Detection of the ɛ phase and the Headley-Brooks orientation relationships upon α → γ transformation in the Fe-32% Ni alloy

The structure of an Fe-32% Ni alloy preliminarily quenched for martensite and subjected to α→ γ transformation upon a slow heating to different temperatures (430–500°C) has been studied by the electron-microscopic method. There has been observed an intermediate ɛ phase with an hcp lattice and rarely encountered Headley-Brooks bcc/fcc orientation relationships, which differ from the Kurdjumov-Sachs relationships. The networks of reflections of the ɛ phase have been observed in electron-diffraction patterns of the Fe-32% Ni alloy after both a slow heating to 430°C without annealing and a slow heating to 500°C with a subsequent annealing at 280°C; the Headley-Brooks relationships between the α matrix and the γ phase, which are typical for increased temperatures of phase transformations, have been observed in the samples after a slow heating to 500°C with annealing.     

Crystallographic laws governing the formation of the structure of pseudo-single crystals of some 3<Emphasis Type="Italic">d</Emphasis> metals and related alloys

Methods of metallography, X-ray diffraction, transmission electron microscopy, and dilatometry have been used to study the formation of the structure of pseudo-single crystals of nitrogen-bearing steel Kh18AG20 upon the δ → γ (bcc → fcc) transformation, in pseudo-single crystals of pure cobalt and binary alloy Co-29.7% Ni upon the β → α (fcc → hcp) transformation, and in pseudo-single crystals of zirconium upon the β → α (bcc → hcp) transformation. It has been established that the precipitation of austenite from ferrite during the δ → γ transformation in a single crystal of the Kh18AG20 steel occurs via a crystallographically ordered mechanism with the fulfillment of orientation relationships close to the Kurdjumov-Sachs orientation relationships. In the volume of the pseudo-single crystal there were realized six orientations of austenite with the retention at room temperature of a significant fraction of residual δ (α) ferrite. It has been shown that in the process of cooling of the β single crystals of cobalt, Co-29.7% Ni alloy, and zirconium to below the temperature of the β → α transition there formed several crystallographic orientations of the α phase that are grouped into packets. In each packet, there exist crystals of the α phase of one orientation. In accordance with the Wassermann orientation relationships, in the pseudo-single crystals of cobalt and Co-29.7% Ni alloy there are realized packets of four variants. In the pseudo-single crystal of zirconium, six variants of packets based on the Burgers orientation relationships are realized.     

Phase and structural transformations in the Ti<Subscript>49.5</Subscript>Ni<Subscript>50.5</Subscript> alloy with a shape-memory effect during torsion under high pressure

Results of investigations of structural and phase transformations that occur in the titanium-nickelide-based alloy Ti_49.5Ni_50.5 with a shape memory effect during severe plastic deformation by torsion under high pressure (HPT) are reported. The studies were performed using transmission and scanning electron microscopy, neutron and X-ray diffraction, and measurements of temperature dependences of electrical resistivity. The martensitic B2 → B19′ transformation was found to be induced in the alloy when applying a high pressure. After unloading, the martensitic B19′ phase is retained in the alloy. The fine structure of the B19′ martensite and its evolution into nanocrystalline and, subsequently, amorphous state during HPT with 1/4, 1/2, 1, 5, and 10 rev have been studied. It was shown that, after HPT, all nanosized crystallites whose sizes are less than 30–50 nm have a B2-type structure and, therefore, the reverse martensitic B19′ → B2 transformation is realized in the alloy at room temperature after unloading.     

Structure formation upon the β→α transformation in a pseudo-single crystal of the Co-29.7% Ni alloy

Optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, and dilatometry have been used to investigate structure formation upon the β→α (fcc-hcp) transformation in a Co-29.7% Ni alloy. During cooling of an fcc single crystal of the Co-29.7% Ni alloy to the temperature of the β→α phase transformation, the single crystal, because of the polymorphic fcc-hcp transformation, is converted into a two-phase (β + α) pseudo-single crystal. The growth direction of the crystal coincides with the crystallographic direction [112]_β. It is shown that four variants of orientations of the α-martensite crystals are realized during the β→α transformation in a single crystal; the entire volume of the initial crystal is divided into packets, which contain plates of the α phase of predominantly one orientation. The thickness of the α-martensite plates determined by the method of electron microscopy is from 0.1 to 1.0 μm. After a cycle of the α→β→α transformations, in the pseudo-single crystal investigated there is observed a restoration of the initial macro- and microstructure; i.e., the fact of structural heredity is confirmed. The calculated value of the volume changes (ΔV/V) in the Co-29.7% Ni alloy upon the fcc-hcp transformation is close to that experimentally determined from dilatometric data. The hysteresis in the temperature of the onset of the β→α transformation in this alloy is 195 K.     

