Effect of solute atoms on fracture toughness in dilute magnesium alloys

The effect of alloying elements on the toughness and the fracture behaviour was investigated on seven kinds of Mg-0.3?at.% X (X?=?Ag, Al, Ca, Pb, Sn, Y and Zn) alloys with a grain size of 3–5?μm. The fracture toughness and fracture behaviour in magnesium alloys were closely related to the segregation energy. The Mg–Al and –Zn alloys that had small segregation energy showed high toughness and ductile fracture in most regions, while the Mg–Ca alloy with large segregation energy exhibited low toughness and intergranular fracture. These different tendencies resulted from solute segregation at grain boundaries (GBs). The change in the lattice parameter ratio was the influential material parameter regardless of whether the GB embrittlement was for enhancement or suppression.     

Bi，Sb合金化对AZ91镁合金组织、性能影响机理研究

利用大角重位点阵模型建立了AZ91镁合金α相［0001］对称倾斜晶界原子结构模型，应用实空间的连分数方法计算了Mg合金的总结构能，合金元素引起的环境敏感镶嵌能及原子间相互作用能，讨论了主要合金元素Al及Bi，Sb在AZ91中的合金化行为.计算结果表明，Al，Bi，Sb固溶于α相内或晶界区使总结构能都降低，起到固溶强化作用；合金元素在AZ91α相内趋于均匀分布，在晶界区易占位于三角椎上部.AZ91镁合金中加入Bi或Sb时，Bi或Sb比Al容易偏聚于晶界，从而抑制了Al在晶界的偏聚，促进基体中连续的Mg₁₇Al₁₂相的析出，提高AZ91合金室温性能； AZ91合金中(α相内和晶界区)主要合金元素Al和微加元素Bi，Sb都能够形成有序相Mg₁₇Al₁₂，Mg₃Bi₂或Mg₃Sb₂，且在晶界区形成的量大.Bi，Sb加入AZ91合金中，由于Bi，Sb抑制Al在晶界的偏聚，晶界区主要析出相为Mg₃Bi₂或Mg₃Sb₂，提高镁合金高温性能. 关键词： 电子理论 合金化 晶界偏聚 镁合组织与性能     

A first principles calculation of site occupancy of the B2 phase in Ti2AlX (X=V,Cr, Fe,Mo, Ta,Nb, Zr,Hf and Re) intermetallics

The site occupancy of the B2 phase in Ti₂AlX (X=V, Cr, Fe, Mo, Ta, Nb, Zr, Hf and Re) intermetallics have been studied using first principles pseudo potential plane wave method.The Ti, Al and X atoms are arranged in five different ways, in the lattice sites corresponding to B2 structure of Ti₃Al. In Ti₃AlX, the X atoms are substituted at the Ti and / or Al sites. Further, the equilibrium lattice constants and the formation energy (E_for) of these intermetallics with different site occupancies in the B2 phase have been predicted. The formation energy values suggest that the B2 phase is stable in all alloys. Amongst the five cases in a particular alloy, stable configuration is identified with the minimum E_for and is further considered for the calculations of mechanical properties. All the alloys are mechanically stable in terms of Born stability criteria and show anisotropic behaviour. All the alloys display ductile behaviour in terms of G/B ratio.     

Normal and excess nitrogen uptake by iron-based Fe–Cr–Al alloys: the role of the Cr/Al atomic ratio

Upon nitriding ferritic iron-based Fe–Cr–Al alloys, containing a total of 1.50 at. % (Cr?+?Al) alloying elements with varying Cr/Al atomic ratio (0.21–2.00), excess nitrogen uptake occurred, i.e. more nitrogen was incorporated in the specimens than compatible with only inner nitride formation and equilibrium nitrogen solubility of the unstrained ferrite matrix. The amount of excess nitrogen increased with decreasing Cr/Al atomic ratio. The microstructure of the nitrided zone was investigated by X-ray diffraction, electron probe microanalysis, transmission electron microscopy and electron energy loss spectroscopy. Metastable, fine platelet-type, mixed Cr_{1? x} Al _x N nitride precipitates developed in the nitrided zone for all of the investigated specimens. The degree of coherency of the nitride precipitates with the surrounding ferrite matrix is discussed in view of the anisotropy of the misfit. Analysis of nitrogen-absorption isotherms, recorded after subsequent pre- and de-nitriding treatments, allowed quantitative differentiation of different types of nitrogen taken up. The amounts of the different types of excess nitrogen as function of the Cr/Al atomic ratio are discussed in terms of the nitride/matrix misfit and the different chemical affinities of Cr and Al for N. The strikingly different nitriding behaviors of Fe–Cr–Al and Fe–Cr–Ti alloys could be explained on this basis.     

