Martensite Bands Research Articles

Microstructural analysis and phase transformation in Ni–Mn–Ga ferromagnetic shapememory alloys

The martensitic thermoelastic and magnetic transformations have been investigated for theNi–Mn–Ga ferromagnetic shape memory alloys. The martensitic phase of the shapememory alloy responds to the application of stress and, as a result, the martensite variantsare rearranged to accommodate the applied stress, which results in the macroscopic shapechange. The shape recovery property of Ni–24.7 Mn–24.8 Ga (at.%) alloy with martensitictransformation temperatures has been analysed. The mechanisms of the magneticmicrostructure, which cause the martensite band domains and run diagonally in amicrograph from left to right, have been observed. The micrograph also showsthat the martensitic transformation of Ni–24.7 Mn–24.8 Ga (at.%) alloy not onlydepends on the structural rearrangement but also responds to a magnetic field. It isobserved that upon cooling from the high-temperature austenite parent phase, theferromagnetic shape memory alloys exhibit transition from paramagnetism toferromagnetism. It is studied that the Ni–Mn–Ga alloy is an intrinsically suitable ductilematerial.

Various magnetic domain structures in a Ni–Mn–Ga martensite exhibiting magnetic shape memory effect

Magnetic domain structures of the Ni–Mn–Ga martensite were observed by means of type I and type II magnetic contrast in scanning electron microscope. The different configuration of magnetic domain patterns coupled together with the twin structures were studied in multivariant, two-variant, and single-variant martensite. The martensitic band contains broad stripelike magnetic domains following the easy axis of magnetization, i.e., the crystallographic c axis. These stripe domains are connected by 90° domain walls creating a staircaselike structure in the adjoining bands. It is found that the internal twins, substructures of the martensite twin domains, are distorted into a zig–zag shape in order to accommodate the main band magnetization. Furthermore, the dagger-shaped stripe domains occur only when the internal twins are present. When the sample exhibits the single-variant state, the internal twins disappear totally and the stripe magnetic domains spread over the whole specimen. The configuration observed here for the magnetic microstructure together with the crystallographic microstructure can help in understanding the magnetic shape memory effect.

From crystallographic properties to macroscopic detwinning strain and magnetisation of Ni-Mn-Ga magnetic shape memory alloys

In this present work, all the known crystallographic structures of Ni-Mn-Ga alloys (i.e. 5M, 7M and NMT martensitic structures) are considered and their properties are used to deduce the macroscopic detwinning strain and magnetisation of a single crystal by a change of scale. The microstructure configurations of twin martensitic bands in a single crystal are worked out using the nonlinear kinematic theory. The process of rearrangement (or detwinning) of twin martensites is rigorously studied. Detwinning strain, material rotation induced by detwinning and orientation between twin martensitic structures are investigated for a single crystal. Numerical applications shows that results are very sensitive to lattice parameters. A comparison with estimated values shows slight difference, as bigger as, lattice parameter a is greater (or smaller) than c. Magnetic properties are also examined. In order to deduce macroscopic physical quantities from microscopic ones, a Representative Volume Element (RVE) which is representative of the whole material is introduced. By a change of scale, partial detwinning strain and magnetisation are expressed as functions of microstructure parameters, such as, volume fraction of one of twin martensites, length ratio of two consecutive magnetic domains and lattice parameters. These results will be used to performed modeling of magneto-mechanical behavior of Ni-Mn-Ga alloys initially proposed by authors.

Effect of elastic matrix constraint on the tensile deformation of NiTi superelastic fiber

