Instability of the crystal lattice of CuPd alloy

The results of structural investigations (x-ray structural analysis, small-angle scattering and electron microscopy) of the B2→Al deformation phase transition and the kinetics of the recovery processes of deformed CuPd alloy are presented. It is established that the phase transition occurs in regions of local plastic deformation and the B2→Al deformation transition helps to enrich these regions with palladium. Tomsk State Architecture-Construction Academy. V. D. Kuznetsov Siberian Physico-Technical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 93–102, March, 1997.     

Determining orientation relationships for the <Emphasis Type="Italic">B</Emphasis>2 → <Emphasis Type="Italic">B</Emphasis>19′ transformation in single crystal titanium nickelide according to the texture of <Emphasis Type="Italic">B</Emphasis>19′ martensite

A precision method for determining orientation relationships upon the B2 → B19′ transformation in titanium nickelide has been developed. The method is based on analyzing the martensite texture formed in the initial high-temperature B2 phase of a single crystal having a high degree of perfection. The orientation relationships between the B19′ lattice and the initial B2 lattice of the TiNi single crystal were established from the B19′ martensite texture formed in single crystals of titanium nickelide upon the B2 → B19′ transformation and from measurements of the lattice parameters. Crystal mechanism of B19′ martensite was proposed which included the shear in the (21-1) B2 plane in the direction [−11-1] B2 by 10°. Such a shear system is typical of the bcc crystals at deformation by twinning. Absolute shear values are in a ratio of 1:4 for B2 → B19′ transformation and for twinning, respectively. Martensite deformation at an invariant lattice is accompanied by small rotations of martensite crystals (±1.6°), that increases the quantity of martensite orientations from 12 to 24.     

Phase transitions in TiNi(Rh) alloys

In this paper we report the results of x-ray structure analysis of how gamma rays and slight plastic strain affect phase transitions in the alloy Ti₅₀Ni₄₈Rh₂. Analysis of the published data on the temperature dependences of the rhombohedral angle in the R phase during the B2-R phase transition showed that not a single rhombohedral angle curve reflects the distortion relative to the cubic lattice of the B2 phase from the reduced temperature in the alloys TiNi and Ti₅₀Ni_50–xMe_x (Me=Fe, Co, Rh).Tomsk State Architectural-Construction Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 67–75, October, 1995.     

Premartensitic and martensitic transformations in Ti49Ni51 single crystal: Ageing effect

The influence of isochronous annealing on the phase composition, long-range atomic-order parameter S in the B2 phase, premartensitic structure, and martensitic transformation sequence and temperatures in a Ti₄₉Ni₅₁ single crystal was studied by X-ray and neutron-diffraction. Ageing the test alloy was found to bring about concurrent changes in the type of the intermediate shear structure involved and in the sequence of martensitic transormations in the premartensitic temperature range studied. The increase in the order parameter for the B2 phase was most pronounced in passing from the zonal stage of ageing to the growth stage of the Ti₃Ni₄ precipitates, as revealed by X-ray diffraction. No apparent correlation between the type of the premartensitic structure, R-transformation temperature, and order parameter variations was observed. At the same time, the martensite start temperature into the B19′ phase was directly proportional to variations in S. Institute of Strength Physics and Materials Science, Siberian Branch of the Russian Academy of Sciences. Institute of Physics of Metals, Urals Division of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 64–70, July, 1999.     

Pressure-induced amorphization of Gd2(MoO4)3: A high pressure Raman investigation

High pressure Raman spectroscopic studies on Gd₂(MoO₄)₃(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO₄ tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.     

First-principles study of high pressure phase transformations in Li<Subscript>3</Subscript>N

The lattice dynamics of lithium nitride (Li₃N) under high pressure are extensively investigated to probe its phase transformations by using the pseudopotential plane-wave method within the density functional theory. A new second order α↦α^′-Li₃N phase transition is identified for the first time. The newly proposed α^′-phase possesses a hexagonal symmetry with four ions in the unit cell having a space group of P-3m1. Further enthalpy and phonon calculations support the existence of this phase, which stabilizes in a narrow pressure range of 2.8 – 3.6 GPa at zero temperature. Upon further compression, transitions to denser packed phases of β-and γ-Li₃N are typical first order. The analysis of the electronic densities of states suggests that all the high pressure modifications of Li₃N are insulators and, interestingly, the typical behavior of compression is to broaden the band gap.     