Microstructure and mechanical properties of nano-Y2O3 dispersed ferritic steel synthesized by mechanical alloying and consolidated by pulse plasma sintering

Ferritic steel with compositions 83.0Fe–13.5Cr–2.0Al–0.5Ti (alloy A), 79.0Fe–17.5Cr–2.0Al–0.5Ti (alloy B), 75.0Fe–21.5Cr–2.0Al–0.5Ti (alloy C) and 71.0Fe–25.5Cr–2.0Al–0.5Ti (alloy D) (all in wt%) each with a 1.0?wt% nano-Y₂O₃ dispersion were synthesized by mechanical alloying and consolidated by pulse plasma sintering at 600, 800 and 1000°C using a 75-MPa uniaxial pressure applied for 5?min and a 70-kA pulse current at 3?Hz pulse frequency. X-ray diffraction, scanning and transmission electron microscopy and energy disperse spectroscopy techniques have been used to characterize the microstructural and phase evolution of all the alloys at different stages of mechano-chemical synthesis and consolidation. Mechanical properties in terms of hardness, compressive strength, yield strength and Young's modulus were determined using a micro/nano-indenter and universal testing machine. All ferritic alloys recorded very high levels of compressive strength (850–2850?MPa), yield strength (500–1556?MPa), Young's modulus (175–250?GPa) and nanoindentation hardness (9.5–15.5?GPa), with up to 1–1.5 times greater strength than other oxide dispersion-strengthened ferritic steels (<1200?MPa). These extraordinary levels of mechanical properties can be attributed to the typical microstructure of uniform dispersion of 10–20-nm Y₂Ti₂O₇ or Y₂O₃ particles in a high-alloy ferritic matrix.     

Elastic properties, thermal expansion coefficients and electronic structures of Ti0.75X0.25C carbides

Elastic properties, thermal expansion coefficients and electronic structures of Ti_0.75X_0.25C carbides (X=W, Mo, Ta, Nb, V, Hf, Zr, Cr and Al) were systematically investigated using ab initio density functional theory (DFT) calculations. The calculated elastic moduli, electronic structures and thermal expansion coefficients α(T) of pure TiC are in good agreement with experimental data and other DFT calculations. Based on a phenomenological formula, the trends of elastic properties and ductile/brittle behavior of Ti_0.75X_0.25C were analyzed. It was found that alloying elements W, Mo, Ta, Nb, V and Hf can increase elastic moduli, while Zr, Cr and Al reduce moduli. The nearly free electron model and Debye approximation were applied in the evaluation of α(T). The anharmonic effect was taken into account by including volume-dependent elastic moduli and Debye temperature. Results show that alloying additions of 3d V, 4d Zr and Mo slightly reduce α(T), while 3d Cr increases α(T), Al, 4d Nb, 5d Hf and W almost keep α(T) unchanged in Ti_0.75X_0.25C at high temperatures. The electronic structures of Ti_0.75X_0.25C were calculated and analyzed, and the electronic density of states was used to interpret variations of elastic properties and ductile/brittle behavior induced by alloying additions.     

The nature of distribution of atoms,and its connection with electron structure in a number of ternary alloys on Ni3Mn basis

The influence of alloying the Ni₃Mn alloy with titanium, vanadium, or copper, on the superstructure of the alloy, was studied by the neutron diffraction method. It was shown that Ti, V, and Al stabilize the superstructure of Ni₃Mn, and Cu actively destroys it. The distribution of atoms of the alloying elements along the sites of fcc lattice and its connection with the electron structure of the alloy was analyzed. The different influence of alloying with a third element on the superstructure in alloys Ni₃Mn and Ni₃Fe is explained.     

