace amount of Al on these processes. Our results show that addition of proper amount of Al significantly increases the purity of Ti₃SiC₂ in the MA and subsequent SPS products, and remarkably reduces the sintering temperature for Ti₃SiC₂. Ti₃SiC₂ sintered compact with a purity of 96.5 wt% was obtained by 10 h of MA and subsequent SPS from a starting mixture composed of n(Ti):n(Si):n(Al):n(c) = 3:1:0.2:2 at 850 °C. At 1100 °C, Ti₃SiC₂ with a purity of 99.3 wt% and a relative density of 98.9% was obtained.     

细晶Bi2Te3块体材料的制备及其热电性能

采用惰性气体保护蒸发-冷凝(IGC)法制备了纳米Bi及Te粉末,结合机械合金化(MA)和放电等离子烧结(SPS)工艺,在不同烧结温度(663～723K)下制备出了n型Bi2Te3细晶块体材料。利用X射线衍射分析(XRD)确定机械合金化粉末和SPS烧结块体的物相组成,借助TEM观察了粉体的粒度及形貌,SEM观察了块体试样断口显微组织结构。在323～473K温度范围内测试了烧结块体的电热输运特性。实验结果表明:纳米粉末合成的细晶Bi2Te3与粗晶材料相比,电输运性能变化不大,热导率大幅度降低,在423K时,热导率由粗晶材料的1.93W/m·K降至1.29W/m·K,并且在693K烧结的细晶块体的无量纲热电优值(ZT)在423K时取得最高ZT值达到0.68。     

Concentration dependence of Cr for alleviating environmental embrittlement in Fe30Ni20Mn35Al15

Previously, it has been shown that Fe₃₀Ni₂₀Mn₃₅Al_15, which consists of alternating submicron B2 and f.c.c. lamellae, exhibits a room temperature yield strength of 770 MPa and an elongation of ∼10% at a strain rate of 3 × 10⁻³ s⁻¹ in air, but at the slower strain rate of 3 × 10⁻⁶ s⁻¹ the alloy exhibits an elongation <1% [1]. An addition of 6 at% Cr has been proven to not only solve this environmental problem, but also to increase the elongation to 15–18% irrespective of strain rate [2]. Since we do not know whether Cr additions less than 6 at% can suppress this environmental embrittlement, in this paper we examined the room temperature mechanical properties of several alloys based on Fe₃₀Ni₂₀Mn₃₅Al₁₅ with Cr additions ≤6 at%. We show that additions as low as 0.5 at% Cr alleviate the environmental embrittlement and that additions of ≥2 at% completely suppress the embrittlement with little change in microstructure. X-ray photoelectron spectroscopy examination suggested that the suppression is mostly due to the formation of protective oxide scales on the surface that provide rapid passivation. The lower yield strength when Cr is present may also contribute to the improved ductility, possibly by easing dislocation cross-slip in the deforming f.c.c. phase where most of the Cr resides.     

Effects of WC particle size on densification and properties of spark plasma sintered WC–Co cermet

The WC–Co cermet bulks were prepared by spark plasma sintering (SPS) using powder mixtures with different-scaled WC particles. The SPS densification process was studied by calculating the current distribution between the powder sample and the die in the SPS system. The microstructures were characterized and compared for different samples by the WC grain size, Co mean free path and contiguity of WC grains. In spite of a weak effect of WC particle size on the SPS densification stages, the WC particle size plays a significant role in the homogeneity of the cermet microstructure. Good mechanical properties of the SPSed cermet were obtained with an optimized WC and Co particle-size combination. The effects of scale combination of WC and Co particles on the microstructure hence the properties of the SPSed cermet were discussed.     

