Martensite Crystals Research Articles

Dynamic theory of possible morphological characteristics of austenite nanocrystals formed upon the α–ε–γ transformation in Fe–Ni alloys via deformation and shuffling of {110}α planes

In dynamic theory, one of the possible variants of the formation of martensite crystals upon the reverse α–γ transformation in Fe–Ni alloys connected with the participation of the intermediate e phase is discussed. The mechanism of planar tension–compression deformation of {110}α planes is considered. A set of characteristic morphological features for crystals of the γ phase has been found, which indicates the participation of the e phase in their formation and includes the material orientation relationships close to the ideal Kurdjumov–Sachs orientation relationships.

The exploration of the effect of microstructure on crackling noise systems

A wide variety of physical systems respond to changing external conditions through discrete impulsive events called jerks, typically leading to collective “crackling noise” behaviour. Statistical distributions of jerky events often exhibit a universal scale-invariant power law, regardless of the specific mechanisms that are responsible for crackling noise processes and microstructural features that affect them. Here, we analyse uniaxial compression loading curves of two different physical systems that exhibit jerky behaviour: a martensitic NiMnGa single crystal and a stack of corrugated fiberboards. The jerky response is attributed to a non-uniform twin boundary motion along the NiMnGa crystal and to a local buckling of individual fiberboard layers. In both cases, our analysis reveals that different variables exhibit different statistical distributions. While the velocity of temporal processes within jerky events exhibits scale invariant distribution, the irreversible displacements induced throughout complete events are distributed around a characteristic value. In the case of NiMnGa, the displacement of a twin boundary is directly related to the length-scale of the internal magneto-mechanical microstructure. Similarly, the displacement of the fiberboard stack corresponds to the thickness of individual board layers. These observations reveal the effect of the internal microstructure on crackling noise systems and demonstrate an analysis approach for uncovering the details of the jerk mechanism.

Morphological features in B2→B19 martensitic transformation and their description based on the concept of controlling wave process

The paper analyzes the possibility of describing morphological features in the B2→B19 transformation in the framework of a dynamic theory based on the concepts of heterogeneous nucleation and wave growth of martensite crystals. It was shown, using Ti-Ni-Cu alloys as an example, that the predicted and observed orientations of habit planes agree well. The issue that strain ratios remain valid in going from threshold to final strains is considered, and calculation results for orientation relations and their fulfilment conditions are discussed.

Possible Wave Processes Controlling the Growth of Martensite Crystals at B2-B19, B2-B19' and B2-R Transformations

Basic directions in the theory of martensitic transformations are briefly listed. Within the framework of the dynamic theory based on the synthesis of concepts of heterogeneous nucleation and wave growth of martensite crystals, the possibilities of description of morphological parameters during the В2→B19, В2→B19′, В2→R transformations are analyzed. It is demonstrated that the calculated and observed habit planes and orientation relationships can be matched.

Pseudoelasticity effect in amorphous—crystalline Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy

The pseudoelasticity effect was studied in amorphous–crystalline Ti40.7Hf9.5Ni44.8Cu5 shape-memory alloy with different volume fractions of the crystalline phase. Pseudoelastic behaviour was found in the samples with a volume fraction of the crystalline phase of 60% and above. An increase in the volume fraction of the crystalline phase led to a rise in the strain accumulated due to stress-induced martensite, an increase in the mechanical hysteresis and a decrease in the stress needed for the appearance of the stress-induced martensite. It was found that the dependences of the pseudoelasticity effect parameters on the volume fraction of the crystalline phase were due to the influence of the surface energy on the formation of martensite crystals with preferable orientation. An increase in the volume fraction of the crystalline phase led to an increase in the grain sizes and, as a result, a decrease in the surface energy. It provided an increase in the number of preferably oriented martensite variants appearing in the grain under load.

Microstructural, thermal stability and wear resistance properties of X153CrMoV12 tool steel subjected to deep cryogenic treatment

This paper analyses the cause of the exothermic effect occurring in the heating cycle of X153CrMoV12 tool steel samples subjected to deep cryogenic treatment (DCT) process. The differential scanning calorimetry (DSC) studies showed the presence of energetic effect with a maximum at a temperature of about 218 K, that is, the temperature at which the occurring phenomena are associated with the presence of the carbon atoms. The energetic effect occurring at this temperature was more intense in case of samples subjected to the DCT. Observations of the structure of martensite showed that the DCT promotes refinement of martensite sub–grains, which is connected with the state of stresses in martensite crystals produced by sub–zero treatment. Above–mentioned changes in the structure of the material matrix and the way of precipitation of the strengthening phases in deep cryogenically treated X153CrMoV12 tool steel found their reflection in the improved resistance to wear.