Ab-initio investigation of electronic properties and magnetism of half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In)

The electronic structures and magnetism of the half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In) have been investigated to search for new candidate half-metallic materials. Here, we predict that NiCrAl, and NiCrGa and NiCrIn are possible half-metals with an energy gap in the minority spin and a completely spin polarization at the Fermi level. The energy gap can be attributed to the covalent hybridization between the d states of the Ni and Cr atoms, which leads to the formation of bonding and antibonding peaks with a gap in between them. Their total magnetic moments are 1μ_B per unit cell; agree with the Slater-Pauling rule. The partial moment of Cr is largest in NiCrZ alloys and moments of Ni and Al are in antiferromagnetic alignment with Cr. Meanwhile, it is also found that FeCrAl is a normal ferromagnetic metal with a magnetic moment of 0.25μ_B per unit cell and CoCrAl is a semi-metal and non-magnetic.     

Site preference and elastic properties of ternary alloying additions in B2 YAg alloys by first-principles calculations

First-principles calculations were preformed to study the site preference behavior and elastic properties of 3d (Ti–Cu) transition-metal elements in B2 ductility YAg alloy. In YAg, Ti is found to occupy the Y sublattice whereas V, Cr, Co, Fe, Ni and Cu tend to substitute for Ag sublattice. Due to the addition of 3d transition metals, the lattice parameters of YAg is decreased in the order: V<Cu<Cr<Ni<Co<Fe<Ti. The calculated elastic constants show that Cr, Fe, Co and Cu can improve the ductility of YAg alloy, and Fe is the most effective element to improve the ductility of YAg, while Ti, Ni and V alloying elements can reduce the ductility of YAg alloy, especially, V transforms ductile into brittle for YAg alloy. In addition, both V and Ni alloying elements can increase the hardness of YAg alloy, and Y₈Ag₇V is harder than Y₈Ag₇Ni.     

Experimental evaluation of phonon–phason coupling in icosahedral quasicrystals

By measuring phonon strain introduced in crystal approximants, the sign and magnitude of the phonon–phason coupling constant have been evaluated for icosahedral quasicrystals of Mg–Ga–Al–Zn and Al–Cu–Fe systems. The evaluated coupling constants are approximately ?0.04μ and 0.004μ (μ?=?shear modulus) for the former and the latter, respectively. They are in good agreement with the results of a previously reported theoretical calculation. Possible effects of phonon–phason coupling on the onset of phasonic elastic instability in icosahedral quasicrystals are discussed.     

Solute-second phase interaction for Mg,Ag and Zn in Al–Li alloys

ABSTRACT

Early experiments have shown the promises of alloying with Mg?+?Ag (or Mg?+?Zn) on the performance of Al–Li alloys. To better understand the interaction between solutes and second phases in Al–Li alloys, Mg, Ag and Zn segregation to Al/δ′ interface as well as their substitution in δ′ bulk were investigated at the atomic level using first principles modelling and calculations. Energetics results and local charge analyses revealed that Mg, Ag and Zn can segregate to Al/δ′ interface by different preference, but have no significant influence on the interface adhesion. Ag and Zn can also dissolve into δ′ bulk, and enhance the local metallic bonding with nearest-neighboring Al atoms. Based on these results, a multi-fold benefit mechanism was suggested for the combined alloying with Mg?+?Ag (or Mg?+?Zn) in Al–Li alloys.     

Spin‐polarized electron gas in Co2M Si/SrTiO3 (M = Ti,V, Cr,Mn, and Fe) heterostructures

Spin‐polarized density functional theory is used to study the TiO₂ terminated interfaces between the magnetic Heusler alloys Co₂Si (M = Ti, V, Cr, Mn, and Fe) and the non‐polar band insulator SrTiO₃. The structural relaxation at the interface turns out to depend systematically on the lattice mis‐ match. Charge transfer from the Heusler alloys (mainly the M 3d orbitals) to the Ti d_xy orbitals of the TiO₂ interface layer is found to gradually grow from M = Ti to Fe, resulting in an electron gas with increasing density of spin‐polarized charge carriers. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Influence of alloying elements on the elastic properties of ternary and quaternary nickel-base superalloys

