Austenitic State Research Articles

Effect of disorder on magnetic properties and martensitic transformation of Co-doped Ni-Mn-Al Heusler alloy

Influence of Co addition on magnetic, transport properties and electron structure of Ni50−xCoxMn31+yAl19−y (x = 0-10, y = 0-3) Heusler alloys were investigated experimentally and theoretically by first-principles calculations in the framework of density functional theory with an account of supercell approach. Cobalt doping from 0 to 10 at. % was found to increase the martensitic transition temperature T0 = (Ms + Af)/2 from 280 to 365.5 K. Magnetization of austenite phase sharply increases and magnetic order changes from antiferromagnetic to ferromagnetic state, while in martensite phase magnetization slightly growing with cobalt content. From theoretical point of view, Co-doping leads to formation of additional exchange interactions between magnetic moments of Co-Mn and Ni-Co atoms, which leads to ferromagnetism in austenite. Theoretical calculations show that from 4.68 at. % of Co content ferromagnetic order is preferred. Martensitic transformation is suppressed in alloys with more than 9.5 at. % of cobalt. It was supported by theoretical calculations of the energy difference between austenite and martensite states as a function of cobalt content. We show that the energy difference between austenite and martensite phases in L21 structure from x ≥ 6.25 at. % of Co became negative, what points on the absence of martensitic transformation. For B2 structure, the energy difference is positive for all concentrations of cobalt, that proves that phase transition is likely to occur.

Fracture toughness of NiTi–Towards establishing standard test methods for phase transforming materials

A new test methodology for measuring the fracture toughness of shape memory alloys using the critical value of J-integral as the fracture criterion is proposed. The method relies on the ASTM standard method for measuring the fracture toughness of conventional ductile materials extended to account for the martensitic transformation/martensite orientation-induced changes in the apparent elastic properties. A comprehensive set of nominally-isothermal fracture experiments is carried out on near-equiatomic NiTi compact tension specimens at three distinct temperatures: (i) below the martensite-finish temperature, Mf; (ii) between the martensite-start temperature, Ms, and the martensite desist temperature, Md, above which the stress-induced martensitic transformation is suppressed; and (iii) above Md. At these temperatures, the material either remains in the martensite state throughout the loading (martensitic material, case (i)) or transforms from austenite to martensite close to the crack tip (transforming material, case (ii)) or remains always in the austenite state (austenitic material, case (iii)), respectively. The critical J-values for crack growth, i.e., the fracture toughness, reported in all three cases, result in extrapolated stress intensity factors that are much higher than the corresponding values reported in literature on the basis of linear elastic fracture mechanics. Moreover, contrary to literature, the fracture toughness of martensitic and transforming materials is found to be approximately the same while the fracture toughness of stable austenite is considerably higher. This mechanics-aided test method can be potentially utilized for measuring the fracture toughness of martensitically transforming materials beyond shape memory alloys.

Combined effects in quasidisimilar NiTi joints manufactured by pulsed laser welding

The results of joining quasidissimilar NiTi shape memory alloys (i.e. whereas one component is initially in martensite state and the other one is in austenite state at room temperature) in analyzed in view of the possible use of the combined superelastic and shape memory properties in particular devices with application in the biomedical, automotive or aerospace industry. Alloys with the diameter in the range of 400 µm, were welded using Nd:YAG pulsed laser with 25 or 50% overlap for a spot size of 400 µm. The resulting joints were characterized by tensile testing at RT and at temperatures in the transition range of the shape memory alloy wire. The tensile strength of the joint is in the range of 70–75% of the components. It is also shown that the thermomechanical characteristics of both alloys are preserved in the joint and are influenced by the temperature.

Changes in structural, magnetic and magnetocaloric properties due to homogenization annealing in Ni54Mn19Ga27

Structural and magnetic transformations in the Heusler system Ni54Mn19Ga27 due to different annealing time are studied by X-ray diffraction (XRD) and magnetization measurements. The developing of inter-martensitic and second order phase transition are investigated as a function of the annealing time. Analysis of the XRD data reveals that the austenite state has an L21 structure, whereas the structures of the martensite is a 14M (tetragonal) and the alloy crystallizes in the mixture phase at room temperature. In magnetic measurements, these martensite and austenite phases are ferromagnetic and due to ferromagnetic exchange caused different Curie temperatures for the austenitic and martensitic states in the sample with as prepared, 0.5 and 1 h except 5 h. The saturation magnetization changed from 35 to 68 A m2 kg−1 and the coercivity of these decreased from 1.8 to 0.3 mT as a function of annealing time. And also, the magnetic entropy change was determined to be 2.4 J kg−1 K−1 for as-prepared sample and 5.4 J kg−1 K−1 for 5 h sample. At 2 T magnetic field, the calculated relative cooling power (RCP) values for all alloys were found to be 85.8, 113.8, 120.8, 169.1 J kg−1 as annealing time increase.

