Einfluß der Stapelfehlerenergie auf den kristallographischen Umgitterungsmechanismus der γ/α-Umwandlung in hochlegierten Stählen

The mechanism of lattice change in the phase transformation fcc γ → bcc α depends on the stacking fault energy (sfe). In the case of large sfe a translation of (111)γ planes by Shockley half-dislocations of b = 1/6 [211]γ type is not possible, and the direct lattice change γ → α happens by two homogeneous shearings by 19°28′ or 10°32′, respectively. In the case of an sfe below 15 — 20 ergs/cm² this gliding occurs, and the ϵ-phase forms. Dependent on the thermodynamic stability the ϵ → α transformation can take place by a further shearing. The transformation γ → α preferably is caused by external elastic stresses, the γ → ϵ → α transformation likewise, but also by plastic deformation on the base of slidable half-dislocations.     

Orientierung der α-Martensitkristalle beim K.-S.-Zusammenhang

During the quenching of steel the fcc γ-phase transforms without diffusion to bcc α-martensite. Accurate relations of orientation exist between the two different lattices. At the KURDJUMOW -SACHS relation 24 α-crystals with different positions can originate from one γ-crystal because of the cubic symmetry. These 24 K.-S.-variants as well as the change of directions during the γ → α transformation have been calculated by means of matrices.     

Umwandlungsplastizität und Form-Gedächtnis-Effekt in einer Eisenlegierung mit 16% Mangan

During the quenching of wire shaped specimens of steel X8Mn16 fec austenite changes under reduction of length in hcp ϵ-martensite. If the transformation takes place under the influence of an external tensile stress an elongation results. This transformation plasticity consists of a reversible and an irreversible part. The reversibel part rises by the formation of stress-induced ϵ-martensite in preferred orientation and is built back under reduction of length during the following heating over the Af-temperature. By this it is verified that the strain-memory-effect can occur in iron alloys too. A plastic deformation of the austenite below the recristallisation temperature prevents the γ → ϵ-transformation.     

Bildung von α-Martensit im Schnittbereich von ϵ-Martensitplatten

In steels the nucleation of crystals of α martensite from austenite is facilitated in the presence of ϵ martensite. Particularly within the range of crossing platelets of ϵ martensite with 〈110〉_γ direction α martensite is formed easily. That has been demonstrated by metallography in alloys with suitable composition and pretreatment. Also within the range of crossing of other shear products, as stacking faults or twins, the atomic arrangement for nuclei of α martensite is favourable. By the aid of matrices the strain in crossing 2 ϵ martensite plates, ϵ martensite plate and twin, and 2 twins resp., has been calculated and the result compared with the strain during the martensitic γ → α transformation.     

Zur Kristallographie der martensitischen ε → α-Gitterumwandlung

Many iron and cobalt alloys transform to the martensitic reaction γ → ε → α. The platelets of the hex. ε-martensite are very thin. Thus the growth of the crystals is limited in the direction [0001]_ε and the transformation ε → α can be treated as a plane problem. Metallographic investigations demonstrated the size, form and orientation of the α-crystals after cooling without and with external tension. On the basis of connection of the hex. and the bcc. lattice formulae are discussed of the directions of the lattices, and the changes of the volume and the shape, resp.     

In-situ Observations on Cu-SiO2 and Stainless Steels

Specimens of Cu SiO₂ and of several high purity stainless steels have been observed during deformation in an HVEM. The behaviour of the Cu SiO₂ specimens is dominated by the free surfaces, even in the thickest regions which can be examined, but in the stainless steels it has been found that if the specimen is thicker than about 0.5 μm, bulk behaviour is observed. The formation of ϵ- and α-martensite has been directly observed and essentially identical observations of martensite formation have been made during cooling. It has been shown that ϵ-martensite is always formed in regions where appropriately, but usually irregularly spaced faults, are generated and detailed analyses of isolated and overlapping faults show that these faults contain close-packed planes of the appropriate eph spacing. α-martensite is formed in association with dislocation pile-ups.     

Crystallography of the Martensitic transformation in Indium-Thallium Alloys

On cooling a single crystal of Indium containing 20.75 atomic per cent Thallium may transform martensitically by the migration of a single interface, covering the whole cross-section, from the high temperature face-centred cubic β-structure to the facecentred tetragonal α-structure. Applied external mechanical stress is able to reverse the direction of interface motion during transformation. The α-martensite consists of a stack of fine parallel twins which are suficiently wide to be visible under an optical microscope. There are no longrange stresses, provided the twins have mean thickness ratio 1.6:1. By extension or compression the whole specimen may undergo a conversion into one or the other orientation by movement of the twin boundaries, leading to ferroplasticity or ferroelasticity. The geometry and crystallography of and the effects of external stresses on the β → α transformation have been calculated on the basis of the phenomenological crystallographic martensite theory.     