Different structural states and root-mean-square atomic displacements in CuPd alloys

Results of X-ray diffraction studies onCu-39 at.%Pd andCu-36 at.%Pd are discussed. Root-mean-square (rms) atomic displacements in an ordered B2 phase and a disordered A1 phase are determined at various temperatures. The data obtained are indicative of an anomalous state of the crystalline structure of the disordered A1 phase prior to phase transitions both in the high-temperature range of its existence and in the low-temperature metastable range. During the stress-induced phase transformation, the rms atomic displacements are found to be larger in the A1 product phase, containg abundant defects, than in the parent B2 phase. Tomsk State University of Architecture and Building. Siberian Physicotechnical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 40–46, July 1999.     

On the mechanism of phase transformation in Ag2Se

A differential thermal analysis ΔT _y (T) in vacuum has been performed, and the temperature gradient ΔT _x (T) along the Ag₂Se sample during the transition α → β has been studied. It has been shown that the transitions α → α′ and β′ → β are displacive transitions and that the transition α′ → β′ is a reconstructive transition. It has been found that the temperature gradient along the sample during the transition α′ → β′ passes through a deep minimum due to a strong increase in the specific heat capacity.     

Stress-induced martensitic transformations in [001] crystals of titanium nickelide and its relation to mechanical twinning in the <Emphasis Type="Italic">В</Emphasis>2-phase

Methods of transmission electron microscopy were used to investigate the mechanisms of plastic deformation of TiNi(Fe, Mo) [001] crystals on compression in the interval of stress-induced martensitic transformations. It has been revealed that the formation of {113} and {114} twins of the В2-phase by the В19′→В2 reverse martensitic transformation over another path is, alongside with mechanical (100) twinning of В19′ martensite, an important deformation mechanism in this interval.     

Order-disorder transitions: structural and magnetic ordering in double perovskites

Using an effective lattice gas-like model for B-B′ site ordering we study order-disorder transitions in double perovskite materials A₂BB′O₆. We motivate this effective model from a microscopic hamiltonian. Using this model, we are able to address several experimentally observed issues including nonmonotonic dependence of the degree of order on annealing temperature, and the rapid decrease of order upon overdoping with either B or B′ species. We also study ordering in the ‘ternary’ compounds A₂BB′_1−yB″_yO₆, using a variant of the Blume-Emery-Griffiths model. Several issues related to structural and magnetic ordering are discussed.     

Local strain distribution in Ni3Ge crystals

We have used a net method to study the distribution of stresses on two facets of Ni₃Ge crystals deformed compressionally to a deformation ε=14 and 16% (near destruction) at experimental temperatures of 77 and 673 K, respectively. It is shown that the stress distribution is inhomogeneous over the sample. Stresses which exceed the average are distributed randomly over the sample at low temperature and in more localized fashion at T=673 K. The temperature has a significant influence on the nature of the deformation distribution. It is shown that the Shannon entropy of the normal distribution of local deformation values is determined only by the variance of random quantities. It is observed that as the temperature is raised to 673 K the Shannon entropy falls or we observe a self-organization of the local volume deformations. State Architecture and Construction Academy, Tomsk. V. D. Kuznetsov Physicotechnical University, Tomsk State University, Siberia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 87–92, March, 1997.     

Structural analysis of the Γ phase in cholesterol-distearoylphosphatidylcholine multilamellar vesicles