Effect of Magnetic state of Austenite on Martensitic transformation in Fe–Ni Alloys in High and Zero Magnetic Fields

The effect of high pulsed magnetic fields on the martensitic γ → α transformation in Fe–Ni and Fe–Ni–Mn alloys characterized by dual kinetics has been analyzed. It has been shown that the higher the iron content and the lower the martensitic transformation temperature, the more substantial the effect of pulsed magnetic field on the athermal martensitic transformation (MT) in the Fe–Ni alloys. The possibility of athermal martensite nucleation at magnetic inhomogenities of austenite, which is characterized by disordered magnetic moments (antiferromagnetic contribution) within a local area that assumes coherent conjugation at the interface, has been discussed. It has been suggested that athermal MT in Invar Fe–Ni alloys characterized by dual kinetics occurs in a pulsed magnetic field via a first-order magnetic phase transformation.

Characteristics of martensite transformed from deformed austenite with various states of ultrahigh strength 300 M steel

300 M steel was isothermally compressed on a Gleeble 1500 simulator at a deformation temperature of 1173 K, a strain rate of 0.1 s−1 and strains ranging from 0.11 to 1.20. The morphologies and structure characteristics of martensite transformed from deformed austenite during water quenching were observed and analysed via scanning electron microscope and transmission electron microscope. Subsequently, effects of prior austenite state on martensitic transformation behavior of 300 M steel were analysed. The experimental results showed that predominant lath martensite and complemental twinned martensite and retained austenite coexisted in deformation-quenched 300 M steel at various strains. The volume fraction of lath martensite and packets of lath martensite decreased with the increasing of strain, and the length and width of the blocks in lath martensite concurrently decreased. Prior austenite with the state of severely work-hardening or just completed dynamic recrystallization would hamper the nucleation and subsequent growth of lath martensite. Additionally, as for austenite with the state of work-hardening, the morphologies and width of laths changed little with the increasing of strain, whereas dislocations in the interior of laths dramatically increased. However, as for austenite with the state of dynamic recrystallization and subsequent growth, the width of laths obviously increased with the increasing of austenite grain size.

Evolution of multiple Martensite variants in a SMA thick-walled cylinder loaded by internal pressure

The stress and deformation fields in a SMA ring or a thick-walled cylinder loaded by internal pressure at constant temperature (over the start temperature of the martensitic transformation) are determined in closed form under plane stress loading conditions. The phenomenological SMA constitutive model incorporates the volume fractions of multi-variants Martensite, which are assumed to evolve linearly with the Tresca effective stress, according to the associative flow rule and the corner flow rule. Initially, the cylinder is everywhere in a state of Austenite. The application of an internal pressure then triggers the martensitic transformation starting from the inner radius of the cylinder wall and extending towards the outer radius. If the wall thickness is large enough, the tangential stress may vanish at the inner radius and correspondingly the stress state may reach a corner of the Tresca transformation condition, thus originating two different Martensite variants according to the corner transformation rule. The admissible phase partitions within the wall thickness originating during the loading process have been systematically investigated according to the ratio between the outer and inner radii. The results obtained here suggest that the loading process should be interrupted soon after the complete martensitic transformation is achieved at the inner radius of the cylinder to avoid permanent plastic deformations.

Influence of Cobalt and Molybdenum Additives on the Structure and Shape Memory Parameters of Reaction-Sintered Porous Nickel Titanium Alloys

We have studied the structure and properties of porous nickel titanium (TiNi) alloys obtained upon reaction sintering of Ti and Ni powders with Co and Mo additives. It is established that Co and Mo doping additives retain the compaction of Ni powder achieved at the initial stage of sintering. The maximum deformation of porous samples loaded in the austenite state was observed upon adding Co, while the addition of Mo resulted in minimum deformation. The addition of Co leads to single-stage martensitic transformation in TiNi phase, while the addition of Mo leads to the two-stage transformation that is more homogeneous over the volume. Both Co and Mo additives lead to increase in the maximum accumulated strain due to the formation of favorably oriented stress-induced martensite and reoriented quench-induced martensite.