Using new supercells, the influence of the alloying elements Co, Cr, Ta, Re, and Ru on the elastic properties of ternary and quaternary nickel-base superalloys are systematically studied by the first-principles pseudopotential method. The calculated results of the five ternary and three quaternary superalloys suggest that the alloying elements can increase the elastic moduli. Re is found to be most effective in increasing the moduli of ternary alloys. Among the quaternary alloys, the Ni-Al-Re-Ru system has the largest moduli. Our calculated values of the elastic moduli agree very well with the experimental results. The charge density redistribution and partial density of states are calculated for the quaternary system including both Re and Ru. Covalent-like bonding between Re and its nearest neighbor is observed. The hybridization between the Re-d orbital and the host Ni-p, d orbitals are the origin of the modulus strengthening effect.     

Elastic properties of Nb-based alloys by using the density functional theory

A first-principles density functional approach is used to study the electronic and the elastic properties of Nb₁₅X(X = Ti, Zr, Hf, V, Ta, Cr, Mo, and W) alloys. The elastic constants c₁₁ and c₁₂, the shear modulus C', and the elastic modulus E_〈100〉 are found to exhibit similar tendencies, each as a function of valence electron number per atom (EPA), while c₄₄ seems unclear. Both c₁₁ and c₁₂ of Nb₁₅X alloys increase monotonically with the increase of EPA. The C' and E_〈100〉 also show similar tendencies. The elastic constants (except c₄₄) increase slightly when alloying with neighbours of a higher d-transition series. Our results are supported by the bonding density distribution. When solute atoms change from Ti(Zr, Hf) to V(Ta) then to Cr(Mo, W), the bonding electron density between the central solute atom and its first neighbouring Nb atoms is increased and becomes more anisotropic, which indicates the strong interaction and thus enhances the elastic properties of Nb-Cr(Mo, W) alloys. Under uniaxial 〈100〉 tensile loading, alloyed elements with less (more) valence electrons decrease (increase) the ideal tensile strength.     

退火诱导机械合金化Fe42.5Al42.5Ti5B10合金的结构演变与晶粒生长动力学

利用XRD和TEM方法研究Fe_42.5Al_42.5Ti₅B₁₀合金在机械合金化及等温热处理过程中的结构演变及晶粒生长动力学,讨论了机械合金化合成机理和热处理过程中的晶粒生长机理.结果表明,球磨过程中Al,Ti,B原子向Fe晶格中扩散,形成Fe(Al,Ti,B)固溶体.机械合金化合成Fe(Al,Ti,B)遵循连续扩散混合机理.球磨50h后,金属Fe,Al,Ti,B已完全合金化,球磨终产物为纳米晶Fe(Al,Ti,B).球 关键词： XRD TEM 42.5Al_42.5Ti₅B₁₀合金')" href="#">Fe_42.5Al_42.5Ti₅B₁₀合金 机械合金化     

A Dilute-Limit Heat of Solution of 3d Transition Metals in Iron Studied with 57Fe Mössbauer Spectroscopy

The room-temperature ⁵⁷Fe Mössbauer spectra for binary iron-based solid solutions Fe_1?x D _x with D=V, Cr, Mn and Co, were analysed in terms of binding energy E _b between two D atoms in the Fe–D system. The extrapolated values of E _b for x=0 were used for computation of the dilute-limit heat of solution of D metals in iron. The results were compared with those derived from calorimetric data concerning the heat of formation of the systems mentioned as well as with those resulting from the Miedema's model of alloys. The comparison shows that our Mössbauer spectroscopy findings are in a qualitative agreement with the available calorimetric data and they are at variance with corresponding Miedema's values for Fe–Mn and Fe–Co systems.