Summary The interaction of cholesterol with membranes is particularly interesting as the cholesterol concentration influences membrane stability and permeability by inducing changes in the structure and dynamics of the constituent lipid molecules. On the other hand, cholesterol is present in the atherosclerotic plaques and in some skin diseases, like xanthomas and xantelasmas. An investigation was performed on the Γ phase in a cholesterol-distearoyl phosphatidyl choline (DSPC) mixture by using X-ray diffraction and differential scanning calorimetry. The cholesterol/DSPC molar ratio was 35/65 and DSPC-to-water weight ratio was 1/3. The Γ phase appears at sufficiently high concentration of cholesterol and it is characterized by interchain positional order considerably lower than in theL _β′ andP _β′ phases but higher than in theL _∝ liquid crystalline phase of pure lecithin in excess water. The results suggest that a lateral phase separation exists in the temperature range of existence of the Γ phase. The other phase Γ′ appears to be richer in cholesterol. The (Γ+Γ′) toL _∝ phase transition was investigated and the temperature dependence of the in-plane correlation length in the Γ phase was determined. A very weak enthalpy peak was observed at the chain melting transition, confirming the highly disordered nature of phases Γ and Γ′. Work presented at the First USSR-Italy Bilateral Meeting on Liquid Crystals held in Portonovo, Ancona (Italy), September 30–October 2, 1987.     

Evolution of the crystal structure during phase working of titanium nickelide

Using x-ray diffraction we have measured the dynamic and static atomic displacements and the unit cell parameters of monoclinic martensite in the intermetallide 50.1 at. %Ti-Ni, during thermal cycling through the interval of the martensitic transition (280 to 400 K). It is shown that along with a change in the sequence of transitions from B2→B19′ to B2→R→B19′, there is also a change in the atomic displacements from predominantly dynamic on the first cycles to static displacements during the later ones. The martensite unit cell has a larger volume on cooling than on heating, and cycling leads to a lowering of its monoclinic angle. Institute of Strength Physics and Materials Production, Siberian Division of the Russian Academy of Sciences.     

Thermal effects of transformations B2 ⇄ Al in CuPd alloy

We used high-temperature x-ray diffraction and thermography to investigate phase transitions B2 ⇄ Al in alloys Cu-Pd around 40 at. % Pd concentration. In phase transition region B2 → Al in alloy Cu-39.5 at. % Pd, we observed the maxima in the heat capacity curve. Based on structural data, we show that the first maximum is due to structural transition bcc-fcc (B2-Al) and the second is due to a decrease in the atomic long-range order in superlattice B2 and to a continuing bcc-fcc transition. Using structural information, we analyzed the thermograph of the quenched alloy Cu-36 at. % Pd. Structural Engineering Institute, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 35–38, February, 1993.     

Crystallographic analysis of the <Emphasis Type="Italic">B</Emphasis>2 → <Emphasis Type="Italic">B</Emphasis>19′ martensite transformation in titanium nickelide

A phenomenological theory of martensite transformation is used to determine (select) the mechanism of B2 → B19′ transformation. The most realistic mechanism corresponds to the minimum additional rotation of a martensite plate needed to maintain the invariance of the habit plane. Equal values of the macroscopic shear direction and extent, the habit plane, the deformation for the invariant lattice, and the general deformation of shape are obtained for four different deformations. However, the supplementary rotation for each option is different. The minimum angle of rotation is observed for deformation by a martensite transformation with {21-1}B2 plane shear in the 〈-11-1〉 direction.     

Study of graded Ni-Ti shape memory alloy film growth on Si(100) substrate

In-situ X-ray diffraction (XRD) was employed to study the effect of the deliberate change of the Ti/Ni ratio during the deposition of Ni-Ti films. Thus, graded films were deposited exhibiting distinctive composition and crystalline structure along the growth direction. The as-sputtered films were ex-situ characterized by Auger electron spectroscopy (AES), cross-sectional transmission electron microscopy (XTEM), and electrical resistivity (ER) measurements (during thermal cycling). In this paper results are presented concerning a film (thickness of ≈ 420 nm) with a Ti-rich composition in the central part (ranging from 50 to ≈60 at. %) and near-equiatomic composition in the extremities, following four distinct deposition periods (different Ti target powers). During the initial deposition step (near-equiatomic composition) the Ni-Ti B2 phase starts by stacking onto (h00) planes on the naturally oxidized Si(100) substrate due to the presence of the native Si oxide (2–3 nm). The increase of the power of the Ti target in the second and third steps induced the precipitation of Ti₂Ni. When stopping the Ti co-sputtering, Ti₂Ni dissolves and, thus, plays the role of a Ti reservoir for the formation of B2 phase now preferentially stacking onto (110) with the system approaching again the equiatomic composition. The ex-situ study of the morphology of the interface has shown the presence of NiSi₂ silicides (A-NiSi₂ and B-NiSi₂), Ti₄Ni₄Si₇, Ti₂Ni and a non-identified phase constituted by Ni, Ti and Si, most likely amorphous. During thermal cycling, ER measurements revealed phase transitions associated with the B2, R-phase and B19^′ phases. These type of studies allow the identification of intermediate states during deposition and annealing, and the correlation with the final structure of the film, being useful for the optimisation of the deposition parameters in order to fabricate films with a two-way reversible actuation. PACS 81.15.Cd; 61.10.Nz; 68.55.Jk     