Creep deformation and microstructural change in extruded zn-al based alloy

Microstructural evolution and related phase transformation of an extruded eutectoid Zn-Al based alloy are studied in detail under creep deformation at 150°C. Lamellar structure in the extruded alloy spheroidized partially into fine grain structure. Also decomposition of a metastable ή_T phase and a four phase transformation. α+ε→T+η, were observed in the creep deformed alloy specimens. Creep rupture of the extruded Zn-Al alloy was studied in correlating with the creep induced phase transformation and microstructural changes.     

Crystal dynamics of the BCC-HCP martensitic transformation: II. Morphology

A dynamic theory of the formation of martensite crystals upon the bcc-hcp transformation based on the concepts of the heterogeneous nucleation and related controlling wave process is presented. It is shown that the entire set of the observed morphological characteristics is unambiguously connected with the trajectory of the martensitic reaction assigned by the controlling wave process. In particular, for the orientation relationships between the phases involved in the transformation there was found an analytical dependence of the angle between the directions of close-packed atomic rows on the elastic properties of the initial bcc lattice. Quantitative estimates are given based on the values of the elastic moduli for bcc titanium. A discussion of the results obtained is conducted and special attention is paid to the new possibilities of experimental studies.     

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

Structural phase transformations in quasicrystal-forming Al-Cu-Fe alloys and defects of the icosahedral phase

Transmission electron microscopy was used to investigate structural and phase transformations and defects of the icosahedral (ι) phase that is formed upon isothermal annealings (T _ann = 550–700°C) of quenched quasicrystal-forming alloys Al₆₁Cu₂₆Fe₁₃ and Al₆₃Cu₂₅Fe₁₂ (β solid solution + ι phase). It has been established that in the Al₆₃Cu₂₅Fe₁₂ alloy there occurs a reversible ι-R-approximant transformation, whereas in the Al₆₁Cu₂₆Fe₁₃ alloy there is formed a single-phase ι structure with regions with a high density of randomly distributed planar defects (T _ann = 550°C), which are partially annealed at 650°C. The observed defects are, mainly, ultrathin interlayers (“intergrowths” to 3–5 nm in thickness) on quasicrystal planes with A5_ι axes with an imperfect decagonal structure. As the basic mechanism, the growth mechanism of the formation of defects during the β → i transformation is proposed. The role of the alloy composition and low-temperature β → 3C-phase transformation in the realization of this mechanism is discussed.     

Magnetic and structural transitions in crystals with a structure of the NaCl type

A model of simultaneous magnetic and structural first-order transitions in antiferromagnets with a strong cubic magnetic anisotropy has been constructed on the basis of a synthesis of magnetic modified 6-state and 8-state Potts models and the theoretical model of structural phase transitions in cubic crystals. A revised scheme has been suggested for the derivation of possible magnetic structures in the fcc lattice with allowance for competing interactions between the nearest and next-nearest neighbors. A calculation of the temperature evolution of high-temperature diffuse magnetic scattering of neutrons has been carried out to show that the mechanism of a magnetic transition at the Néel point is caused by the transformation of diffuse magnetic scattering into magnetic Bragg peaks.     

Neutron diffraction study of diffuse scattering in Cu2−δSe superionic compounds

Crystal structure and short-range order in Cu_2−δSe compounds were studied in superionic and non-superionic phases using high-resolution neutron diffractometer Echidna at ANSTO. In diffraction patterns of β-Cu_1.98Se (ordered phase at ambient T), both Bragg peaks and diffuse background change sharply through the β → α structural phase transition at T = 414 K during heating. In case of α-Cu_1.75Se (disordered superionic phase at ambient T) the changes are monotonic, showing gradual shifts of Bragg peaks and increased intensity of the diffuse background as a function of temperature. On cooling, both compounds undergo a β → β′ transformation. Diffuse scattering in the α-phase shows an oscillating dependence on wavevector, with broad peaks centred at Q ∼ 3, 5.5 and 8 Å⁻¹. The measurements taken in energy dispersive mode show that the oscillating diffuse background arises from correlated thermal displacements of the ions. Diffuse scattering is higher for compositions close to stoichiometry and increases with temperature. Theoretical calculations show that the increase in diffuse intensity both with temperature and Cu content is related to correlated thermal vibrations of Se and Cu atoms, with Se-Cu(8c, 32f) and Cu(8c)-Cu(8c) correlations being the most important.     

