Strain behavior of Fe—Ni—C martensite

This article examines the effect of carbon on the structure of slip traces and characteristics of the deformation resistance of quenched iron-nickel-carbon alloys deformed at 77°K. It is established that an increase in the carbon concentration in the martensite within the range 0.03–0.30 wt. % is accompanied by the formation of channels of easy slip during low-temperature deformation, which explains the simultaneously observed increase in the plastic strain of the alloys. Localization of slip in these channels leads to a reduction in the nonconservative component of resistance to deformation.Translated from Izvestiya Vysshikh Uchehnykh Zavedenii, Fizika, No. 6, pp, 33–38, June, 1985.     

Reply to ‘On substructure in titanium alloy martensite’

Thermal stability of nanocrystalline Cu prepared by compacting nanoparticles (mean grain size about 50?nm) under high pressure has been studied by means of positron annihilation lifetime spectroscopy and X-ray diffraction. A gradual increase of mean grain size in the sample is observed with an increase in ageing time at 180°C, indicating an increase of volume fraction of the ordered regions. Furthermore, during the ageing, the increase in average size of the vacancy clusters in grain boundaries is confirmed by the positron lifetime results. The recrystallization is observed at the temperature of about 180°C, and becomes significant above 650°C. Three annealing stages, which are at the intervals 180–400°C, 400–650°C and 650–900°C have been characterized by positron average lifetime. The average volume of the defects almost remains constant in the interval 400–650°C but becomes considerably smaller in the interval 650–900°C.     

Microstructure and properties of plastic deformed martensite induced by laser shock processing

Firstly, 45# steel was quenched by the NEL-2500A rapidly axial flow CO2 laser. The experimental parameters were the laser power of 750 W, the laser beam diameter of 4 mm, the scanning velocity of 7 mm/s. The thickness of coating layer was 0.1 mm and the width was 8 mm. Secondly, the martensite induced by laser quench was shocked by Nd:YAG laser. The parameters of laser shock processing were the wavelength of 1.06 μm, the pulse duration of 23 ns, and the output energy of 16-20 J. The laser was focused on a spot of φ7 mm. K9 optical glass was used as confinement. The sample was coated with black paint 86-1 (the thickness is about 0.025 mm). By testing and analysis of samples which were treated by laser quench and laser quench+shock with transmission electron microscope (TEM), it was discovered that the surface layer of martensite was deformed plastically by laser shock processing. In the secondary hardened zones, there were a lot of slender secondary twin crystal martensites, dislocation tangles, and cel     

Dislocation centers in α-martensite formation and dual couplings of thinly-laminated martensite crystals

The spectrum of observable dual couplings of crystals of thinly-laminated martensite with habiti {3 15 10} is discussed within the framework of the concept of the dislocation formation of -martensite crystals. A spectrum of dislocation centers of formation arising together with the developing martensite crystal is proposed. Here it is essential to divide the arising dislocation centers of formation into two groups associated with different displacement mechanisms. This difference permits understanding of the difference in the frequency of observation of couplings and the introduction of the concept of a comb structure — a set of secondary parallel crystals of martensite, associated with the same initial crystal.Urals Forestry Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 64–67, June, 1994.     

A structure created by intersecting ϵ martensite variant plates in a high-manganese steel

Microstructural evidence for a new intersection structure, which has been observed in grains oriented to near ?101? in a polycrystalline Fe–30Mn–4Si–2Al TRIP/TWIP steel, was obtained by combined use of electron backscatter diffraction and transmission electron microscopy, and the responsible intersection mechanisms are discussed from crystallographic and thermodynamic viewpoints. A γ phase with the orientation 90° rotated from the matrix with respect to the intersection axis is produced at the intersection of two dense ? plates, whereas the twinning of the obstacle ? plate occurs when the incident plate is a fine γ/? lamella or a stacking fault bundle. The results are compared with various structures reported previously, and the wide variation of the intersection reactions is interpreted by their dependence on thermodynamic stability of the phases, grain orientation and shear amount.     

Stability and stabilization of 2H martensite in Cu–Zn–Al single crystals

The stabilization of the 2H martensitic phase in Cu–Zn–Al single crystals with an electron concentration e/a?=?1.53 was investigated. This orthorhombic 2H martensite was first induced from the cubic β phase by the direct β?→?2H or the indirect β?→?18R?→?2H transformations. On loading the 2H martensite, a transition without hysteresis is observed at a stress which was denoted σ_T1. It was found that this stress is associated with a change in the behaviour of the 2H martensite. A high stabilization of the 2H martensite, around 300?K, is only obtained if an ageing is performed at a stress above σ_T1. Additionally, the stresses of the transformation to another martensitic phase, called 18R₂, were found to be constant when the value of σ_T1 is below the retransformation stress. The 2H martensite and its behaviour on ageing were studied by dilatometry, calorimetry, mechanical testing, optical microscopy and transmission electron microscopy (TEM). Models accounting for the stabilization of the 2H martensite on ageing are proposed.     