In this paper we study the effect of elastic matrix constraint on the tensile deformation of an active NiTi shape memory alloy fiber, which, when no matrix constraint is present, will experience stress-induced phase transformation by nucleation and growth of a macroscopic martensite band. The effect of the constraint is measured by two factors: the relative Young's modulus (by dimensionless parameter E (2)/ E (1)) and the relative dimension (by dimensionless parameter h/ a) of the fiber and the matrix. The transformation process of the fiber through the martensite band growth under tension is modeled as an embedded elastic fiber containing growing cylindrical transformation inclusions. By Love's stress function, the elastic solutions of the inclusion–fiber–matrix system as well as the internal elastic energy during the transformation are obtained. Analytical expressions of the free energy of the system during the transformation are also formulated for the case of uniaxial tension. After introducing the band nucleation and growth criteria, the growth capability of a martensite band is examined. The results demonstrate that, depending on the magnitude of the matrix constraint, three distinct deformation patterns of the fiber exist: (1) with weak matrix constraint, single band growth dominates the transformation process of the fiber; (2) with intermediate matrix constraint, sequential bands nucleation and growth prevails in the fiber; and (3) with strong constraint, numerous bands form and grow, and macroscopically the fiber tends to deform homogeneously. Parametric studies on the macroscopic stress–strain response of the fiber–matrix system are performed and the obtained results are discussed.

Damping and mechanical properties of nitrogen-containing Fe-Mn alloys

An effect of nitrogen on damping in Fe-Mn alloys is studied. It is shown that nitrogen provides unchanged value of damping capacity in the alloy Fe-17Mn (wt. %) in spite of a decrease in the fraction of the thermally-induced h.c.p. ∈-martensite. This effect is attributed to an enhanced mobility of Shockley partials at the interface between the ∈-martensite bands and residual austenite. At the same time nitrogen increases the yield and ultimate strengths at nearly the same value of plasticity of the Fe-14Mn alloy and increases plasticity of the Fe-17Mn alloy by more than three times.

Growth kinetics of an athermal martensitic transformation by time-resolved optical diffraction from a Ni63AI37(001) surface

The vertical and lateral growth process of single martensitic bands on the (001) surface of a Ni 63 Al 37 single crystal was investigated using time-resolved diffraction of He-Ne laser light. Our extended kinephotometric setup provided spatial information through an analysis of the Fraunhofer pattern created by diffraction from the surface relief, while temporal resolution was achieved with a fast-response photodiode. On continuous cooling through the martensitic phase transition at a rate of 10 K/min, discontinuous changes in the diffraction pattern occurred on time scales ranging from 100 microseconds up to some tens of milliseconds. Each change in the pattern corresponded to a burst-like upheaval of the relief by a few hundred nanometers. The bands were fully developed within 150 - 250 ms and reached final heights of about 1.5 μm.

The initiation and growth of macroscopic martensite band in nano-grained NiTi microtube under tension

The superelastic behavior of polycrystalline nano-grained NiTi shape memory alloy microtube under uniaxial tension is studied in this paper. The nominal stress–strain curve of the microtube during superelastic deformation is recorded. Both direct surface observation and observation by using a special surface coating show that the deformation of the tube is via the nucleation and propagation of macroscopic stress-induced martensite band. It is also found that the martensite nucleates in the form of a spiral lens-shaped narrow band that inclines at about 33o to the plane of cross section of tube when the stress reaches the peak of stress–strain curve. The spiral band grew via gradual increase in both width and length of the band and finally merged into a single cylindrical band. The subsequent deformation of the tube is realized by the growth of this cylindrical martensite band. Several other deformation features of the tube are also observed and the results are discussed and compared with the theoretical analysis in this paper.

The effect of tempering on susceptibility to stress corrosion cracking of AISI 4140 steel in 33% sodium hydroxide at 80°C

The stress corrosion cracking (SCC) behaviour of low alloy steel AISI 4140 was investigated in 33% sodium hydroxide solution at 80°C under freely corroding conditions. Tempering temperatures of 450, 500, 550, 600 and 650°C were used to produce a range of microstructures and strength levels. The relation between the crack growth rate and the stress intensity factor was obtained by constant displacement rate tests conducted on pre-cracked compact tension specimens. The crack growth behaviour and fracture mode were found to be sensitive to the presence of inclusions and to the microstructure of the steel. In case of insufficient tempering, intergranular cracking was observed in the band zone adjacent to the sulfide inclusions. This was attributed to hydrogen embrittlement of martensitic band structure. Disappearance of intergranular cracking in specimens tempered above 550°C was explained by the difference of hydrogen diffusivity in various microstructures of quenched and tempered steels. The results were also interpreted with a model developed for low alloy steels in which the relation between the grain size and the plastic zone size was emphasized.