Phase equilibria and phase transformation of Co−Ni−Ga ferromagnetic shape memory alloy system

Phase equilibria and martensitic and magnetic phase transformations of the β phase in the Co−Ni−Ga system have been investigated. It has been shown that the β phase is in equilibrium with the α-phase over a wide range compositions at 600–1100°C. The β phase exhibits both a paramagnetic-ferromagnetic transition and a thermoelastic martensitic transition from B2 to L1₀ structure. The Curie temperature T _C increases with decreases with decreasing Ni content and with increasing Ga content. The composition region of the β phase exhibiting the thermoelatstic martensitic transformation from ferromagnetic austenite is located near the α+β two-phase region. T _C and M _s of α+β two-phase alloys increase with increasing annealing temperature. This paper was presented at the International Symposium on User Aspects of Phase Diagrams, Materials Solutions Conference and Exposition. Columbus, Ohio, 18–20 October, 2004.     

Structural properties in flame quenched Cu−9Al−4.5Ni−4.5Fe alloy

The flame quenching process has been employed to modify the surfaces of a commercial marine propeller material, aluminum bronze alloy (Cu−9Al−5Ni−5Fe), and the material’s microstructure and hardness properties have been studied. The thermal history was accurately monitored during the process at various surface temperatures and holding times. XRD and EDX analyses have shown that aboveT _β temperature the microstructure consisting of α and κ phases changes into α and β’ martensite due to an eutectoid reaction of α+β→κ and a martensitic transformation of β→β’. The β’ martensite phase formed has a face-centered cubic (FCC) crystal structure with typical twinned structure. The hardness of the flame-quenched layer having the α+β’ structure is similar to or lower than that of the α+κ structure, depending highly on the size and distribution of β’ and κ phases. It is noted that the sliding wear resistance of the flame-quenched layer is enhanced with the formation of β’ martensite.     

New Concepts of the Nature of Bainitic Transformations in Steels

Special features of theγ → transformation occurring in a wide range of cooling rates (0.1 – 500,000 K/sec) in pure iron and in carbon and chromium steels are considered. It is shown that four stages of the γ → transformation are possible in the steels, the first of which has a diffusion or massive nature, and the other three can be characterized as displacement transformations. The genetic tie between the upper and lower bainite and transformations occurring through stages II and III, respectively, in pure iron is shown. The difference between the upper and lower bainite is determined by the difference in the mechanisms of motion of the phase boundary, which can be accompanied by redistribution of carbon atoms. Stage IV corresponds to athermal martensitic transformation. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 24 – 29, July, 2005.     

The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α₂ (ordered close-packed hexagonal D0₁₉ structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti₂AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α₂+B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O+B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α₂+B2+O field. Below 875°C, an O+β field existed. All heat treatments at or above 875°C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O+β platelets at O/B2 and α₂/B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.     

The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α₂ (ordered close-packed hexagonal D0₁₉ structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti₂AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α₂+B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O+B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α₂+B2+O field. Below 875°C, an O+β field existed. All heat treatments at or above 875°C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O+β platelets at O/B2 and α₂/B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.     

Composition and non-equilibrium crystallization in partially devitrified co-rich soft magnetic nanocomposite alloys

The body-centered cubic (bcc) phase tends to preferentially nucleate during solidification of highly undercooled liquid droplets of binary alloy systems, including Fe–Co, Fe–Ni and Fe–Cr–Ni. We investigate a similar tendency during the partial devitrification of Co-rich amorphous precursors of composition (Co_1?xFe_x)₈₈Zr₇B₄Cu₁ by identifying the structure and composition of the nanocrystalline grains. The Co:Fe ratio of the bcc nanocrystals varies linearly with the Co:Fe ratio of the amorphous precursor, and can lie well within the single-phase face-centered cubic (fcc) region of the Fe–Co phase diagram at the crystallization temperature. Classical nucleation theory therefore suggests several potential explanations for the preferential nucleation of bcc phase from an amorphous precursor, including: (i) a reduced amorphous/bcc interface energy as compared to the close-packed phases; (ii) a lower strain of precipitation for bcc nuclei as compared to close-packed fcc and hexagonal close-packed nuclei; and (iii) stabilization of the bcc phase by dissolved glass-formers such as Zr and B.     