Theoretical Prediction of Forming Limit Diagrams for B1500HS Steel at Elevated Temperature Based on Modified Arrhenius and M–K Models

Abstract Boron steel B1500HS deforms at elevated temperature under the complete austenite state, and it accords with the 0-angle necking type, irrespective of the strain state on the left- or right-hand side of the forming limit diagram (FLD), which is in accordance with the M–K simplified model. However, the constitutive equation significantly affects the prediction of the FLD of the sheet metal. In this study, the modified Arrhenius model, including the deformation activation energy and the deformation temperature, describing the hot deformation of the sample with austenitic microstructure combined with the Logan–Hosford yield criterion and the Von Mises yield criterion, was used to predict the forming limits of boron steel B1500HS. The theoretical calculation of the forming limits for B1500HS steel at elevated temperature and under different conditions was carried out, and the FLD predicted by the theoretical calculation was found to be in good agreement with the experimental results. It was concluded that with decrease in the initial inhomogeneity of the material, the hot forming limit curves for B1500HS steel at elevated temperature decreased, and the initial inhomogeneity coefficient f0 = 0.997 was better for the accurate prediction of the hot forming limits of boron steel. With increasing forming temperature, the forming limit curves calculated for B1500HS steel at elevated temperature increased. With increasing mean strain rate, the forming limit curves of B1500HS steel at elevated temperature increased slightly.

Three-dimensional atom probe analysis of boron segregation at austenite grain boundary in a low carbon steel - Effects of boundary misorientation and quenching temperature

Segregation of boron at random austenite grain boundaries with known misorientation angle in a boron-added low carbon steel quenched from austenite state at different temperatures was investigated quantitatively using three-dimensional atom probe. Boron segregation is reduced at low angle boundaries or low quenching temperatures. The latter result indicates that non-equilibrium boron segregation plays an important role and is enhanced at higher quenching temperature due to more excess vacancies and longer diffusion distance during quenching from higher temperature.

Effect of the Thermomechanical Treatment on Structural and Phase Transformations in Cu–14Al–3Ni Shape Memory Alloy Subjected to High-Pressure Torsion

The possibilities of controlling the structure and properties of a Cu–Al–Ni shape memory alloy due to the use of different schemes of the thermomechanical treatment, including forging, homogenizing in the austenitic state and subsequent quenching, and high-pressure torsion have been found. For the first time, an ultrafine-grain structure has been produced in this alloy via severe plastic deformation using high-pressure torsion. It has been detected that high-pressure torsion using ten revolutions of the anvils leads to the formation of a nanocrystalline structure with a grain size of less than 100 nm. The subsequent short-term heating of the alloy to 800°С (10 s) in the temperature region of the existence of the homogeneous β phase made it possible to form an ultrafine-grain structure with predominant sizes of recrystallized grains of 1 and 8 μm. The quenching after heating prevented the decomposition of the solid solution. The refinement of the grain structure changed the deformation behavior of the alloy, having provided the possibility of the significant plastic deformation upon mechanical tensile tests. The coarse-grained hot-forged quenched alloy was brittle, and fracture occurred along the boundaries of former austenite grains and martensite packets. The highstrength ultrafine-grained alloy also experienced mainly the intercrystalline fracture along the high-angle boundaries of elements of the structure, the grain size of which was less by two orders than that in the initial alloy. This determined an increase in its relative elongation upon mechanical tests.

Effect of Pre-strain and High Stresses on the Bainitic Transformation of Manganese-boron Steel 22MnB5

During the last decade, the use of press-hardened components in the automotive industry has grown considerably. The so-called tailored tempering, also known as partial press hardening, employs locally heated tools seeking to obtain bainitic transformations. This leads to (seamless) zones within the formed parts with higher ductility. Due to the intrinsic nature of this process, phase transformations happen under the influence of high loads and in pre-deformed austenite. The austenite pre-strain state and applied stresses affect the kinetics of the bainitic transformation. Moreover, stresses have an additional relevant effect in this process, the so-called transformation plasticity. Linear transformation plasticity models have been successfully used to predict the behavior in the presence of low stresses. Nonetheless, because of the process’s severe conditions, these tend to fail. A strong nonlinearity of the transformation plasticity strain is observed for applied stresses above the austenite yield strength. Using thermomechanical tests on sheet specimens of a manganese-boron steel (22MnB5), widely utilized in the industry, the effect on the bainitic transformation of various degrees of deformation in the range of 0 to 18 pct, applied stresses in the range of 0 to 250 MPa and the transformation plasticity effect are investigated in this work.

