亚稳态β钛合金的成分设计、变形机制与力学性能研究进展

本文介绍了当前孪晶塑性/相变塑性(TWIP/TRIP)钛合金的研究现状和设计方法,统计了TWIP/TRIP钛合金中发生{332}<113>、{112}<111>双孪晶钛合金β相的晶粒尺寸、屈服强度和加工硬化率。讨论了Bo-Md图在多组元钛合金设计上的应用和局限性,特别是析出不同二次相(α相与ω相)对基体β相稳定性的影响。从基体β相稳定性、析出相、β相晶粒尺寸和晶体学取向三方面归纳了亚稳态β钛合金TWIP/TRIP变形机制的影响因素,并对其中存在的一些问题和不足进行了分析,简要总结了双孪晶机制对钛合金力学性能的影响。通过综述最新的研究进展及相关问题,对未来高强韧钛合金的发展提出新见解。     

FeMnCoCr系亚稳高熵合金的研究进展

高熵合金是一种原子排列有序,化学无序的新型多主元合金.通过改变合金元素的种类和浓度,能够调控合金系统层错能及显微组织的相稳定性,进而诱发形变孪晶、马氏体相变等塑性变形机制,最终使合金获得突出的综合力学性能.这种高熵合金的设计理念称为"亚稳工程".亚稳高熵合金的显微组织、相结构及变形机制与合金体系的层错能密切相关.在Fe...     

Effect of quenching temperature on structure and properties of titanium alloy: Structure and phase composition

X-ray diffraction analysis and transmission and scanning electron microscopy have been used to study regularities of the formation of structure and phase composition of a VT16 alloy during its quenching. The formation of an athermal ω phase in the VT16 alloy with the initial (α + β) structure during quenching of the alloy from 800°C was found to be possible. Quenching temperatures (T _q) at which various metastable phase compositions, such as the metastable β solid solution, β + α″ + ω, β + α″, and α″ martensite, are formed have been determined to be 750, 800, 750–850, and ≤850°C, respectively. Dependences of variations in the volume fractions of phases were plotted. It has been shown that, at quenching temperatures close to the β-transus, the active growth of β-phase grains takes place at the expense of a decrease in the α-phase volume fraction.     

Effects of Cd and Sn on double-peak age-hardening behaviors of Al-Si-Cu-Mg cast alloys

The effects of trace elements Cd and Sn on precipitation process of Al-Si-Cu-Mg cast alloys were investigated in the present research.It is shown that the addition of Cd and Sn not only increases remarkably the aging peak hardness and reduces the time to reach aging peak,but also eliminates the double-aging-peak phenomenon which appears in Al-Si-Cu-Mg alloys.In Al-Si-Cu-Mg alloys the first aging peak corresponds to GP zones(especially GPⅡ) ,and the second one is caused by metastable phases.The obvious time interval of transition from GPⅡ to metastable phases associates with the double-aging-peak phenomenon.The results of DSC and TEM show that Cd/Sn elements suppress the formation of GPⅠzone,stimulate the formation of θ",θ' and θ phases,and then shorten remarkably the temperature intervals of each exothermic peak.Because the transition interval between GPⅡzone and metastable phases is shortened by Cd/Sn in Al-Si-Cu-Mg cast alloys,θ' phase coexists with θ" phase in matrix of ageing peak condition,which causes effective hardening on the alloys,and at the same time,eliminates the double-aging-peak phenomenon.     

MICROSTRUCTURE AND PROPERTIES OF Mn-CONTAINING TiAI ALLOYS

The microstructure of Ti-33wt-％ Al-(1.6-4.5)wt-％ Mn alloys are composed ofγ+α_2 phases,but the volume of x_2-phase is a little.Mn alloying decreases the latticeparameters a and c of γ-phase and the c/a ratio becomes nearly to one,because the atomicradius of Mn is less than the atomic radius of Ti or Al.Mn alloying promotes the formation oftwins in γ-phase and increases the ability of plastic deformation of TiAl alloys at room tem-perature.     