     

Shear-strain induced structural relaxation of Cu Σ3 [110](112) symmetric tilt grain boundary: The role of foreign atoms and temperature

Grain boundaries (GBs) relaxation is a promising and effective strategy to improving GB stability or stabilizing nanocrystalline metals. However, previous studies mainly focused on nanocrystalline pure metals and GB behaviors therein, without considering the role of foreign atoms such as impurity or alloying atoms in GB relaxation. In this work, the shear-strain induced structural relaxation of pure Cu Σ3 [110](112) symmetric tilt GBs (STGBs), and the effects of foreign elements (Fe and Ni) and temperature on the GB relaxation were investigated in detail by molecular dynamics method. The results show that shear strain can trigger the structural relaxation of pure, Fe- and Ni-containing Cu GBs by the emission of Shockley partial dislocations from Cu GBs. Both Fe and Ni have impediment effects on the shear-strain induced GB relaxation, though the content of Fe or Ni atom (0.00165 at.%) is quite low in the GB model. The temperature cannot trigger GB relaxation independently within the considered temperature range, but play a positive role in the shear-strain induced structural relaxations of pure, Fe- and Ni-containing Cu Σ3 [110](112) STGBs. Our work might gain new insights into the mechanically induced GB relaxation in nanocrystalline copper and could be beneficial for improving the stability of Cu GBs.     

Thermal and structural characterization of Cu–Al–Mn–X (Ti,Ni) shape memory alloys

In this study, the Cu–Al–Mn–X (X = Ni, Ti) shape memory alloys at the range of 10–12 at.% of aluminum and 4–5 at.% manganese were produced by arc melting. We have investigated the effects of the alloying elements on the transformation temperatures, and the structural and the magnetic properties of the quaternary Cu–Al–Mn–X (X = Ni, Ti) shape memory alloys. The evolution of the transformation temperatures was studied by differential scanning calorimetry with different heating and cooling rates. The characteristic transformation temperatures and the thermodynamic parameters were highly sensitive to variations in the aluminum and manganese content, and it was observed that the nickel addition into the Cu–Al–Mn system decreased the transformation temperature although Ti addition caused an increase in the transformation temperatures. The effect of the nickel and the titanium on the thermodynamic parameters such as enthalpy and entropy values was investigated. The structural changes of the samples were studied by X-ray diffraction measurements and by optical microscope observations at room temperature. It is evaluated that the element Ni has been completely soluble in the matrix, and the main phase of the Cu–Al–Mn–Ni sample is martensite, and due to the low solubility of the Ti, the Cu–Al–Mn–Ti sample has precipitates, and a martensite phase at room temperature. The magnetic properties of the Cu–Al–Mn, Cu–Al–Mn–Ni and Cu–Al–Mn–Ti samples were investigated, and the effect of the nickel and the titanium on the magnetic properties was studied.     

Comparison among the local atomic order of amorphous TM-Ti alloys (TM = Co,Ni, Cu) produced by mechanical alloying studied by EXAFS

We have investigated the local atomic structure of amorphous TM-Ti alloys (TM = Co, Ni, Cu) produced by Mechanical Alloying by means of EXAFS analyses on TM and Ti K-edges. Coordination numbers and interatomic distances for the three alloys where found and compared. EXAFS results obtained indicated a shortening in the unlike pairs TM-Ti as the difference between d electrons of TM and Ti atoms increases, suggesting an increase in the chemical short range order (CSRO) from TM = Co to Cu.Received: 12 October 2003, Published online: 9 April 2004PACS: 61.43.Dq Amorphous semiconductors, metals, and alloys - 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. - 81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation     

Effect of alloying elements on room temperature tensile ductility in magnesium alloys

The impact of alloying elements on the room temperature tensile behaviour was investigated for a wide range of strain rates using eight types of extruded Mg-0.3 at.% X (X = Ag, Al, Li, Mn, Pb, Sn, Y and Zn) binary alloys with an average grain size of 2–3 μm. The solid solution alloying element affected not only tensile plasticity but also rate-controlling mechanism for these fine-grained magnesium alloys. Most of the alloys exhibited an elongation-to-failure of 20–50% , while the alloys with a high m-value exhibited large tensile plasticity, such as an elongation-to-failure of 140% in a strain rate of 1 × 10^?5 s^?1 for the Mg–Mn alloy. This elongation-to-failure is more than two times larger than that for pure magnesium. This is due to the major contribution of grain boundary sliding (GBS) on the deformation. Microstructural observations reveal that grain boundary segregation, which is likely to affect gain boundary energy, plays a role in the prevention or enhancement of GBS. The present results are clearly expected to open doors to the development of magnesium alloys with good secondary formability at room temperature through the control of alloying elements.