Variant Of Phase Research Articles

Temperature behavior of ZrPt compound studied by PAC with 181Ta and 111Cd probes

ZrPt intermetallic compound was studied by perturbed angular correlation (PAC) method. The hyperfine interaction parameters and their temperature dependences for 181Hf/ 181Ta and 111In/ 111Cd radioactive probes were determined. An unusual temperature dependence of hyperfine interaction parameters for 181Ta probe atoms points to the appearance of a monoclinic variant of CrB-type phase below 573 K and confirms the existence of the orthorhombic B f-type phase at temperatures up to 1023 K.

Unusual mesomorphic behaviour in bent-core compounds derived from 5-cyanoresorcinol

We describe the synthesis and mesomorphic properties of the first two homologous series of bent-core compounds derived from 5-cyanoresorcinol. The lower homologues in both series show a normal B1 phase, and the mesophase of higher homologues exhibits very unusual textures and electro-optical switching behaviour. The observation of spiral growth patterns under a polarizing microscope, and only layer reflections from X-ray diffraction studies for these homologues, suggests that this mesophase is different from a B2 phase. This mesophase shows chiral domains of opposite handedness which can be identified from electric field experiments. Under a triangular-wave electric field, at lower voltages although the mesophase shows a ferroelectric type switching behaviour it is actually antiferroelectric in nature. The observation of helical structures and spontaneously formed chiral domains suggests that the mesophase is a variant of the B2 phase.

Theory, experiments and numerical modelling of phase transformations with emphasis on TRIP

This paper presents an overview of the co‐operative efforts aiming at the correct characterisation of the thermo‐mechanical behaviour of materials during the process of a phase change. In the first section the physical conditions for the onset of transformation processes, either diffusive or massive or displacive, expressed in terms of the chemical driving forces in a multi‐component system are derived on a very general basis. Introducing appropriate expressions for the chemical as well as the mechanical dissipation based on jump conditions of quantities such as the deformation rate and the diffusive fluxes at the moving interface allows to formulate proper transformation criteria. No fluxes will occur in the case of displacive, i.e. martensitic transformation which is responsible for the TRIP phenomenon. The mechanism governing the selection of a particular martensitic variant of the product phase out of a discrete number of possible variants is described in the paper. The underlying ideas and tools supplied by continuum mechanics eventually leading to a transformation condition for martensitic transformation are summarised in the appendix.The second section of the paper shows some aspects of a comprehensive experimental program investigating the thermo‐mechanical behaviour of a maraging steel with very advantageous properties in the transformation regime. It allows to filter out the TRIP strain evolution during transformation from the total strain measured by means of a multiaxial tension torsion dilatometer equipment. The focus is put on finding a material law that is valid also for non‐proportional loading paths. Unlike the predictions of traditional constitutive relationships the TRIP strain rate exhibits a significant drop if the external load is removed during the progress of transformation suggesting the existence of a transformation related backstress.Finally a method is demonstrated how to validate the experimental findings by means of a numerical algorithm. Based on the physical principles explained in the first part of the paper a subroutine can be devised and implemented into a commercial finite element code that allows to simulate the behaviour of the material represented by a unit cell. The simulations yield realistic results for the transformation kinetics, the load‐displacement curves as well as the material response for non‐proportional loading paths.

Structural and magnetic ordering in the two-dimensional coordination polymer Co(ox)(bpy-d8), (ox=C 2O 42−, bpy-d8=4,4′-bipyridine-d8)

Neutron diffraction and magnetic susceptibility studies of the two-dimensional coordination polymer Co(ox)(bpy-d8) are presented, where ox=C 2O 4 2− and bpy-d8=4,4′-bipyridine-d8 (fully deuterated). The neutron powder diffraction data reveal a second-order crystallographic phase transition at 290 K. Above 290 K, a disordered structure, space group Immm, is observed that is closely related to the ordered structure previously proposed on the basis of single crystal X-ray diffraction. At low temperatures, the structure is an ordered variant of the high-temperature phase with space group I222. In both phases, the Co ions are linked by the oxalate forming infinite chains that are crosslinked by the bpy ligands. The magnetic susceptibility follows qualitatively a quasi one-dimensional chain behavior. It exhibits a broad maximum around 35 K, corresponding to a strong antiferromagnetic coupling through the oxalate bridges. A kink at 9 K marks the onset of long-range antiferromagnetic ordering due to much weaker interchain magnetic interactions. The magnetically ordered structure determined from the low-temperature neutron diffraction data can be described with the propagation vector (1/2, 1/2, 1/2), i.e. a doubling of the unit cell in each principal direction. It is concluded that a significant antiferromagnetic interaction is mediated through the bpy ligands, although the Co–Co distance along these bridges is 11.4 Å.