Magnetic properties and domain structure of magnetic shape memory Ni–Mn–Ga alloy

Magnetic domain structures of single-crystalline Ni48Mn31Ga21 prism with {1 0 0} faces was studied using scanning electron microscopy and by Bitter technique. The sample faces parallel to magnetizing direction contain more or less a stripe domain structure with 180° walls. On the face perpendicular to magnetizing direction, the labyrinth domain structure is established due to out of plane magnetization under strong uniaxial anisotropy. Residual martensitic bands on the same sample face, however, owing to different crystal orientation, contain stripe domains. Magnetization curves of the prism exhibit an abrupt magnetization change and a transient hysteresis in the first quadrant. The abrupt magnetization change is due to field-induced rearrangement of martensitic twin variants preceded by magnetization rotation.

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The evolution of microstructure and texture across the fusion boundaries of a cladding produced by gas tungsten arc welding process was examined by optical microscopy, X-ray diffraction, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy. The cladding comprised of high strength low alloy steel substrate, austenitic stainless steel buffer layer, and super duplex stainless steel top coating. Results indicated that the heat affected zone of the substrate mainly consisted of coarse polygonal ferrite grains. The transition zone, which was separated from the buffer layer by Type II boundaries, was comprised of a narrow austenitic–martensitic band. The microstructure of the buffer layer was characterized by vermicular ferrite which was replaced by lathy morphology adjacent the top coating. The buffer layer grains had <001>║ND, i.e. normal direction, orientation near their interface with the substrate. The top coating was separated from the buffer layer by a distinct epitaxially grown austenitic layer. In none the cladding layers, the texture of the austenite was as strong as that of the ferrite. While in the buffer layer, the austenite grains had close Nishiyama–Wasserman orientation relationship with respect to the parent ferrite, Kurdjumov–Sachs relationship was observed across the top coating. Both the ferrite and austenite phases had significant fraction of low angle grain boundaries across the cladding, but the mount of austenite grain boundaries with high angle grain boundaries with low Σ special character was higher. Moreover, the austenite showed higher levels of stored energy in both layers. The total fraction of low Σ boundaries was also higher in the top coating compared to the buffer layer.

On instabilities and autocatalytic effects associated with the nucleation of martensite bands in polycrystalline SMA flat tensile specimens

The mechanical response in the pseudoelastic range of polycrystalline NiTi SMA specimens in displacement controlled tensile tests exhibits a load drop associated with the nucleation of martensitic bands, a phenomenon which is similar to the formation of Lüders bands in mild steel. A continuum mechanical energy approach allows to calculate the nominal stress–strain curve incorporating the characteristic load drop. The influence of the geometrical aspect ratio, i.e., thickness-to-width ratio of the flat tensile test specimens on the load drop is discussed and compared to the experimental data.

Fatigue behavior and development of microcracks in F82H after helium implantation at 200°C

The ferritic/martensitic steel F82H-mod has been push–pull fatigue tested at 200°C before and after homogeneous implantation of 400 appm helium at about 0.5 dpa and 250°C to assess the growth characteristics of short fatigue cracks and their impact on lifetime. Crack initiation and propagation on carefully polished specimen surfaces were monitored by a high resolution optical microscope during fatigue testing. Subsequent metallographic and microstructural investigations revealed crack initiation mainly along martensitic laths and transgranular crack propagation, partly affected by the orientation of martensitic bands in adjacent grains. A statistical analysis of the crack formation and propagation processes was performed. Irradiation has led to pronounced fatigue life reduction by a factor of ∼5 at higher strain ranges and to a moderate fatigue life enhancement at low strain ranges. This behavior can be understood by different types of microstructural barriers and their effect on crack formation and growth.

