Peculiarities of the phase composition and structure of the high-entropy FeWMoCrVSiMnC multicomponent steel

We report on the results of analysis of the phase composition, structure, and hardness of high-entropy FeWMoCrVSiMnC multicomponent steel subjected to synthesis and subsequent thermal treatment. Analysis is carried out using the methods of analytic transmission and scanning structural and orientational electron microscopy, optical metallography, X-ray energy-dispersive spectroscopy, X-ray phase and structural analyses, and Rockwell measurements of hardness. It is found that steel has a high hardness (62 HRC) and is in ultra-fine-disperse composite martensite-multicarbide state. All carbides are distributed uniformly over the steel volume and have nano- and submicrometer scales depending on the carbide type.

     

Low Temperature Characteristics of Electronic Density of States in Epitaxial Graphene

We propose a novel approach which considers the positions of defects in graphene structure to describe how electronic density of states and the type of graphene conductivity are affected by electron scattering by certain configurations of foreign atoms in graphene matrix. Despite the fact that there is still insufficient experimental data concerning the effect of short-range order on graphene physical properties, we assume that local disorder can play a decisive role in the low-temperature behavior of graphene’s electronic properties.     

High-temperature shape memory effect and the B2-L10 thermoelastic martensitic transformation in Ni-Mn intermetallics

The properties and structure of the martensitic phase of alloys with a near-stoichiometric equiatomic Ni₅₀Mn₅₀ composition, as well as martensitic transformations in them, are investigated in a wide temperature range by measuring the resistivity and thermal expansion coefficient and applying transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. It is found that Ni₅₀Mn₅₀ and Ni₄₉Mn₅₁ alloys experience the B2 → L1₀ highly reversible thermoelastic martensitic transformation and its related high-temperature deformation of the transformation and shape memory effect. Critical temperatures, volume (ΔV/V = ?1.7%) and linear size effects attributed to the direct and reverse martensitic transformations, and the high-temperature dependences of the martensitic and austenite lattice parameters are determined. It is found that the morphology of tetragonal L1₀ martensitic represents a hierarchy of thin coherent sheets of submicrocrystallites and nanocrystallites with plane near-{111}L1₀ habit boundaries, the crystallites being pairwise twinned according to the {111}〈11 $\bar 2$ 〉 _L10 ∥ {011}〈-1 $\bar 1$ 〉 _B2 twinning shear scheme.     

Fine structure and mechanical properties of the shape-memory Ni<Subscript>50</Subscript>Ti<Subscript>32</Subscript>Hf<Subscript>18</Subscript> alloy rapidly quenched by spinning

We have reported the results of investigations of the structure and chemical and phase compositions of the amorphous Ni₅₀Ti₃₂Hf₁₈ alloy prepared by rapid quenching from melt by spinning and subjected to heat treatments. The specific features of the fine polycrystalline alloy structure formation depending on the heat-treatment mode have been studied by transmission and scanning electron microscopy, chemical microanalysis, electron diffraction, and X-ray diffraction analysis. According to the data on the temperature behavior of electrical resistivity, critical temperatures of devitrification and subsequent thermoelastic martensitic transformation B2 → B19′ have been determined. The mechanical properties in different heat-treatment modes have been investigated.     

Crystallographic features of the structure of a martensite packet of the NiMn intermetallic compound

Optical microscopy, scanning electron microscopy, and X-ray diffraction are used to show that a pseudosingle crystal forms upon cooling of an alloy Ni₄₉Mn₅₁ single crystal below the temperature of the β→θ (bcc → fct) transformation. At room temperature, this pseudosingle crystal has the structure of tetragonal L1₀ martensite with parameters a = 0.3732 nm and c = 0.3537 nm and a tetragonality c/a = 0.94775. The temperatures of the forward and reverse B2 → L1₀ transformations are determined. The crystallographic features of martensite packet formation are analyzed. As shown by EBSD, neighboring martensite packets always have three kinds of tetragonal martensite plates, which are in a twin position and have different tetragonality axis directions. Repeated heating and quenching of the pseudosingle crystal result in recrystallization with the formation of coarse grains. The packet structure of the tetragonal martensite is retained in this case, and the sizes of the packets formed within a grain decrease by a factor of 2–3 as compared to the initial pseudosingle crystal.     

Peculiarities of Structure and Phase Composition of Ternary NiMn–NiTi Alloys with a Quasi-Binary Cross-Section

Russian Physics Journal - The influence of a chemical composition on the phase composition, stability, and crystal structure type of the austenitic and martensitic ternary NiMn–NiTi alloys...     

Effect of gallium alloying on the structure,the phase composition,and the thermoelastic martensitic transformations in ternary Ni–Mn–Ga alloys

The effect of gallium alloying on the structure, the phase composition, and the properties of quasibinary Ni₅₀Mn_50–zGa_z (0 ? z ? 25 at %) alloys is studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures is analyzed. A general phase diagram of the magnetic and structural transformations in the alloys is plotted. The temperature–concentration boundaries of the B2 and L2₁ superstructures in the austenite field, the tetragonal L1₀ (2M) martensite, and the 10M and 14M martensite phases with complex multilayer crystal lattices are found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to {011}_B2. Martensite crystals are twinned along one of the 24 \(24\left\{ {011} \right\}{\left\langle {01\bar 1} \right\rangle _{B2}}\) “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses.     

Magnetically controlled thermoelastic martensite transformations and properties of a fine-grained Ni<Subscript>54</Subscript>Mn<Subscript>21</Subscript>Ga<Subscript>25</Subscript> alloy

Comparative studies of physical characteristics (the electrical resistivity, the magnetic susceptibility, the magnetization, the bending deformation, and the degree of shape recovery during subsequent heating) of the Ni₅₄Mn₂₁Ga₂₅ ferromagnetic alloy as-cast and rapidly quenched from melt have been performed in the temperature range 2–400 K. The results are compared to the results of studying the structural–phase transformations by transmission and scanning electron microscopy and X-ray diffraction. It is found that the rapid quenching influences the microstructure, the magnetic state, the critical temperatures, and the specific features of thermoelastic martensite transformations in the alloy. It is found that the resource of the alloy plasticity and thermomechanical bending cyclic stability demonstrates a record-breaking increase in the intercritical temperature range and during subsequent heating.