Calorimetric study of the effect of linear strain on the shape memory properties of Nitinol

The effect of linear strain on the transformation behaviour of 40% cold worked Nitinol wire has been studied using a differential scanning calorimeter (DSC), for two heat-treat temperatures, one in which there exists a fully developed martensitic phase (M) at room temperature and another in which there exists a mixture of M-phase and intermediate R-phase at room temperature. Samples have been subjected to a linear strain in the range 0–30% at room temperature by applying tensile stress. The effect of linear strain on the transformation temperatures, the energy of transformation and the thermal width of the transformation is discussed on the basis of the orientation effect which facilitates the growth of a single variant of the martensitic phase, the internal stress effect which opposes the growth of the martensitic phase and the conversion effect which increases the size of the martensitic phase. An attempt is made to correlate the shape of the endotherm peaks with the recovery of the applied linear strain. The DSC results are compared with the observations obtained through the tensile stress–strain curves.

Variant selection in heterogeneous nucleation on defects in diffusional phase transformation and precipitation

In heterogeneous nucleation on lattice defects in the matrix during diffusional phase transformation and precipitation reactions, the variant of product phase with the specific orientation relationship is strongly selected by the nature of the defects. For dislocations, effective accommodation of the transformation strain by the strain field of dislocations occurs, leading to the variant selection in which the direction of the maximum misfit is nearly parallel to the Burgers vector of the dislocations. For high-angle grain boundaries and subgrain boundaries, precipitates tend to select the variant with a low-energy interface (often the parallel close packed planes) inclined at the smallest angle to the grain boundary plane. On the other hand, precipitates that nucleate on the incoherent inclusions embedded in the matrix do not hold any specific orientation relationship with respect to the matrix. It is important to produce many variants locally for the effective refinement of microstructures that improves the mechanical properties of steel and titanium alloys.

Evolution and thermal stability of Ni3V and Ni2V phases in a Ni-29 at. pct V alloy

Eutectoid decomposition of the disordered fcc Ni-V solid solution in the composition range of 25 to 33.3 at. pct V gives rise to a mixture of the ordered Ni3V and Ni2V phases. In the present work, the evolution and thermal stability of these phases were studied in a Ni-29 at. pct V alloy. Solution-treated and water-quenched specimens, when aged at 850 °C, were found to exhibit two types of microstructure. In the first, the Ni2V phase precipitated in a lamellar Ni3V matrix where a pair of conjugate lamellae corresponded to two variants of the Ni3V phase. In the second morphology, the Ni2V phase precipitated within a Ni3V matrix comprising a single variant of the Ni3V phase. The Ni2V phase was observed to precipitate in a plate-shaped morphology, exhibiting {120}fcc-type, habit planes. The precipitation of the Ni2V plates in the Ni3V lamellae resulted in zigzag interfaces between adjacent Ni3V domains. Both the microstructures were found to be thermally quite stable and did not coarsen appreciably on prolonged aging. However, the prolonged aging caused the renucleation of the Ni3V and the Ni2V phases in the vicinity of the grain boundaries in a manner similar to “recrystallization.” The stability of the aged microstructure could be attributed to the nature of the interfaces between different domains of the Ni3V and Ni2V phases.

New stacking variant of laves phase found in (Ti0.95V0.05) Co2 alloy

Microstructures of the Laves phase alloy, (Ti0.95V0.05) Co2, were studied by high resolution electron microscopy and electron diffraction. In this alloy system, coexistence of several kinds of layered structures was observed. Among these structures, a new stacking variant was found and was analyzed to be ABCAB′A′C′BCA′C′B′. This structure belongs to the trigonal system of the space group P3m1 (no. 164). The lattice parameters presented in the hexagonal system are a = 0.4727 ± 0.0009 nm and c = 4.628 ± 0.008 nm. This structure is called 12T in the Ramsdell notation and 4323 in the Zhdanov symbol, and is classified into hP3m1-(6i)5(3f)(3e)(2d)12(2c)6using the Wyckoff notation. Microsc. Res. Tech. 40:277–283, 1998. © 1998 Wiley-Liss, Inc.