Effect of Pre-cold Rolling-Induced Twins and Subsequent Precipitated ω-Phase on Mechanical Properties in a β-Type Ti-Mo Alloy

This paper reported an effectiveness of pre-cold rolling-induced {332} < 113 > twins combined with subsequent isothermal ω-phase formation for enhancement of uniform elongation in a β-type Ti-15Mo alloy with high yield strength level. Mechanical {332} < 113 > twins were induced by cold rolling with an thickness reduction of 5%, which had little effect on ω-phase precipitation after aging at 573 K for 3.6 ks. Twinning after the cold rolling was further activated during tensile deformation, even with the presence of isothermal ω-phase. This combination of twins and ω-phase enhanced uniform elongation from 0 to 9% at yield strength level of 890 MPa. The high yield strength was mainly dominated by dislocation slip due to the isothermal ω-phase formation, and early onset of plastic instability after yielding was hindered due to the pre-cold rolling-induced twins. Dynamic microstructural refinement was induced by further twinning activation during deformation, which resulted in high work hardening rate corresponding enhancement of uniform elongation.     

Geometrical Model of Polymorphous Transformations in Titanium and Zirconium

A geometrical model of transformation of a body-centered cubic lattice of α-phase into a hexagonal close-packed lattice of α-phase is developed with the aim of explaining the special features of the crystal geometry of formation of martensite phases in titanium and zirconium and in alloys based on them. The transformation is described as mutual reconstruction of coordination polyhedra of the cubic and hexagonal lattices through an intermediate configuration of the crystal structure of ω-phase. In the language of algebraic geometry the transition is implemented as reconstruction of an 11-atom cluster that represents a union of three octahedra around a common edge into an 11-atom cluster composed of 11 tetrahedra united over faces. Experimentally observed orientation relations and habit planes at α → ω and β → α transformations are describable by elements of the structure of the mentioned clusters.     

Structure and properties of ion-plasma deposited Ni-C films in a metastable state

A modified ion-plasma sputtering technique was used to produce metastable states, in particular, amorphous (from the standpoint of X-ray diffraction) and nanocrystaline phases in nickel-carbon film alloys in a wide composition range from 7 to 61.4 at. % C. Short-range order parameters of amorphous phases and lattice parameters of metastable phases were determined; the sequential formation and transformation of intermediate phases during the transition of the Ni-C alloys into the equilibrium state are studied. Electrical properties of the deposited alloys in different states were measured; the results obtained are explained.     

The thermodynamics of and strengthening due to co-clusters: General theory and application to the case of Al–Cu–Mg alloys

Co-clusters in ternary or higher order metallic alloys are metastable structures involving two or more distinct alloying atoms that retain the structure of the host lattice. A thermodynamic model based on a single interaction energy of dissimilar nearest neighbour interaction energy is presented, and a model for the strengthening due to these co-cluster dimers is derived. The model includes a new treatment of (short-range) order strengthening relevant to these co-clusters and further encompasses modulus hardening and chemical hardening. The models are tested against data on a wide range of Al–Cu–Mg alloys treated at temperatures between 20 and 220 °C. Both quantitative calorimetry data on the enthalpy change due to co-cluster formation and strengthening due to co-clusters is predicted well. It is shown that in general (short-range) order strengthening will be the main strengthening mechanism.     

Effect of carbon addition on solidification behavior,phase evolution and creep properties of an intermetallic β-stabilized γ-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition.     

Interface-driven alloying and metallic glass formation in nano-multilayers in an immiscible Y–Nb system

Several new amorphous alloys were obtained in an immiscible Y–Nb system by room temperature 190 keV xenon ion mixing of Y–Nb multilayered films, which were designed to include a sufficient fraction of interfacial atoms and thus possessed a high free energy comparable to that of the amorphous state. In addition, two metastable f.c.c. phases were formed in Y-rich and Nb-rich multilayered films, respectively. The growth kinetics of the f.c.c. phases and their effect on the composition range of amorphization are also discussed. To understand the observed unusual alloying behaviors, a Gibbs free-energy diagram was constructed based on Miedema's model. The diagram included the free-energy curves of all the involved phases as well as that of the Y–Nb multilayered films including the extra interfacial free energy and thus gave a qualitative and reasonable interpretation to the formation of metastable alloys upon ion mixing. Furthermore, some multilayered films with a sufficient fraction of interfacial atoms were subjected to steady-state thermal annealing, which also resulted in the formation of the Y–Nb amorphous phases, suggesting that alloying in this immiscible system was actually driven by the interfacial free energy.     

