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.     

Verformungsinduzierte Martensitbildung in metastabilen austenitischen Stählen

The volume of strain induced α-martensite in metastable steels depends on the stacking fault energy. If its value makes more than ∼100 ergs/cm², a direct γ → α transformation occurs, this being not or little quantitatively influenced because of the complicated shearing mechanism of lattice change by external stresses or strains. The tendency to form extended stacking faults or hexagonal ϵ-martensite increases to the degree as by adding of certain alloying elements, as e. g. Cr + Ni, Cr + Mn or Mn + C, the stacking fault energy is lowered. The indirect γ → ϵ → α or γ → SF → α-transformation occurs, where ϵ-martensite and stacking faults only gradually differ from eachother. Because of the simple shearing mechanism of the γ/ϵ- and γ/SF transformation external stresses are strongly favouring the indirect transformation. Especially in crossing regions of ϵ-plates or stacking faults, but also in their internal parts, shearings are generated which are prestages or nuclei of α-crystals. If the SFE is below ∼20 ergs/cm² a complete strain induced α-martensite formation becomes possible, if this is in accordance with the thermodynamic stability relations.     

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.     

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.     

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.     

Positron annihilation study of defects in martensite transformation of Fe?Mn alloy

It has been observed that a small number of repetitions of the transformation by thermal cycling enhanced the formation of ϵ-Martensite whereas a large number of repetitions of transformation decreased the amount of ϵ-Martensite formed. PAT analysis and electron microscope show that a few repetitions of the transformation produced less defects, including vacancies and dislocations, whereas a large number of repetitions of transformation produced progressively more defects. The PAT analysis also shows that these accumulated defetcs are mainly dislocations, which form a network of dislocations or a Lomer-Cottrell dislocation net. A small number of dislocations enhanced the nucleation of ϵ-Martensite whereas a large number of dislocations prevented the nucleation. So the dislocations-produced by cycling transformation thermally — has a great effect on the ϵ-formation.     

Dielectric studies on binary mixtures containing 4-trans-4′ -n-hexyl-cyclohexyl-isothiocyanato-benzene

We have studied binary mixtures with an induced smectic phase of 4-trans-4′ -n-hexylcycohexyl isothiocyanatobenzene (6CHBT) with 4-n-ethyl-4′-pentylazoxybenzene (EPAB) by means of dielectric, microscopic and refractometric measurements. The effective values of components p ⟂ and p∥ of the dipole moment per one molecule of the mixture have been calculated from values of ϵ ⟂ and ϵ ∥. As a result of our studies, we have established that the heterocomplexes formed in the mixture are so weak that they do not cause any essential changes in the distribution of molecule charges. The observed deviations from additivity of the dielectric constant of the mixture are caused solely by a non-additive behavior of the mixture density.     

Formation of a blocky ferrite in Fe–Cr–Ni alloy during directional solidification

Formation and evolution details of a blocky microstructure in AISI 304 stainless steel are studied by quenching method during directional solidification. Results show that a coupled growth microstructure, consisting of lathy ferrite and austenite, forms first from the melt. At solid-state transformation stage, most lathy ferrite disappears due to the phase transformation from ferrite to austenite. With further decreasing of the temperature, plenty of fine ferrite colonies occur in the original austenite region. The formation of the blocky ferrite indicates that reverse solid-state transformation from austenite to ferrite takes place. This transformation is due to the segregation and the instability of austenite during the growth of austenite under low cooling rate. The fine ferrite colonies transform into blocky ferrite at room temperature. TEM and EDS analyses were carried out to identify the phases and determine the phase composition, respectively.     

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.     

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.     

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.     

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.     

On the formation and technical importance of the microsegregation in austenitic chromium-nickel alloyed steels

The austenitic chromium nickel alloyed steels are showing different types of concentration-distributions of the elements chromium and nickel in the austenite in dependance on the content of alloys. As a consequence of the crystallization of the heat to austenite the “equal directed” type of segregation forms. The ferritic solidification of a heat and the following ferrite to austenite transformation leads to “unequal directed” segregation in the austenite. Heats having both the types of segregation solidify by way a 3-phase reaction. For heats ferritically solidified better hot plasticity, lower susceptibility to hot cracking in the weld and better polishability is observed compared with heats solidified austenitically. The advantages are accounted to the lower separation during the ferritic solidification as consequence of the higher diffusivity of the alloying and accompanying elements in the ferrite compared with the austenite.     

Thermodynamische Betrachtungen zur Nitridbildung auf Eisen bei Gasnitrieren

The instructions about the calculation of the thermodynamic conditions for the formation of iron-nitrogen-phases (nitrided α-iron, γ′-nitrides, ϵ-nitrides) are compiled. The results calculated according to these instructions are in good correspondence with the experimental results stated in the technical publications. It is pointed out that there is a want of knowledge on the relation of equilibrium between the nitrogen content of ϵ-nitride and the conditions of nitriding (temperature, pressure, composition of the gas). Resulting from the experimental investigations a valid relation is set up for the nitrogen content of ϵ-nitride which adjusts at equilibrium with the gas phase in the range of temperatures from 550 to 570°C. By means of X-ray tests the dependence of the hexagonal ϵ-nitride lattice constant relation on the content of dissolved nitrogen is mathematically determined.     