EFFECT OF THE FLUENCE OF HIGH-ENERGY CRYPTON IONS IRRADIATION ON Ni-Ti ALLOY HARDENING

The patterns of changes in microhardness and phase composition of Ni-Ti alloy in the austenitic structural-phase state have been experimentally studied depending on the fluence of irradiation with 84Kr15+ ions with the energy of 147 MeV. It was established that hardening with a maximum in the range of the projected range Rp and in the out-of-range region (h > Rp) increases with the growth of the irradiation fluence up to 5·1019 ion/m2. It was shown that the reason of hardening is formation of the radiation-introduced hardened defective structures. The formation of nano-sized particles of the martensitic R-phase and the decrease in NiTi phase content with the B2 structure are also characteristic in the process of irradiation up to maximum fluence due to radiation effects.

Molecular dynamics simulation on the shape memory effect and superelasticity in NiTi shape memory alloy

Molecular dynamics (MD) simulations Ni-Ti based on a second nearest neighbor modified embedded-atom method potential were performed to study atomic-scale mechanical behavior and microstructural evolution of Ni-Ti alloy at different temperatures. By using four calculated characteristic transformation temperatures, the optimum simulated temperature ranges were determined. At temperatures lower than Ms, a thermally induced self-accommodating martensitic phase consisting of three variants formed. Each of the two variants was twin structures. In the subsequent loading process, the most favorable variant grew with the movement of interfaces among the variants, which produced remnants of the residual strain after unloading. After heating up to over Af, the residual strain disappeared and the shape recovered to the original. The shape memory effect under austenite state was also performed, wherein the sample exhibited a typical stress–strain temperature curve and remained in a martensitic state after unloading. After heating at a high temperature, the martensite transformed back into its parent phase accompanied with the recovery of residual strain. The material exhibited superelastic behavior above Af, and the critical transformation stress increased with increased temperature. Some parts of B2 structures remained at the end of the plateau stage and continued transforming into martensites during the hardening stage for both T > Af and Ms <T <Af. The critical stresses for transformation satisfied the Clausius–Clapeyron relationship at T > Ms.

Microstructural origin of hysteresis in Ni-Mn-In based magnetocaloric compounds

Microstructures of magnetocaloric Ni-Mn-In-based Heusler alloys, Ni50.2Mn35.0In14.8 and Ni46.1Mn37.9Fe3.0In13.0, were studied to understand the origin of a large difference in thermal hysteresis in these two alloys. In-situ transmission electron microscopy (TEM) observation showed that the Fe containing sample with a large hysteresis shows a discontinuous phase transition due to the existence of nano-scale Fe-rich bcc phase, along with Fe-lean B2 and L21 phases in the austenite state. The Fe-free sample with a low hysteresis shows a uniform phase transition from martensite to austenite initiated by the nucleation of austenite at the twin boundaries. Ni segregation was found at the twin boundaries of the low hysteresis sample that is considered to facilitate the nucleation of the austenite. The phase transition progresses by the growth of the nucleated austenite to the neighboring twins. 5M and 7M modulated martensites in the low hysteresis sample give rise to a slight difference in the phase transition temperatures in the twin bands contributing to the small hysteresis of 4.4 K in the Fe-free sample. Based on these results, we conclude that to minimize the thermal hysteresis of the Ni-Mn-In based magnetocaloric compounds, one of the key factors is to achieve a uniform composition and crystal structure in the alloy.

Ni-Mn-Ga high temperature shape memory alloys: Function stability in β and β + γ regions

Record-breaking values of the tensile superelastic strain (about 12%) have been found previously in Ni-Mn-Ga single crystalline alloys at 400 °C which placed such materials ahead of known high temperature shape memory alloys (HTSMAs) promising in the automotive or aerospace industries operating in the range of 400–500 °C and above. This paper addresses two main issues that commonly affect Ni-Mn-Ga HTSMAs and limit their application, namely cycling stability of the transformation temperatures and thermomechanical actuation. These issues have been studied systematically by using six different Ni-Mn-Ga HTSMAs. The results show initial transformation temperatures up to 500 °C, which evolve, together with transformation strains, during more than 300 thermal cycles with and without mechanical loading. The specific evolution of a given sample depends on the microstructure, heat treatment prior to the cycling and whether the initial state of austenite is a single (β) or a dual phase (β + γ). The cycling protocol employed can be considered as an innovative training procedure to achieve the stabilization of the functionality and longer lifetime of these materials.