Structure of quenched Zr-Pd alloys and the formation of the ω phase in zirconium alloys

Methods of X-ray diffraction, optical metallography, transmission electron microscopy, and measurements of hardness and microhardness have been used to study the structure of quenched alloys of the zirconium-palladium system with a palladium concentration from 1 to 10 at %. It has been established that at the palladium content below 4 at % in the process of quenching in these alloys a eutectoid decomposition occurs and a two-phase structure is formed, which consists of laths of the α phase and particles of an intermetallic compound Zr₂Pd arranged predominantly at the boundaries of laths of the α phase. In the alloys with a palladium content from 5 to 8 at %, there is formed a metastable ω phase. In the alloys with a palladium concentration of 9 and 10 at %, a metastable β phase is revealed. A method of determining the concentration of the alloying component in the ω phase is proposed. It is shown that in the quenched state this concentration is less than the content of the alloying component in the alloy. The behavior of the concentration boundary corresponding to the formation of the ω phase in binary zirconium alloys has been determined depending on the type of the alloying metal.     

Effect of nitrogen on stacking fault energy of f.c.c. iron-based alloys

A method is proposed to estimate the stacking fault energies of face-centered-cubic (f.c.c.) iron-based alloys. The segregation of alloying elements to stacking faults and the interaction of substitutional and interstitial alloying elements in solid solution and their effect on stacking fault energy have been taken into account. It is shown that at low nitrogen concentrations (e.g. 0.05 wt%), the stacking fault energy is increased mainly due to the effect arising from the bulk of the alloy. At high nitrogen concentration (e.g. 0.5 wt%), the stacking fault energy is decreased due to the segregation of the alloying elements (mainly nitrogen) on the stacking faults of the alloy. Moreover, it is shown that in nitrogen alloying of f.c.c. iron-based alloys the magnetic contribution of the nitrogen to the stacking fault energy is negligible. The method shows reasonable agreement with existing experimental data.     

Microstructural Characterization of Co-Based ODS Alloys

Co-based ODS alloys, strengthened by nanosized oxide dispersion and γ′ precipitates, are potential high-temperature structural materials. The characteristics of the mechanically alloyed powder and the microstructural evolution of the Co-based ODS alloys were investigated. The results revealed that mechanical alloying had induced the formation of supersaturated solid solution in immiscible Co-Al-W-based alloys, originating mainly from extensive grain boundary region, high dislocation density, and ample point defect. Chemical compositions of mechanically alloyed Co-Al-W-based ODS alloys easily deviate from the γ/γ′ two-phase region, leading to the existence of Al _x Co, Co₃W, Co₇W₆, and W phases in addition to the γ and γ′ phases. Nonuniform distribution of alloying elements brings about the differences in morphologies and sizes of γ′ precipitates. Microstructural formation process is impelled by spinodal decomposition mode, and spinodal decomposition behavior has been accelerated in the fine-grained alloy because of the presence of short-circuited diffusion paths for atomic movement.     

Effect of Cr on Microstructure and Properties of a Series of AlTiCr x FeCoNiCu High-Entropy Alloys