F.C.C. Rolling Texture Transitions in Relation to Constraint Relaxation

A complete set of f.c.c. rolling texture predictions, obtained with the use of a relaxed constraint model, is presented in this paper. The whole spectrum of textures between the copper and brass types can be found if some components of reaction stresses (between grains and the matrix) are relaxed. The best prediction of the copper type texture is found if ϵ₁₃ is a free parameter and the other components of grain deformation tensor are controlled by reaction stresses. The best prediction of the brass type texture is obtained if ϵ₁₂, ϵ₁₃ and ϵ₂₃ are free parameters. However, a decisive factor for the texture transition from the copper to the brass type is the relaxation of the ϵ₁₂ component. We present for all the considered cases the orientation distribution functions (O.D.F.) and also the pole figures for some selected cases. Nous présentons pour tous les cas les fonctions de distribution d'orientations cristallines ainsi que dans certains cas — les figures de pǒles.     

Far Infrared and Raman Optical Study of CdInGaS4, CdIn2S4, HgInGaS4 and CdIn2S2Se2 Crystals

Raman and infrared reflectivity spectra of CdInGaS₄, CdIn₂G₄, HgInGaS₄, and CdIn₂S₂Se₂ crystals have been investigated. The fiindamental phonon parameters, the limiting dielectric constants ϵ₀ and ϵ∞ and the reflectivity spectrum contours have been calculated using classical dispersion relations.     

Electrical Characterization of Irradiation plus Annealing – induced VAsZnGa Pairs in p-type GaAs (Zn) Crystals

Effect of fast electron irradiation (E =2.2 Mev, ϕ_c = 1 × 10¹⁶ el/cm²) and subsequent annealings (T = 150 to 350 °C, t = 10 to 600 min) of zinc-doped p-type GaAs crystals on the formation and dissociation of V_AsZn_Ga, pairs is studied. An analysis of the formation and dissociation kinetics of V_AsZn_Ga pairs permitted to find the diffusion coefficient of radiation-induced arsenic vacancies D(D = 1.5 × 10⁻¹⁸, 1 × 10⁻¹⁷ and 5 × 10⁻¹⁷ cm²/s at 150, 175 and 200 °C accordingly), their migration energy ϵ_m(ϵ_m = 1.1 eV), the binding energy of V_AsZn_Ga, pairs ϵ_b(ϵ_b = 0.5 eV), and also their dissociation energy ϵ_d(ϵ_d = 1.6 eV).     

Korrespondenzen bei Scherumwandlungen in Stählen

Shear transformations in iron and steels (twinning and martensitic transformations) take a leading part at the plastic deformation and the different heat treatments. They are characterized by a military motion, and between the original and the product phase there are lattice orientation relationships. By the acceptance of a pattern the atomic motions may be described by a matrix, and from this the correspondence matrix can be calculated. The correspondence matrices enable the changes of indices of directions and planes during the lattice transformation. In the work the required matrices are stated for the twinning of α-Fe and γ-Fe crystals, the martensitic transformations f.c.c. to h.c.p., h.c.p. to b.c.c., f.c.c. to h.c.p. to b.c.c., and f.c.c. to b.c.c. (BAIN -, NISHIYAMA -, KURDJUMOV -SACHS -mechanism), and the possible variants of orientation are described.     

Dielectric constant,loss factor and ac conductivity of Ni2+‐doped K2ZnCl4 crystals in the ferroelectric‐commensurate,incommensurate and normal phases

The dielectric constant (ϵ), dielectric loss (tanδ) and ac conductivity (σ_ac) of virgin and thermally cycled undoped and Ni²⁺‐doped K₂ZnCl₄ (KZC) crystals in the ferroelectric‐commensurate (FC), incommensurate (IC) and normal phases (N) have been studied. Anomalous behaviour at the two‐phase transition points was observed while measuring ϵ along the polar a‐axis for the undoped sample. With increasing Ni²⁺concentration a systematic shift of the phase transition temperature towards lower values and a continuous inhibition of the peak height were detected. ϵ changed linearly with lnf up to f =10⁵ Hz. tanδ along the a‐axis declared the phase transitions by peak changes. After Ni²⁺‐doping this behaviour was preserved at the FC‐IC phase transition point while the IC‐N phase transition was manifested by a change in the slope of the straight line representing the tanδ‐T dependence. The ac conductivity changed lineally with frequency according to a relation of the form σ_ac = σ_o f ^β where 0>β>1.9. σ_ac increased monotonically with increasing temperature and doping concentration in the low‐temperature phases tending to merge in one straight‐line with high activation energy that might be due to superionic dc conduction in the high temperature N‐phase. Doping with Ni²⁺ pinned stripples, decreased the soliton‐soliton interaction, weakened discommensuration effects, shortened the IC‐phase and strongly affected the ‘chaotic' behaviour at the T_C‐IC. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Influence of Surface Modification upon the Angular Distribution of Slip Traces on the Surface of Polycrystalline F.C.C. Metals

Surface modification by ion implantation leads to a smaller portion of grains of a polycrystalline metal showing slip traces as a result of cyclical deformation. The analysis of the azimuthal distribution of the slip traces gives an approach to explain this experience in more detail. Analytical and numerical calculation of the azimuthal distribution of slip traces under consideration of the slip geometry relative to the free surface leads to azimuthal distributions as experimentally determined for polycrystalline nickel after implantation with N⁺-ions and following cyclic fatigue with constant amplitude of plastic strain ϵ_pa = 10⁻⁴.