Structure of As-Cast Hafnium

The structure of a hafnium crystal grown by the method of floating zone melting which underwent upon cooling the β → α (bcc → hcp) polymorphic transformation has been studied using the metallography, EBSD analysis, and electron microscopy. It has been shown that the α-phase structure of as-cast hafnium consists of lath-shaped crystals grouped into packets. As a rule, the α-phase grains contain several packets of different orientations from 12 orientations that can arise according to the Burgers orientation relationships. The boundaries of the α-phase grains and packets differ significantly. The grains have smooth boundaries, whereas the boundaries of packets are wavy. The misorientations between separate laths in a packet are less than 1°. Extended twins with a \({\left\{ {10\;\bar 1\;2} \right\}_\alpha }\) twinning plane have been discovered in the structure of hafnium. The presence of twins in the hafnium crystal is due to the influence of the total internal stresses caused by the temperature gradient that arise upon zone melting and by the β → α phase transformation. The fracture of the hafnium crystal at room temperature occurs along the basal (0001) plane. The typical fracture is brittle-viscous with a predominance of the brittle component.     

Features of Isothermal Formation of Carbide-Free Bainite in High-Carbon Manganese-Silicon Steel

Change in the stability of overcooled austenite of high-carbon manganese-silicon steel relative to decomposition via the intermediate stage upon isothermal holding and martensitic γ → α transformation upon subsequent cooling has been studied using dilatometry and X-ray diffraction. The kinetics of low-temperature bainite transformation has been studied. The tetragonality of the crystal lattice of bainite α-phase caused by the superequilibrium content of carbon was found.     

Thermal vibrations and α → β polymorphic transformation in lanthanum

Neutron diffraction was used to determine temperature dependence of the Debye-Waller factor and thermal atomic displacements for two polymorphic modifications of lanthanum, namely, α-La with a double hcp (dhcp) crystal structure and β-La with an fcc crystal structure. No substantial changes in the mean-square thermal atomic displacements were shown to occur; the Debye temperatures of the two polymorphic modifications, α-La and β-La, are equal to 135 and 130 K, respectively. However, the relative (with respect to the lattice parameters) displacements along the axes change substantially. The transition from the anisotropic hexagonal modification to the isotropic cubic modification leads to a decrease in the maximum thermal atomic displacements and a weakening of their temperature dependence because of their redistribution over the crystallographic axes. Upon the α → β transformation, the volume decreases and the anharmonicity increases as demonstrated by a twofold increase in the volumetric thermal expansion coefficient. An analysis of literature data available for the polymorphic transformations shows that the enhancement in the anharmonicity is related to an increase in the specific volume in the course of transition from the low-temperature to the high-temperature modification. The explanation of this phenomenon is suggested.     

Study of the structure of cobalt single crystals during the β → α transformation

Methods of metallography, transmission electron microscopy, and X-ray diffraction have been used to study the specific features of the structure formation during the fcc → hcp transformation occurring under different thermokinetic conditions in a single crystal of cobalt. It is shown that only one of four possible versions of hcp-phase crystal orientations is predominantly realized upon the β → α transformation occurring in the single crystal at a low cooling rate (V _cool ～ 1–2 K/min). It has been established that several cycles of slow heating to temperatures of 600, 800 and 1000°C and subsequent cooling of the single crystal do not increase the number of α-phase orientations. The restoration of the initial fcc-phase single-crystal orientation was observed after each heating cycle of the oriented sample, while after cooling the restoration of the preferred hcp-martensite orientation was observed; in this case, the quantity of the retained β phase fixed in the structure at room temperature increases with increasing number of cycles. After rapid quenching from 530°C into salt water (V _cool ～ 600–700 K/min), α-phase crystals of all four possible orientations are formed in the structure. Upon high-temperature quenching from 1000°C, the volume of crystal is divided into packets each containing martensite plates of predominantly one orientation. The transformation-induced recrystallization of the cobalt pseudo-single crystal quenched in salt water has been observed during repeated heating to temperatures above the β → α transformation temperature.     

Features of forming structure of titanium pseudosingle crystal during β → α (bcc → hcp) polymorphic transformation

The structure of a titanium iodide single crystal obtained by zone melting has been studied by metallography, X-ray diffraction, and electron microscopy. It has been shown that the initial bcc titanium single crystal becomes a pseudosingle crystal upon cooling below the temperature of the β → α polymorphic transition. The pseudosingle crystal consists of macroscopic packets, i.e., crystals of lath morphology with a size of 0.1–0.5 cm² in different crystal sections. Each packet consists of α-phase laths of the same orientation, which are separated by dislocation boundaries. A total of six different types of packets in the pseudosingle crystal volume is realized in accordance with the Burgers orientation relationships. The structural heredity in the titanium pseudosingle crystal after the cycle of the α → β → α transformations is confirmed.     