Constitution of the systems {V,Nb,Ta}-Sb and physical properties of di-antimonides {V,Nb,Ta}Sb2

The binary phase diagrams {V,Nb,Ta}-Sb below 1450 °C were studied by means of XRPD, EPMA, and DTA measurements. In the V-Sb system, five stable binary phases were observed in this investigation: V_3+xSb_1−x, ℓT-V₃Sb₂, hT-V_2−xSb, V_7.46Sb₉, V_1−xSb₂. The V-Sb phase diagram is characterized by two degenerate eutectic reactions: L↔V_3+xSb_1−x+(V) (T > 1450  °C at 18.1 at.% Sb) and L↔V_1−xSb₂+(Sb) (T=(621 ± 5)°C at ∼99 at.% Sb), three peritectic reactions: L + V_3+xSb_1−x↔hT-V_2−xSb (T=(1230 ± 10)°C at ∼42 at.% Sb), L + hT-V_2−xSb↔V_7.46Sb₉ (T=(920 ± 10)°C at ∼87 at.% Sb), and L + V_7.46Sb₉↔V_1−xSb₂ (T=(869 ± 5)°C at ∼88 at.% Sb), a peritectoid reaction: V_3+xSb_1−x + hT-V_2−xSb↔ℓT-V₃Sb₂ at (875 ± 25)°C, a eutectoid reaction: hT-V_2−xSb↔ℓT-V₃Sb₂+V_7.46Sb₉ at (815 ± 15)°C and congruent melting of V_3+xSb_1−x (T > 1450 °C). An X-ray single crystal study of V₅Sb₄C_1−x proved the existence of interstitial elements in the octahedral voids of a partially filled Ti₅Te₄-type structure (x∼0.5; R_F2 = 0.0101), therefore this phase (earlier labeled “V₅Sb₄”) was excluded from the binary equilibrium phase diagram. V₅Sb₄C_1−x is the first representative of a filled Ti₅Te₄-type structure.A re-investigation of the Nb-Sb system removed the contradiction between the hitherto reported phase diagrams and confirmed the version derived by Melnyk et al. (see ref. [1]).Three binary phases exist in the Ta-Sb system: Ta_3+xSb_1−x, Ta₅Sb₄, TaSb₂. Due to instrumental limits (≤1450 °C), only the peritectic reaction of TaSb₂: L + Ta₅Sb₄ ↔ TaSb₂ ((1080 ± 10)°C at ∼92 at.% Sb) and a degenerate Sb-rich eutectic reaction (L↔TaSb₂+(Sb); (622 ± 5)°C; ∼99 at.% Sb) have been determined.Physical properties (mechanical and transport properties) of binary di-antimonides were investigated with respect to a potential use of these metals either as diffusion barriers or electrodes for thermoelectric devices based on skutterudites. All group-V metal di-antimonides have low metallic-type resistivity and relatively high thermal conductivity. Magnetic field has little influence on the resistivity of V_1−xSb₂ at low temperature, while on {Nb,Ta}Sb₂ it increases the resistivity, especially on NbSb₂. The coefficient of thermal expansion (CTE) decreases from V_1−xSb₂ to TaSb₂, particularly the CTE value of NbSb₂ is in the range of average n-type filled skutterudites. In contrast to the CTE value, elastic moduli increase from V_1−xSb₂ to TaSb₂. The value for V_1−xSb₂ is in the range of Sb-based skutterudites, whereas the values for {Nb,Ta}Sb₂ are significantly higher.     

Phase equilibria and mechanical properties of the Ir–W–Al system

Phase equilibria in the Ir–W, Ir–Al and Ir–W–Al systems at temperatures between 1100 °C and 1600 °C were experimentally investigated using diffusion couples and two- or three-phase alloys, and the mechanical properties of γ′ (L1₂) strengthened Ir–W–Al alloys were examined by hardness and compression tests at room and elevated temperatures. The phase boundaries between the γ(A1)/ε′(D0₁₉), ε′/ε(A3) and ε/ε″(B19) in the Ir–W system at 1400 °C–1600 °C and those between the γ/β(B2) and β/Al_2.7Ir in the Ir–Al system at 1100 °C–1400 °C were determined. The phase diagrams in the Ir-rich corner of the Ir–W–Al ternary system at 1300 °C and 1400 °C were also determined. The existence of the γ′ phase of the Ir₃(W,Al) ternary compound was confirmed, and this system was found to consist of the γ, γ′, ε, ε′ and β phases in the Ir-rich portion. It was also found from hardness and compression tests up to 1200 °C that Ir–Al–W alloys having the γ + γ′ structure with a small lattice misfit show high hardness and strength at room and high temperatures.