A series of AlTiCr _x FeCoNiCu (x: molar ratio, x = 0.5, 1.0, 1.5, 2.0, 2.5) high-entropy alloys (HEAs) were prepared by vacuum arc furnace. These alloys consist of α-phase, β-phase, and γ-phase. These phases are solid solutions. The structure of α-phase and γ-phase is face-centered cubic structure and that of β-phase is body-centered cubic (BCC) structure. There are four typical cast organizations in these alloys such as petal organization (α-phase), chrysanthemum organization (α-phase + β-phase), dendrite (β-phase), and inter-dendrite (γ-phase). The solidification mode of these alloys is affected by Chromium. If γ-phase is not considered, AlTiCr_0.5FeCoNiCu and AlTiCrFeCoNiCu belong to hypoeutectic alloys; AlTiCr_1.5FeCoNiCu, AlTiCr_2.0FeCoNiCu, and AlTiCr_2.5FeCoNiCu belong to hypereutectic alloys. The cast organizations of these alloys consist of pro-eutectic phase and eutectic structure (α + β). Compact eutectic structure and a certain amount of fine β-phase with uniform distribution are useful to improve the microhardness of the HEAs. More γ-phase and the microstructure with similar volume ratio values of α-phase and β-phase improve the compressive strength and toughness of these alloys. The compressive fracture of the series of AlTiCr _x FeCoNiCu HEAs shows brittle characteristics, suggesting that these HEAs are brittle materials.     

Omega-like diffuse X-ray scattering in Ti–V caused by static lattice distortions

The diffuse scattering of body-centered cubic β-Ti–V was measured using high-energy synchrotron X-rays and two-dimensional detectors. The study included in situ experiments of the equilibrium β-phase and room temperature measurements of the quenched metastable state. The kinematical X-ray scattering revealed details in reciprocal space that could not be detected by the electron diffraction employed in previous studies. The signal was analyzed using a statistical thermodynamic approach based on physically motivated parameters. The characteristic features attributed to an ω-like structure or a “diffuse ω-phase” in the past are explained by static lattice distortions due to atomic size mismatch.     

Formation of nano-sized grains in Cu and Cu−Fe−P alloys by accumulative roll bonding process

Mechanical properties and formation of nano-sized grains in Cu and Cu−Fe−P alloys by the accumulative roll bonding (ARB) process were investigated. Nano-sized grains were successfully obtained in OFC and PMC-90 alloys by the ARB process after the third cycle. Once the 200 nm grains formed, further reduction in the grain size was not observed up to 8 ARB process cycles. For both alloys, the tensile strength values increased drastically in the initial stage of the ARB process. The tensile strength values of both alloys tended to saturate after the third ARB process cycle. The tensile elongation value greatly decreased by 1 cycle of the ARB process due to the strain hardening. After the third cycle of the ARB process, each alloy showed a gradual increase in tensile elongation due to the dynamic recovery. For PMC-90 alloy, the strength value was higher than that of OFC due to addition of the alloying elements. With increased annealing temperature, the nanosized grains tended to grow in OFC at 150°C, and after annealing at 200°C, coarse grains formed. On the other hand, in PMC-90 alloy, there was no grain growth up to 250°C due to the alloying elements (Fe, P).     

The Phase Competition and Stability of High-Entropy Alloys

Phase competition and stability of several typical high-entropy alloys (HEAs) were studied, and the effects of alloying additions and processing conditions on phase formation in these alloys were discussed. Alloying with chemically incompatible elements having a large difference in either the atomic size or enthalpy of mixing with constituting components in HEAs, e.g., Cu and Al in the FeCoNiCr alloy system, inevitably induced phase separation and stimulated formation of duplex solid-solution phases and even intermetallic compounds. The solid-solution phase in the as-cast FeCoNiCrMn HEA is extremely stable due to the good chemical compatibility among constituent components, but in the FeCoNiCrAl and (FeCoNiCrAl)₉₉Si₁ HEAs with the incompatible elements Al and Si, pretreatment and annealing processes could induce phase transitions and the formation of new phases, indicating that the as-cast solid-solution phases were destabilized by quenched-in chemical segregation, resulting from additions of the dissimilar elements.     

Cluster Model of Formation of an Incommensurate ω-Phase in Alloys of the Titanium – Iron System

The structural model of formation of the ω-phase in titanium alloys is refined by considering coordination polyhedra of the matrix b.c.c. phase. It is established that fragments of the crystal lattice of the ω-phase can unite with fragments of the lattice of the intermediate Ti₂Fe phase in Ti – 20 at.% Fe alloy after quenching from the liquid state. The structural model makes it possible to explain the anomalies of diffusion scattering on electron diffraction patterns of quenched alloys of the Ti – Fe system.