Towards the ab initio based theory of phase transformations in iron and steel

Despite of the appearance of numerous new materials, the iron based alloys and steels continue to play an essential role in modern technology. The properties of a steel are determined by its structural state (ferrite, cementite, pearlite, bainite, martensite, and their combination) that is formed under thermal treatment as a result of the shear lattice reconstruction γ (fcc) → α (bcc) and carbon diffusion redistribution. We present a review on a recent progress in the development of a quantitative theory of the phase transformations and microstructure formation in steel that is based on an ab initio parameterization of the Ginzburg–Landau free energy functional. The results of computer modeling describe the regular change of transformation scenario under cooling from ferritic (nucleation and diffusion-controlled growth of the α phase) to martensitic (the shear lattice instability γ → α). It has been shown that the increase in short-range magnetic order with decreasing the temperature plays a key role in the change of transformation scenarios. Phase-field modeling in the framework of a discussed approach demonstrates the typical transformation patterns.     

Elasticity-based model of the variant selection observed in the β to α phase transformation of a Zircalloy-4 sample

The β→α texture inheritance of a Zircalloy-4 sample has been investigated after an α→β→α transformation cycle. The final inherited α texture has been determined from a crystal orientation map determined by electron back-scattering diffraction, whereas the texture of the high temperature β phase has been reconstructed by a method analysing the orientations and misorientations of α variants. The comparison of the α texture calculated from the parent β texture without variant selection with the experimental sharp α texture shows differences due to a strong variant selection mechanism occurring during the phase transformation at cooling.A model of a variant selection mechanism based on the elastic anisotropy of the parent β phase leads to a simulated inherited α texture with the main characteristics of the experimental texture.     

Martensitic transformations and the evolution of the defect microstructure of metastable austenitic steel during severe plastic deformation by high-pressure torsion

It has been shown that, in metastable austenitic Fe–18Cr–10Ni–Ti steel, under conditions of torsion under pressure, local reversible (forward plus reverse) (γ → α′ → γ) martensitic transformations can occur, which are one of the mechanisms of the formation of nanostructured states. An increase in the rotation rate, which leads to an increase in the deformation temperature, stimulates the reverse (α′ → γ) transformation. The evolution of the structural and phase states is represented as the following sequence: (1) mechanical twinning; (2) nucleation of martensitic plates in the microtwinned structure of the austenite with the formation of two-phase (γ + α′) structures, packet α′ martensite, and structural states with a high curvature of the crystal lattice; (3) reverse (α′ → γ)-transformations; and (4) the fragmentation of nanosized crystals via the formation of a nanotwinned structure in the austenite and of a nanoscale banded structure of the ε martensite in the α′ martensite.     

Structure of titanium subjected to dynamic channel-angular pressing

The microstructure of titanium after dynamic channel-angular pressing in two passes is studied by metallography and electron microscopy. This structure is compared to the structure of titanium after one-pass pressing. The high-rate deformation of titanium in this method consists of uniform deformation and localized deformation. Uniform deformation creates a submicrocrystalline structure. An increase in the number of passes from one to two leads to an increase in the grain-subgrain misorientation and the formation of a more homogeneous structure. Localized deformation causes the formation of adiabatic shear bands and cracks. An increase in the number of passes from one to two is accompanied by the accumulation of localized-deformation regions. The presence of regions with a martensitic structure in adiabatic shear bands in a sample deformed in two passes indicates heating of these regions above the α-β transition temperature. ω-phase particles are observed. The orientation relationships between the α and ω phases are such that the basal planes of their hexagonal crystal lattices are mutually perpendicular.     