Crystons: basic ideas and applications

A brief review of the cryston concept and its applications is given. By definition, crystons are shear carriers arising in the contact interaction of dislocations with intersecting slip planes. The notion of standard orientations of habits (SOH) is introduced for shear bands. On the example of fcc crystals with SOH of the {hhℓ}-type, the conditions for generation and propagation of crystons, as well as for a discrete change of habit orientations, are considered. The reasons for deviations from SOH are indicated. With regard to the formation of martensite crystals, сrystons can act as dislocation nucleation centers, carriers of threshold deformation and relaxation shear. The areas of promising investigations are mentioned.

Formation of Packet (Lath) Martensite in Iron-Nickel Alloys

The kinetics of isothermal formation of packet martensite in Fe – Ni alloys is considered. Atheory describing the formation of a packet of martensite crystals is presented as an autocatalytic process initiated by formation of the first laths of a future packet and of blocks in it. The theory predicts progressive change of the “instantaneous” exponent of power in the Avrami equation from 2 or 3 to 1, which has been observed indeed for alloy Fe – 14.95% Ni. The kinetics of incomplete phase transformations is described within the concept that the reaction is stopped due to decrease in the thermodynamic stimulus of the transformation caused by accumulation of defects in the initial phase. An equation describing the C-curve of the transformation is obtained.

Effect of heat treatment and plastic deformation on the structure and the mechanical properties of nitrogen-bearing 04N9Kh2A steel

The effect of the conditions of heat treatment and plastic deformation on the structure and the mechanical properties of low-carbon martensitic nickel steel (9 wt % Ni) with an overequilibrium nitrogen content is studied. The limiting strain to failure of 04N9Kh2A steel is found to be 40% at a rolling temperature of 20°C and 80% at a rolling temperature of 900°C. Significant strengthening of the steel (σ0.2 = 1089 MPa) is obtained after rolling at a reduction of 40% at 20°C. The start and final temperatures of the α → γ transformation on heating and those of the γ → α transformation on cooling are determined by dilatometry. The specific features of the formation of the steel structure have been revealed as functions of the annealing and tempering temperatures. Electron-microscopic studies show that, after quenching from 850°C and tempering at 600°C for 1 h, the structure contains packet martensite with thin interlayers of retained austenite between martensite crystals. The strength of the nitrogen-bearing 04N9Kh2A steel after quenching from 850 and 900°C, cooling in water, and subsequent tempering at 500°C for 1 h is significantly higher than that of carboncontaining 0H9 steel used in cryogenic engineering.

Dynamic Interpretation of Formation of Parallel Thin-Plate Martensite Crystals in Strong Magnetic Fields

Ensembles of crystals of thin-plate martensite initiated by a strong magnetic field are analyzed within the concepts of the dynamic theory. It is shown that the laws detected agree with a specific dislocation scheme of arrangement of austenite subgrains with orientation {110}γ.

The structure of self-accommodation complexes of martensite crystals in alloys with the shape memory effect

The internal structure of self-accommodation complexes defined by employed twinning planes of martensite crystals is discussed. A method for the analysis of the possible structure of self-accommodation complexes depending on the symmetry of martensite crystals and the orientation relationships between the lattices of austenite and martensite is proposed.

The role of martensitic transformation in corrosion fatigue failure of Ti-22Nb-6Ta and Ti-22Nb-6Zr (at %) medical alloys

The effect of cyclic dynamic loading on the kinetics of changes in the electrode potentials of Ti-22Nb-6Ta and Ti-22Nb-6Zr (at %) superelastic medical alloys in a model solution simulating the bone tissue environment in a living organism is studied with the use of electrochemical chronopotentiometry. An increase in the applied load from 450 to 780 MPa is found to affect dramatically the character of changes in the electrode potentials of the alloys during exposure. Ti-22Nb-6Ta alloy exhibits higher resistance to corrosion fatigue cracking than does Ti-22Nb-6Zr alloy. It is shown that the superelasticity of the alloys is responsible for the deceleration of the corrosion fatigue cracking due to the blocking effect of martensite crystals formed in the structure during the loading half-cycle, which is confirmed by the low rate of the potential decrease after the nucleation of corrosion fatigue cracks.