X-ray investigation of the mechanism of phase transformation in single crystals of ZnS,ZnxCd1−xS and ZnxMn1−xS (I). Calculation of diffraction effects by a three parameter model

Hexagonal close-packed (2H) single crystals of ZnS, Zn_xCd_1−xS and Zn_xMn_1−xS are known to undergo solid state transformation to the cubic close-packed (3C) and 6H-structures on annealing at elevated temperatures. The transformations occur by the non-random nucleation of stacking faults on individual close-packed layers parallel to (0001). The nature of the faults involved and their probability distribution during transformation determine the diffraction effects produced along the 10. L reciprocal lattice row by a crystal quenched in an intermediate state of transformation. We have investigated the mechanism of the transformation by comparing the diffraction effects recorded from such crystals on a single crystal diffractometer, with those calculated for an assumed model of the transformation. It is known that in these materials the faults involved in the transformation are deformation faults. To explain the observed diffraction effects we develop a three parameter theoretical model employing a fult probability α for the radom insertion of a deformation fault in the 2H structure, a fault probability β for the deformation faults to occur at three layer separations and a fault probability γ of their occcurrence at 2-layer separations. The probability α corresponds to the development of a fresh nucleus, the probability β to the growth of the 6H nucleus and the probability γ to the growth of a 3C nucleus. This paper develops the necessary theory of X-ray scattering for such a model of the transformation and predicts the diffraction effects for different values of α, β, and γ. The next paper compares these results with experimental observations.     

Anisotrope Längenänderungen bei spannungs- und verformungsinduzierter martensitischer Gitterumwandlung kubisch-flächenzentriert → hexagonal

With the elastic and plastic deformation of the steel X8Mn16, respectively, a stress or deformation induced transformation of the fcc γ-matrix lattice into the hex. ϵ-martensite occurs, being connected with anisotropic changes in length, and leading to a decrease of the apparent elastic modulus of the alloy. A further investigation shows it is obviously the question of the result of the strain-memory effect, until now not yet being observed with iron alloys. Similar anisotropic changes in length can be taken into account with all pure metals and alloys, the martensitic transformation of which turns out under the influence of an outer uniaxial stress.     

On the existence of declared 9<Emphasis Type="Italic">R</Emphasis> phase in Fe–Ni invar alloy

An analysis of recently reported electron diffraction patterns suggests that metastable austenitic Fe–32Ni alloy subjected to α → γ transformation upon slow heating does not exhibit any signs of formation of the 9R phase; the conventional nanocrystalline γ phase with an fcc lattice is formed instead. Extended lamellae with a layered structure, erroneously identified as a new phase of the (3R + 9R) type in Fe–32Ni alloy, are conventional twinning (midrib) regions of each initial α crystal, in which γ-phase twin nanolamellae are formed upon heating.     

Mechanisms of relaxation of internal elastic stress arising during the reverse α → γ transformation upon slow heating of Fe–32 at % Ni alloy

The evolution of structural-phase states during the α → γ transformation upon slow heating of metastable Fe–32 at % Ni alloy has been studied. Experimental evidences in favor of the mechanism of dynamic γ → α → γ transformations as plastic relaxation of internal elastic stress, providing lattice reorientation and leading to the formation of a fine-grained structure, are found.     

Structural aspects of thermal instability of polytypes in the Co?C system

The stability of the irregular long-period structures being formed in Co C alloys during β → α transformation has been studied according to the carbon content. In the concentration range 0.2 to 1.2 at.% C the type of the short-range order concerning the atomic layer stacking sequence remains unaffected on subsequent cooling of the alloy, whereas the α-phase crystalline structure of the alloys with the carbon content of 1.3 to 2.17% C suffers polytype transformation. The transition from one polytype structure to another is found to be due to the decrease in length of the cubic-type stacking sequences.     

Stacking faults in deformed zinc and magnesium single crystals

Using the transmission electron microscopy dissociated dislocations containing stacking faults were observed in Mg single crystals, deformed in (0001) <1120> slip system, and in Zn single crystals deformed in {1122} <1123> slip system. The overlapping flat stacking faults lying in {1102} planes and dissociated triple nodes in (0001) planes are observed in Mg. In Zn flat stacking faults are found in {112 2} planes. The value of stacking fault energy γ has been determined in Mg crystals (γ ⋍ 10 erg/cm²). Such a low value of γ is attributed to the segregation of impurities on dislocations.     

The effect of impurities on the β → α transformation process in tin

This article presents data on the impurities influencing the β → α transformation of high purity tin (99.9995%): temperature of the α → β transition-T₀, nucleation and α-crystal growth.     