TC21合金低温时效过程中的马氏体分解机制

采用透射、X射线及显微硬度分析等方法,研究了TC21合金中淬火马氏体在长时间低温时效过程中的组织演变及马氏体分解机制。研究表明,淬火态TC21合金在400～450℃进行长时间等温处理后,易获得弥散分布的颗粒状α相,显著提高合金性能,而α相颗粒的形核与马氏体中层错的分布密切相关。正交马氏体在低温时效过程中的具体分解方式为α→α+α富→α+β亚稳→α+β。进一步提高时效温度或时效时间α相颗粒将粗化为片层状,降低合金强度。     

BEHAVIOUR OF Co_5Cr_3Si_2 PHASE IN Cu-RICH ALLOYS CONTAINING Co,Cr AND Si

The major phases in the Cu-rich alloys containing Co,Cr and Si are α-Cu(a solid solutionof Co,Cr and Si in Cu),χ-phase(Co_5Cr_3Si_2)and Co_2Si.In comparison with referencesample,it has been detected that the crystal structure of Co_5Cr_3Si_2 is cubic,α-Mn type witha=0.8694 nm.The melting temperature of χ-phase and the alloy are higher than that of thepure Cu,namely,1535 and 1389 K respectively.During ageing treatment,the Co_2Si phaseprecipitates out from α-Cu and χ-phase simultaneously,but the hardening effect is mainlycontributed by the precipitation from α-Cu.     

MICROSTRUCTURE STUDY OF CONSTITUENT PHASES IN 2024 SERIES Al ALLOYS

X-ray microanalysis,convergent beam electron diffraction(CBD)and selected area electrondiffraction(SAD)studies on the structures and compositions of the constituent phases in2024 series Al alloys have been conducted.Partial substitution of alloying elements is found tooccur in all the constituent phases,which cause small deviations from the stoichiometric com-positions reported in these ternary compounds.The dominant phase is α-Al_(12)(FeMn)_3Si whichhas a body center cubic crystal structure with the Im■ space group and a=1.25 nm.The nextdominant phase is Cu_2FeAl_7 which has a primitive tetragonal crystal structure with theP4/mnc space group and a=0.6336 nm,c=1.487 nm.The minor phase is α'-Al_(12)Fe_3Si hav-ing α primitive cubic crystal structure with the Pm■ space group and α=1.27 nm.     

The influence of phase and substructural evolution during dynamic loading on subsequent mechanical properties of zirconium

At high pressures zirconium undergoes a phase transformation from the hexagonal closed packed (hcp) α-phase to the simple hexagonal ω-phase. In high purity Zr and under shock loading conditions the phase transformation has been observed to begin at approximately 7 GPa [1]. Evolution of the plastic response and phase transformation during dynamic loading is not well understood and therefore the contributions of this evolution to strength and damage are not well predicted. Here, through a combination of post-mortem and in situ techniques, different dynamic drive conditions are utilized to create a set of specimens with various volume fractions of retained high pressure ω-phase and stored plastic work. The mechanical properties of these well-characterized microstructures are subsequently examined. The results indicate that while both plastic deformation and the volume fraction of the high pressure phase play important roles in determining subsequent material properties, the effect of texture evolution due to plastic work may be of critical importance in determining these properties. This finding sheds an insight into strength under pressure.     

Variant selection during α precipitation in Ti–6Al–4V under the influence of local stress – A simulation study

Variant selection of α (hexagonal close-packed, hcp) phase during its precipitation from β (body-centered cubic, bcc) matrix plays a key role in determining the microstructural state and mechanical properties of α/β titanium alloys. In this work, we develop a three-dimensional quantitative phase field model to predict variant selection and microstructural evolution during β → α transformation in Ti–6Al–4V (wt.%) under the influence of both external and internal stresses. The model links its inputs directly to thermodynamic and mobility databases, and incorporates the crystallography of bcc to hcp transformation, elastic anisotropy and defects within semi-coherent α/β interfaces in its total free energy formulation. It is found that, for a given undercooling, the development of a transformation texture (also called microtexture) of the α phase due to variant selection during precipitation is determined by the interplay between externally applied stress or strain and internal stress generated by the precipitation reaction itself. For example, the growth of pre-existing α precipitates is accompanied by selective nucleation and growth of secondary α plates of certain variants that may not be the ones preferred by the initially applied stress. Possible measures to reduce transformation texture are discussed.     