Tribological properties of multifunctional coatings with Shape Memory Effect in abrasive wear

The article gives research results of the abrasive wear process on samples made of Steel 1045, U10 and with applied composite surface layer "Nickel-Multicomponent material with Shape Memory Effect (SME) based on TiNi". For the tests we have chosen TiNiZr, which is in the martensite state and TiNiHfCu, which is in the austenitic state at the test temperature. The formation of the surface layer was carried out by high-speed oxygen-fuel deposition in a protective atmosphere of argon. In the wear test, Al2O3 corundum powder was used as an abrasive. It is shown that the wear rate of samples with a composite surface layer of multicomponent materials with SME is significantly reduced in comparison with the base, which is explained by reversible phase transformations of the surface layer with SME. After carrying out the additional surface plastic deformation (SPD), the resistance of the laminated composition to abrasion wear has greatly enhanced, due to the reinforcing effect of the SPD. It is recommended for products working in conditions of abrasive wear and high temperatures to use the complex formation technology of the surface composition "steel-nickel-material with high-temperature SME", including preparation of the substrate surface and the deposited material, high-speed spraying in the protective atmosphere of argon, followed by SPD.

Влияние легирующих добавок кобальта и молибдена на структуру и параметры памяти формы пористого реакционно-спеченного никелида титана

AbstractWe have studied the structure and properties of porous nickel titanium (TiNi) alloys obtained upon reaction sintering of Ti and Ni powders with Co and Mo additives. It is established that Co and Mo doping additives retain the compaction of Ni powder achieved at the initial stage of sintering. The maximum deformation of porous samples loaded in the austenite state was observed upon adding Co, while the addition of Mo resulted in minimum deformation. The addition of Co leads to single-stage martensitic transformation in TiNi phase, while the addition of Mo leads to the two-stage transformation that is more homogeneous over the volume. Both Co and Mo additives lead to increase in the maximum accumulated strain due to the formation of favorably oriented stress-induced martensite and reoriented quench-induced martensite.

Ferromagnetically ordered clusters in austenite as the areas of martensite formation

In this research, the peculiarities of the magnetic state of austenite of carbon steels were studied. The effect of an external magnetic field on the process of martensitic transformation is discussed. It is shown that there are nanoscale regions with ferromagnetic order in austenite. These regions are possible locations for the nucleation of martensite. In the case of magnetic field influence, the ferromagnetic clusters are able to perceive the energy of the external magnetic field by magnetostrictive stresses (which reduce the energy of martensite formation). Ferromagnetically ordered clusters have a certain critical size, below which the single-domain region becomes unstable. The dependence of the critical size of the ferromagnetic cluster on the intensity of the magnetic field was observed. This dependence shows that the higher the intensity of the field, the smaller the size of the clusters that can remain stable. Thus, a strong magnetic field increases the amount of ferromagnetic nanoclusters (consequently, the number of the nucleation centers of martensite increases). The results of the research allow estimating more accurately the effect of a magnetic field on decreasing the formation energy of a nucleation center.

Reversible tuning of magnetocaloric\xa0Ni-Mn-Ga-Co films on ferroelectric PMN-PT substrates

Tuning functional properties of thin caloric films by mechanical stress is currently of high interest. In particular, a controllable magnetisation or transition temperature is desired for improved usability in magnetocaloric devices. Here, we present results of epitaxial magnetocaloric Ni-Mn-Ga-Co thin films on ferroelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (PMN-PT) substrates. Utilizing X-ray diffraction measurements, we demonstrate that the strain induced in the substrate by application of an electric field can be transferred to the thin film, resulting in a change of the lattice parameters. We examined the consequences of this strain on the magnetic properties of the thin film by temperature- and electric field-dependent measurements. We did not observe a change of martensitic transformation temperature but a reversible change of magnetisation within the austenitic state, which we attribute to the intrinsic magnetic instability of this metamagnetic Heusler alloy. We demonstrate an electric field-controlled entropy change of about 31 % of the magnetocaloric effect - without any hysteresis.