New aspects of martensite stabilization in Ni–Mn–Ga high-temperature shape memory alloy

We report on new aspects of martensite stabilization in high-temperature shape memory alloys. We show that, due to the difference in activation energies among various structural defects, an incomplete stabilization of martensite can be realized. In material aged at high temperatures, this gives rise to a variety of unusual features which are found to occur in the martensitic transformation. Specifically, it is shown that both forward and reverse martensitic transformations in a Ni–Mn–Ga high-temperature shape memory alloy can occur in two steps. The observed abnormal behaviour is evidence that, in certain circumstances, thermoelastic martensitic transformation can be induced by diffusion.     

X-ray determination of the volume fraction of the B19′ martensite in nickel-titanium alloys

An X-ray method has been considered for determining the volume fraction of the B19′ martensite in semi-product NiTi alloys, which is based on comparing the total integrated intensity of its strongest reflections with the total integrated intensity of reflections from the matrix B2 phase. It has been shown that the method enables one to determine the volume fraction of martensite with an error of about 1% both in textured and textureless samples.     

Nonelastic behavior of Ni50Ti50-xZrx alloys during B2 ⇆ B19′ martensite transformation

The results of an investigation of martensite transformations (MT), the shape memory effect (SME), and their interaction in Ni₅₀Ti_50-xZr_x alloys (1x50 at. %) are presented. The temperature dependencies of electroresistance and the magnitudes of deformation accumulation and recovery in a torsion mode under conditions of a continuously applied external load were investigated. Structural testing of the alloys was carried out by means of x-ray diffractometry methods. It was revealed that during cooling—heating cycling in alloys containing not more than 15 at. % of Zr, B2 B19 MT occurs. In alloys with 30 and 50 (binary NiZr) at. % Ar, under these conditions phase transformations are not observed. With increase in the concentration of the alloying element an increase in the critical temperatures of the MT was observed, the principal shape of the(T) curve changed, and the sequence of the B2 B19 transition was preserved. The SME in Ni₅₀Ti_50-xZr_x alloys was conditioned by the B2 B19 MT, had a single-stage character, and appeared when the Zr content in the alloy did not exceed 15 at. %. The maximum level of accumulation of inelastic deformation _max did not depend on the quantity of the alloying element and was 10–12%.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 37–39, March, 1995.     

Dislocation centers for nucleation of α-martensite and pairwise joining for martensite crystals with habits {hhl}

We analyze the regular pairwise joinings of -martensite crystals with habits of the {hHl} type. We show that we can consistently interpret all known joinings by considering 60-degree linear dislocations as nucleation centers for crystals with habits {5 7} and {2 5} and by assuming that the directions of the Burgers vectors of the formed nucleation centers of the joined crystals are specified by the directions of the macroscopic or twinning shears of the original crystal.Urals State Wood Technology Academy. Translated from Izvestiya Vysshikh Uchenbnykh Zavedenii, No. 4, pp. 67–70, April, 1994.     

Evolution of magnetic domain structure of martensite in Ni–Mn–Ga films under the interplay of the temperature and magnetic field

Ferromagnetic shape memory Ni-Mn-Ga films with 7M modulated structure were prepared on MgO （001） substrates by magnetron sputtering. Magnetization process with a typical two-hysteresis loop indicates the occurrence of the reversible magnetic field-induced reorientation. Magnetic domain structure and twin structure of the film were controlled by the in- terplay of the magnetic and temperature field. With cooling under an out-of-plane magnetic field, the evolution of magnetic domain structure reveals that martensitic transformation could be divided into two periods： nucleation and growth. With an in-plane magnetic field applied to a thermomagnetic-treated film, the evolution of magnetic domain structure gives evidence of a reorientation of twin variants of martensite. A microstructural model is described to define the twin structure and to produce the magnetic domain structure at the beginning of martensitic transformation; based on this model, the relationship between the twin structure and the magnetic domain structure for the treated film under an in-plane field is also described.     

Violation of the sequence of martensite crystals formation on cooling and their shrinking on heating during B2 ↔ B19′ martensitic transformation in Ti40.7Hf9.5Ni44.8Cu5 shape-memory alloy

An in situ transmission electron microscopy study of the B2 ? B19′ martensitic transformation in Ti_40.7Hf_9.5Ni_44.8Cu₅ shape memory alloy was carried out. It was observed that the sequence of the martensite crystals shrinking on heating differed from the sequence of the martensite crystal appearance on previous cooling. This was shown that strain nanodomain formation on cooling prior to the forward martensitic transformation resulted in accumulation of the elastic energy. This led to the dependences of the elastic energy stored on cooling or released on heating on the volume fraction of the martensite phase became different. In this case, at the same volume fraction of the martensite phase, the configuration of the martensite crystals on cooling and heating was different and it was a reason for a violation of the sequence of the martensite crystal formation on cooling and its shrinking on heating.     

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.     

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.