Structural Defects in the 18R Martensite of CuZnAi Shape Memory Alloys

ABSTRACTThe lattice relaxation at the type I faults in the 18R martensite was studied by high resolution TEM. Local atomic plane rotations and small changes of the interplanar distances were observed over a distance of 1.5 nm. It is shown that this distorted region can be rationalized as an intermediate structure between the martensite and the second variant of the parent phase.

Equivalent least deformation orientation variants by a phase transition between two structures with subgroups of the same point group

Abstract In a phase transition occurring between a structure of a subgroup 1 and another of a subgroup 2, both subgroups of the same point group, the lattice (or unit cell) transformation from one variant of the parent structure to one variant of the daughter structure generally depends on the really appearing variant of the daughter phase. Then all the orientation variants of the daughter structure may not be equivalent from the point of view of the lattice (or unit cell) transformation. It can be expected that the least deformation variant(s) occur(s) preferentially. Generally several of these least deformation variants are equivalent for a transformation from one variant of the parent phase. Their number can be easily determined. When such a transformation occurs from structure 1 to structure 2 and back to structure 1 new variants of structure 1 may occur, with a probability that can be determined in each case. After several direct and reverse phase transitions some minimization of the free energy may ...

Evolution of the two-phase microstructure L12 + DO22 in near-eutectoid Ni3(Al,V) alloy

Transmission electron microscopy and powder x-ray diffraction methods have been used to investigate the evolution of two-phase (L12 + DO22) microstructures from the quenched fcc phase of the Ni-5Al-20V (at. %) alloy. The microstructure after annealing in a temperature range from 650 to 900 °C differs from the eutectoid structure which might be expected for the alloy according to the eutectoid-type phase diagram of the Ni3Al-Ni3V section. This structure results from fast kinetics of ordering in the fcc → L12 and fcc → DO22 phase transitions. Four main stages in the microstructural evolution were observed. Stage I is the formation of spheroidal coherent L12 clusters in a disordered fcc matrix. During stage II the L12 clusters transform into cuboidal precipitates, and the fcc matrix orders into three DO22 variants (which may have interfaces that are wetted by thin fcc layers). In stage III accommodation of misfit (elastic energy) between different phases and variants occurs by formation of (110) twins or a single variant of the DO22 phase and tetragonally strained lamellae of the L12 phase. Stage IV is a discontinuous coarsening process in which a coarse incoherent two-phase structure replaces the fine coherent one. Grains of the coarse structure are nucleated on high-angle boundaries of primary fcc or other surfaces. Many of the grains are found twinned.

Spinelloid in sintered YPSZNi2AlTi cermet

A spinel-derived phase (osp) found at the grain boundary of spinel and zirconia in sintered (1500 °C, 10 h) zirconia with 5mol.% Y 2 O 3 and 4.6 mol.% Ni 2 AlTi additives was characterized by transmission electron microscopy. The selected area diffraction patterns taken from a variant of the osp phase indicated that it can be indexed as an orthorhombic lattice with the lattice parameters a = 0.57 nm, b = 1.14 nm, c = 0.80 nm, or their integer multiples. this osp phase developed along the habit plane {110} of spinel, forming ledge-bearing and dislocation-bearing interfaces. The crystallographic relationships between osp and spinel (sp) are [100] osp ∥[110] so , [010] osp ∥[ 1 10] sp and [001] osp ∥[001] sp . Within the osp variant, anti-phase domain boundary was observed.

Displacive phase transformations in solids

We study diffusionless transformations in solids which involve a sudden change of shape at a certain temperature. We assume the existence of a free energy which depends on the local change of shape and the temperature. Properties of this function reflect the underlying symmetry of the parent and product phases and an exchange of stability from parent to product phase as the body is cooled through the transformation temperature θ 0. We concentrate on two questions: (i) How can loads be applied to cause the body to transform to a particular variant of the product phase at or above θ 0? (ii) Can the parent phase be recovered by applying some system of loads at or below θ 0? Theory and experiment are compared for thermoelastic martensitic transformations in shape-memory materials and for the α-β transformation in quartz.