Investigation of structural transformations in Cu2Se

Structural transformations in Cu₂Se have been studied by the method of hightemperature roentgenography. From a diffraction record of reflection angles and from a Debye-powder pattern the unit-cell parameters of a rhombic α-modification have been calculated at temperatures of 25, 50, 100, and 132 °C. Using the diffraction line (027) disappearing under the α → β transformation and appearing again in the course of the inverse β → α transformation it has been found that the equilibrium temperature between the α- and β-modifications is T₀ = 134 ± 0.5 °C. The unit cell parameter of a hightemperature face-centered cubic β-modification at temperatures of 150, 200, 250, and 300 °C also has been determined. In the course of mutual transformations a rigid orientational linkage has been found to exist between the α- and β-modifications. It has been discovered that when the multiplicity of α → β and β → α transformations n reaches a value of approximately 30, a β-Cu₂Se → Cu₂–_xSe transformation (where x = 0.20) takes place which is not followed by an inverse transformation.     

Gel growth of α and γ glycine and their characterization

The microbial free single crystals of α and γ glycine were grown from gel at room temperature in a new chemical route. These crystals showed a superior quality than the solution grown crystals. The metastable α-form and the stable γ-form of glycine were crystallized in silica gel by solubility reduction method. The form of crystallization is confirmed by single crystal and powder X-ray diffraction analyses. The crystals of α and γ glycine were found to crystallize in monoclinic and hexagonal crystal systems, respectively. For analyzing the functional group and thermal stability of α and γ glycine crystals, spectroscopic and thermal analyses have been carried out. The dielectric studies were performed to find the dielectric constant of the grown crystals and the results are discussed. Second harmonic generation efficiency of the crystal was measured by Kurtz’s powder method using Nd:YAG laser and it was found to be 2.68 times that of potassium dihydrogen phosphate crystals.     

Structural mechanism of α → γ-transformation in Fe-32%Ni alloy

An intermediate 9R paramagnetic phase in the form of large extended plates has been found for the first time in metastable Fe-32%Ni alloy. This phase is formed during slow heating at a rate of 0.3°C/min in the middle of the temperature range of α → γ transformation. The structure of the new 9R phase has been established using electron diffraction.     

On the Transition between Cholesteric and Nematic Phases in Magnetic Field

Singularities at the critical lines bounding the cholesteric and nematic phases on the magnetic field versus temperature plane are investigated. Critical exponents, α and γ, of the specific heat and magnetic susceptibility are found both unities. By regarding the inverse of pitch of the cholesteric structure as an order parameter, it is found that the exponents β and δ are zero and infinity, respectively, and these exponents satisfy the scaling relations, α + 2β + γ = 2 and α + β (δ + 1) = 2. In relation to the discommensurate charge density wave in layer structure materials, it is noted that the transition from the nematic to cholesteric phase occurring with the change of magnetic field and/or temperature is interpreted as a condensation process of solitons.     

Umwandlungsplastizität von Kobalt-Mangan-Legierungen

On quenching Co Mn alloys (4-16.4% Mn) the fcc γ-high temperature phase – while decreasing in volume and length martensiticallytransforms to the hexagonal ε-low temperature phase, the Ms, As, and Af temperatures simultaneously being lowered with increasing Mn contents. If the transformation happens influenced by an external tensile stress transformation plasticity occurs. It is based mainly on a martensite precipitation oriented along the direction of tension and is connected with the final lengthenings after passing a transformation cycle as well as with a decrease of the flow limit and of the elastic modulus. Thus Co Mn alloys behave quite similar as Fe Mn and Fe Cr Ni alloys with a γ/ε transition.     

Crystal structure of [Mn(bpy)3](ClO4)2·0.5(bpy) (bpy=2,2′-bipyridine)

A new monomeric manganese(II) complex with 2,2′-bipyridine (bpy), [Mn(bpy)₃-] (ClO₄)₂·0.5(bpy), has been prepared and characterized by X-ray crystallography. The complex crystallizes in the triclinic space group $P\bar 1$ witha=9.535(2),b=13.194(3),c=14.854(3) Å, α=96.50(3), β=107.26(3), γ=91.19(3)°,V=1770.2(7) ų, andZ=2. The structure comprises discrete [Mn(bpy)₃]²⁺ cations in which the metal atom is coordinated in a highly distorted octahedral environment by three chelate bpy ligands [Mn?N=2.229(3)–2.289(2) Å]. The solvate bpy molecule and a pair of coordinated bpy ligands each from the adjacent cations are arranged in an off-set fashion, showing significant intermolecular stacking interaction with close interplanar contacts ofca. 3.47 Å.