Producing fully-lamellar microstructure for wrought beta-gamma TiAl alloys without single α-phase field

Fine-grained fully-lamellar (FL) microstructure is desired for TiAl components to serve as compressor/turbine blades and turbocharger turbine wheels. This study deals with the process and phase transformation to produce FL microstructure for Mo stabilized beta-gamma TiAl alloys without single α-phase field. Unlike the α + γ two-phased TiAl or beta-gamma TiAl with single α-phase field, the wrought multi-phase TiAl–4/6Nb–2Mo–B/Y alloys exhibit special annealing process to obtain FL microstructure. Short-term annealing at temperatures slightly above β-transus is recommended to produce the desired FL microstructure. The related mechanism is to guarantee the sufficient diffusion homogenization of β stabilizers during single β-phase annealing, and further avoid α decomposition by α → γ + β when cooling through α + β + γ phase field. The colony boundary β phase contributes to fine-grained nearly FL microstructure, by retarding the coarsening of the α phase grains.     

STUDY OF PHASES IN U-10wt%Zr ALLOY BY NEUTRON DIFFRACTION

ＳＴＵＤＹＯＦＰＨＡＳＥＳＩＮＵ－１０ｗｔ％ＺｒＡＬＬＯＹＢＹＮＥＵＴＲＯＮＤＩＦＦＲＡＣＴＩＯＮ￥ＸＩＥＧｕｏｑｉａｎｇ;ＹＡＮＧＪｉｊｉａｎ;ＺＨＡＮＧＢａｉｓｈｅｎｇ;ＬＩＷｅｎｄａｎ（ＣｈｉｎａＩｎｓｔｉｔｕｔｅｏｆＡｔｏｍｉｃＥｎｅｒｇｙ,...     

Phase transitions in crystals with a BCC structure

A theory of bcc-hcp and bcc-fcc structural phase transformations in metals with a high-temperature bcc lattice has been constructed on the basis of a pseudo-spin Ising Hamiltonian, which describes cooperative oscillations of atomic planes in a two-well potential with allowance for interactions between nearest neighbors. The picture of diffuse scattering and of the combined rearrangement of original Bragg reflections and diffuse scattering into Bragg reflections upon transitions into low-temperature phases has been calculated at all temperatures. It is shown that the bcc-hcp and bcc-fcc phase transformations occur in a temperature interval rather than at a point. In the case of the bcc-hcp transformation, as the temperature decreases, the bcc structure first transforms jumpwise into an orthorhombic structure close to the hcp structure and then, with a further decrease in temperature, this structure smoothly changes into the strictly close-packed hexagonal structure at T → 0. In the case of the bcc-fcc transition, there occurs an analogous transformation into a strictly close-packed face-centered cubic structure at T → 0, but now through a monoclinic structure.     

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.     

Transformation mechanism and mechanical properties of commercially pure titanium

In order to establish the rolling process parameters of grade-2 commercially pure titanium(CP-Ti),it is necessary to understand the transformation mechanism and mechanical properties of this material.The β→α transformation kinetics of the grade-2 CP-Ti during continuous cooling was measured and its hot compression behavior was investigated using Gleeble-1500 thermal mechanical simulator.Dynamic CCT diagram confirms that cooling rate has an obvious effect on the start and finishing transformation and microstructures at room temperature.The critical cooling rate for β-phase transforms to α phase is about 15 °C/s.When the cooling rate is higher than 15 °C/s,some β phases with fine granular shape remain residually into plate-like structure.The plate-like α phase forms at cooling rate lower than 2 °C/s,serrate α phase forms at medium cooling rates,about 5-15 °C/s.The flow stress behavior of grade-2 CP-Ti was investigated in a temperature range of 700-900 °C and strain rate of 3.6-40 mm/min.The results show that dynamic recrystallization,dynamic recovery and work-hardening obviously occur during hot deformation.Constitutive equation of grade-2 CP-Ti was established by analyzing the relationship of the deformation temperature,strain rate,deformation degree and deformation resistance.     

Orientation stability in equal channel angular extrusion. Part I: Face-centered cubic and body-centered cubic materials

Stability of crystallographic orientations is a key aspect in the characterization and understanding of texture evolution during plastic deformation. In this study, a rate-dependent crystal plasticity model was applied to investigate orientation stability during equal channel angular extrusion (ECAE) of face-centered cubic (fcc) and body-centered cubic (bcc) crystals. The stability of experimentally observed ideal orientations was examined according to lattice rotation fields computed at and around the orientations. It is shown that these ideal orientations are meta-stable under rate-sensitive conditions, and their stability generally increases with the decrease of strain rate sensitivity. The results also reveal a well-preserved duality in the lattice rotation and orientation stability between the two types of crystal structure. The stability results simulated at low strain rate sensitivities agree well with the experimental observations in one-pass ECAE of Al and Cu single crystals. In Part II of the paper, this analysis is extended to hexagonal materials.