Microstructure of Cu<Subscript>60</Subscript>Zr<Subscript>20</Subscript>Ti<Subscript>20</Subscript> bulk metallic glass rolled at different strain rates

The structural evolution of Cu₆₀Zr₂₀Ti₂₀ bulk metallic glass during rolling at different strain rates and cryogenic temperature was investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy (HRTEM). It is revealed that the deformation-induced transformation is strongly dependent on the strain rate. At the lowest experimental strain rate of 1.0×10⁻⁴ s⁻¹, no phase transformation occurs until the highest deformation degree reaches 95%. In a strain rate range of 5.0×10⁻⁴−5.0×10⁻² s⁻¹, phase separation occurs in a high deformation degree. As the strain rate reaches 5.0×10⁻¹ s⁻¹, phase separation and nanocrystallization concur. The critical deformation degree for occurrence of phase transformation decreases with the strain rate increasing. Supported by the National Natural Science Foundation of China (Grant No. 50471016)     

Influence of nonhydrostatic stresses on the development of martensitic transformations and inelastic strains at various levels in Ti(Ni-Cu-Fe) polycrystalline intermetallic compounds

The levels of inelastic martensitic strain of polycrystals during a thermoelastic martensitic transformation under a load are discussed. The example ofTi(Ni-Cu-Fe) alloys with the B2 structure was used to study the role of microlevel and mesolevels in inelastic martensitic deformation during cooling of polycrystals under a load and loads in different initial structural states. V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Institute of Physics of Strength of Materials and Materials Science, Siberian Branch of the Academy of Sciences of the USSR, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, p. 35–46, January, 1998.     

Effects of current density on electropulsing-induced phase transformations in a Zn–Al based alloy

Phase transformations of an electropulsing-treated ZA22 alloy were studied after tensile deformation by using scanning electron microscopy and transmission electron microscopy. It was found that electropulsing tremendously accelerated phase transformations consequentially in the two stages: (a) quenching from supersaturated state approaching the final stable state, i.e., α+ε→T′+η, and (b) up-quenching from the final stable state to a higher temperature state, i.e., T′+η→α+ε. The mechanism of electropulsing-induced phase transformation is discussed from the point of view of Gibbs free energy, and electropulsing kinetics.     

Effect of annealing on structure and magnetic properties of Fe-N and Fe-Ti-N thin films

The structure and magnetic properties of Fe-N and Fe-Ti-N films have been studied as a function of annealing temperature T_a with a transmission electron microscope and a vibrating sample magnetometer. The as-prepared Fe-N films consist of the γ^′-Fe₄N and α^′′-Fe₁₆N₂ phases, and the Fe-Ti-N films are composed of the γ^′-Fe₄N, α^′′-Fe₁₆N₂, and TiN phases. The structural changes with annealing temperature in the Fe-N films are distinct. The α^′′-Fe₁₆N₂ decomposes into α+γ^′ phases in the Fe-N film annealed at about 300 °C, and it disappears in the film annealed at 350 °C. Annealing of the Fe-Ti-N films shows no structural changes between room temperature (RT) and 500 °C. The saturation magnetization 4πM_S and coercivity H_c of the Fe-N films change drastically with the annealing temperature T_a, whereas those of the Fe-Ti-N films do not change with T_a up to 500 °C. These results indicate that the additon of Ti may improve the thermal stability of Fe-N films. Recieved: 6 Juli 1998 / Accepted: 19 Oktober 1998 / Published online: 10 March 1999