Crystal structures of the α″-cerium phases

Endo et al. [Endo, S., Sasaki, H. & Mitsui, T. (1972) J. Phys. Soc. Jpn. 42, 882-885] have reported x-ray diffraction data for an unidentified phase of cerium metal observed in the 50- to 100-kbar range. It is shown that the unknown phase is a variant of the metastable monoclinic phase, α″-Ce, previously observed. The complete structure of the new form of α″-Ce is deduced.

The Stress-induced omega phase transformation in Ti-V alloys

The deformation behavior of bcc (β) Ti-V single crystals containing the athermal ω phase was studied using tensile tests, optical and electron microscopy and X-ray and electron diffraction. The stress-strain curves were characterized by a marked yield point and an essentially zero work hardening rate. The deformation structures consisted of wavy slip and stress-induced plates. Both X-ray and electron diffraction showed that the stress-induced plates consist of one variant of the omega phase, with the orientation relation to the parents phase close to (0001)ω ‖(001)ω; (•1120)ω ‖(110)β. The habit plane was of the type 554. A small amount of ά (hcp) phase was present within the ω plate, with orientation relation to the β phase close to the Burgers relationshipi.e. (0001)ά, ‖(110)β; [•1120]ά ‖[•111]β. It is suggested that the ω phase may be an intermediate structure in the stress-induced bcc → hep transformation.

THE SOMATIC ANTIGEN OF A PHAGE-RESISTANT VARIANT OF PHASE II SHIGELLA SONNEI

1. A phage-resistant variant of Phase II Shigella sonnei has been isolated and the type-specific antigen from it has been obtained. 2. The antigen of the variant microorganism, II/3, 4, 7, differs from that of the parent Phase II bacillus in serological specificity and in chemical make-up. 3. Although the antigen of Phase II Sh. sonnei inhibits all T phages to which the bacillus is susceptible, that of the variant fails to inhibit T(3), T(4), and T(7).

Traumatic Subdural Hematoma—Acute, Subacute and Chronic

I N the past it has been common practice to class all subdural hematomas as acute or chronic. Acuteness has been estimated on the basis of elapsed time from the injury, or on the degree of associated brain damage. According to one classification, all hematomas that cause death or come to operation before the ~lst day after a cranio-cerebral injury are described as acute (Kennedy and Wortisg). In another classification, cases of subdural hematoma that still show unhealed or acute brain injury are placed in the acute group, the others being classed as chronic (Munrol~). In still another classification, an acute subdural hematoma is regarded as the result of a severe head injury, usually with a fracture of the skull and more or less extensive brain laceration, whereas a chronic subdural hematoma is considered to be unassociated with severe brain injury and the history of trauma, as a rule, is slight or absent (Peet17). As Munro 15 has pointed out, any classification of subdural hematomas into groups is artificial, for as Munro, 13 Laudig, Browder and Watson, n and King TM have clearly demonstrated, the chronic form of t raumatic subdural hematoma, so well described in the literature 3'6' s.12.14.is.19.20 is nothing more than a later variant of the acute phase. Acute hematomas become chronic only gradually, and progressive symptoms appear as the hematoma enlarges by the acquisition of further fluid, usually through the process of osmosis? ''6'~' Therefore if any classification is to be used it should clearly indicate that there is a gradual transition between the so-called acute and chronic hematomas. The purpose of this paper is to combat the present tendency to class all hematomas as acute or chronic. Such a classification is misleading and fails to emphasize the existence of a large group of subdural hematomas which, neither on clinical nor pathological grounds, can be satisfactorily classed as acute or chronic. If this is not clearly recognized, the average observer may fail to diagnose the subdural hematoma that ends in a fatality about the end of the first week to several weeks after injury. In reviewing the cases of subdural hematoma to be presented, it is apparent that they fall roughly into three classes, on the basis of the clinical course, type of hematoma found at operation, and degree of associated brain injury; namely, acute, more or less subacute, and chronic. There is con-