Regulation of Cu precipitation by intercritical tempering in a HSLA steel

Abstract

Variation in kinetics of martensitic transformation during partial thermal cycling of the TiNi alloy

Abstract A method of differential scanning calorimetry has been used for the study of variation in the kinetics of thermal elastic martensitic transformation during the thermal cycling of the equiatomic TiNi alloy through the different fractions of forward or reverse martensitic transformation temperature ranges. The results of the study have shown that “partial cooling” cycling results in damage to the crystalline structure in “transforming” and “austenite” volumes independently of whether or not “austenite” volumes undergo martensitic transformation during partial cycling. The structure damage in “martensite” volumes does not occur during “partial heating” cycling. It is caused by the fact that internal stresses relax through plastic strain in the austenite phase, resulting in an increase in defect density or the reorientation of martensite crystals in the martensite phase without the accumulation of defects.

THE EFFECT OF SIZE STABILIZATION OF CARBON STEELS AUSTENITE

The researches into morphology and faulted substructure of martensite of carbon steels which undergone static (furnace) and dynamic (high-current electron beam) thermal treatment have been conducted using the methods of diffractional electron microscopy. It has been shown that a rapid quenching from a liquid state accompanied by the formation of grain ensemble of submicronsize (0.1–0.5 μm) leads to first – reduction of size of martensite – phase crystals, second – the fixation of surface martensite effect, third – the formation of one ∼0.3 μm-size martensite crystal (grain – martensite crystal) in grains (subgrains).

Правила отбора дислокационных центров зарождения мартенсита в динамической теории реконструктивных мартенситных превращений

In the light of the new paradigm provides a brief overview of the works, reflecting mainly the evolution of the dynamical theory of martensitic transformations applied to the selection rules of dislocation nucleation centers of martensitic crystals. It is noted that the inclusion of quasi-longitudinal waves alters the balance between the ratio of the speeds of the waves and deformations.

Formation of martensite crystals in the limiting case of a supersonic growth rate

Formation of martensite crystals in the limiting case of a supersonic growth rate

Formation of martensite in austenitic steel upon loading by quasi-spherical converging shock waves

Methods of magnetic powder patterns (Bitter technique), optical microscopy, and transmission electron microscopy have been used to study the formation of martensite in a spherical sample of 12Kh18N10T steel upon loading by quasi-spherical converging shock waves. Prior to loading, the steel exhibited carbide banding (TiC and Cr23C6 carbides were located in bands along the axis of the initial sample). It has been shown that, upon loading, disperse crystals of martensite are formed in these carbide-containing bands. No martensite was formed in the central part of the solid sphere, in the surface layers, and in the Altshuler-pattern rays. It has been concluded that, in these regions, the temperature exceeded the M d temperature, above which the deformation does not cause a martensitic transformation.

Vibrational properties of Ni–Mn–Ga shape memory alloy in the martensite phases

Studying the phonon dispersion of the ferromagnetic shape memory alloy system Ni–Mn–Ga gives insight into the mechanism of the martensite transition and the forces driving the transition. Transformation of austenite single crystals under uniaxial stress results in the coexistence of two martensitic variants with perpendicular modulation vector. Here we report on inelastic neutron scattering studies of martensite crystals with off-stoichiometric compositions, varying from non-modulated (NM) to five- (5M) and seven- (7M) layer modulated martensite phases. Both the 5M and 7M crystals show fully commensurate satellite peaks along [0], corresponding to the five- and seven-layer modulation. These superstructure peaks become Γ-points of the modulated structure. Due to the coexistence of two variants within the (001) plane, both new acoustic phonons reflecting the modulation vector [0] and acoustic TA2[ξξ0] phonons corresponding to the non-modulated direction are observed. The latter display a pronounced softening around ξ = 0.2–0.4 when approaching the martensite–austenite transition from above and below, i.e. this soft mode has lowest frequency at the transition temperature. Overall the phonon dispersion of the austenite and martensite phase resemble each other very much. The coexistence of two martensitic variants after uniaxial transformation explains the particular behaviour of the low-energy excitations, in contrast to previous interpretations involving charge-density waves and associated phason modes.

On the Applicability of the Concept of Control Wave Process to the B 2→R Martensitic Transformation in B 2 Titanium Nickelide Alloys

Within the framework of the dynamic theory based on the synthesis of concepts of heterogeneous nucleation and wave growth of martensite crystals, the possibility of description of morphological parameters during the В2→R transformation is analyzed. It is demonstrated that the calculated and observed orientations of the habit planes can be matched. The calculated orientation relationships are given and the results obtained are discussed.