Ferroelastic domains and phases in ferromagnetic nanostructured FePd alloy

Polarization optical, electron microscopic, and thermomagnetic investigations of FePd single crystals undergoing an A1 → L1₀ phase transformation have been carried out. The results obtained have been analyzed proceeding from the concept of ferroelasticity and the experimental criteria of its existence. The features of phase transformations in equiatomic FePt and FePd alloys have been compared. A conclusion has been drawn that the atomic ordering in the FePd alloy is preceded by the formation of a ferroelastic lowsymmetry disordered (BCT) phase, and the A1 → L1₀ phase transformation presents a combination of different types of phase transitions such as cooperative displacement and ordering of atoms.     

<Superscript>63</Superscript>Cu NMR spectra,magnetic susceptibility,and transmission electron microscopy of the rapidly quenched alloy Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript>

Methods of transmission and scanning electron microscopy and nuclear magnetic resonance (NMR) at ⁶³Cu nuclei, as well as measurements of the static magnetic susceptibility χ(T) have been used to study a shape-memory alloy (SMA) Ti₅₀Ni₂₅Cu₂₅, which experiences a thermoelastic martensite transformation. The alloy was obtained from an amorphous ribbon in a bimodal nano- and submicrocrystalline state via a crystallization annealing for 1 h at 770 K with a subsequent quenching to room-temperature water. The resultant B2 austenite is characterized by a fine structure of the ⁶³Cu NMR spectra, which is connected with the different distribution of ⁶³Cu atoms on the second coordination shell. The evolution of the shape of the spectra with decreasing temperature reveals a structural transition B2 → B19. In addition, the ⁶³Cu NMR spectra, just as the transmission electron microscopy, indicate the presence of phase separation in the alloy, with the precipitation of a TiCu (B11) phase. The temperature dependence of the static magnetic susceptibility χ(T) also indicates the occurrence of a structural transition and has a hysteretic nature of “stepped” type. The discovered stepped nature of the χ(T) dependence is explained by the bimodal size distribution of grains of the B2 phase due to the size effect of the martensitic transformation.     

The mechanisms of the fcc–bcc martensitic transformation revealed by pole figures

Face-centered cubic (fcc) to body-centered cubic (bcc) martensitic transformations occur in many materials, such as steels, FeNi meteorites or brass. The phenomenological theory has been the accepted theory for these transformations for more than half a century. However, it cannot explain the continuous singular features in the experimental electron backscatter diffraction or X-ray diffraction pole figures. Here we show that such patterns can be simulated by one discrete orientation relationship and two continuous rotations that correspond to a trace of the transformation mechanisms. A new theory of martensite transformation that is in full agreement with the experimental pole figures is proposed. In this theory, the fcc–bcc transformation results from a fcc–hexagonal close-packed (hcp) step followed by an hcp–bcc step. The advantages of this two-step theory over the phenomenological theory are discussed.     

An investigation of the structural dynamics in the fast ionic conductor Cu2−δSe using neutron scattering

Energy resolved neutron diffraction measurements were performed on Cu_1.75Se and Cu_1.98Se samples for both α- and β-phases. In-situ measurements of the Cu_1.98Se compound during heating show that the β–α phase transition takes place at 420 K and has noticeable temperature hysteresis. In addition to Bragg reflections, the diffraction patterns of the α-Cu_1.75Se at RT and α-Cu_1.98Se at 435 K samples taken in conventional two-axis geometry show a broad maximum related to diffuse scattering. This diffuse background is suppressed in the energy resolved experiment which indicates a strong contribution from inelastic scattering coming from correlated thermal displacements of the ions in the super-ionic phase. On the other hand, the diffraction pattern of the non super-ionic β-phase shows only minor differences between spectra measured with and without the analyser crystal.