Superelastic NiTi Memory Alloy Micro-Tube under Tension— Nucleation and Propagation of Martensite Band

Superelastic NiTi Memory Alloy Micro-Tube under Tension— Nucleation and Propagation of Martensite Band

Microstructure and mechanical properties of low alloy steel T11–austenitic stainless steel 347H bimetallic tubes

A low alloy steel (T11) has been bonded to an austenitic stainless steel 347H by hot coextrusion under industrial conditions. The final product was a seamless bimetallic tube with 347H cladding the exterior for corrosion resistance in severely corrosive environments at high temperatures. The microstructures of the coextruded bonding have been compared to those obtained in the laboratory, after diffusion bonding experiments, using hot isostatic pressing (hipping). In all cases both the interdiffusion of the different elements across the interface and the microstructure have been analysed by optical microscopy, SEM, and TEM. On the 347H side a profuse precipitation, mainly of NbC, was found in a region near the interface. Only in the hipped specimens, as result of nickel and chromium diffusion from the stainless steel to the T11 steel, a martensite band was observed parallel to the interface. The heat treatment performed on the bimetallic tubes, to obtain the optimum combination of mechanical properties and corrosion resistance, consisted of austenitisation between 1050–1100°C, water quenching, and a stabilisation treat ment at 850–900°C, followed by slow cooling.

An inclusion theory for the propagation of martensite band in NiTi shape memory alloy wires under tension

In this paper, we present analytical modelling for the pure mechanical response of uniaxial tensioned NiTi wire that experiences stress-induced martensitic transformation via a propagating martensite band at the superelastic temperature regime. The model aims to predict the overall behavior of the SMA wire as a structural response containing propagating instabilities. Based on the systematic experimental investigation of Shaw and Kyriakides (Shaw, J.A., Kyriakides, S., 1995. Thermomechemical aspects of NiTi. J. Mech. Phys. Solids 43, T243–1281 and Shaw, J.A., Kyriakides, S., 1997. On the nucleation and propogation of phase transformation fronts in a NiTi alloy. Acta Mater. 45(2), 638–700), the wire is modeled as an elastic rod containing a single cylindrical transformation inclusion with a uniform axisymmetric eigenstrain. The analytical expression of the free energy of this special matrix-inclusion system is formulated and the length of the martensite band is identified as the key variable describing the transformation process of the system. Theoretical predictions on the peak stress and the subsequent steady-state propagation stress of the wire during forward and reverse transformations are provided and compared with the available experimental data. Specimen size effect on the nominal stress-strain curves and general deformation features of the wire are discussed.

High resolution electron microscopy studies on the interface structure of deformed stress induced martensite variants in a Ti–Ni–Nb shape memory alloy

Abstract The interfacial structure of deformed stress induced martensite variants in the Ti-44.7 at.% Ni-9 at.% Nb alloy have been investigated by high resolution electron microscopy. Bands conversions were revealed to happen inside the martensite variants to accommodate the macroscopic strain. The evolution of twinning modes from ( 11 1 ) Type I to (001) compound Type, Type II and (111) Type I were found. These newly formed (001) deformation twins nucleate and grow inside the substructural bands to perform the coalescence of substructural bands. It also causes the rearrangement of the unfavorably oriented bands. The appearance of the Type II twinning plate seems to eliminate the stress concentration during the coalescence of martensite bands. The (111) Type I twinning plate is also introduced to accommodate the shape deformation in different directions.

Asymmetry of stress–strain curves under tension and compression for NiTi shape memory alloys

The stress–strain curves of polycrystalline martensitic NiTi shape memory alloys are often different for loading under tension and compression. Under tension, a flat stress-plateau occurs, while under compression, the material is quickly strain hardened and no flat stress-plateau is observed. Cyclic deformation under tension–compression also shows that it is more difficult to deform the material during compression than during tension, where an asymmetric stress–strain loop is obtained. TEM observations show that, under tension to 4% strain, martensite variants are partially reoriented via migration of variant interfaces with formation of dislocation networks mainly along the junction plane areas, and no significantly plastic deformation has been observed inside the martensite twin bands. While under compression to 4% strain, a high density of dislocations has been generated in both the martensite twin bands and the variant accommodation area, and no significant martensite reorientation via variant interfacial migration has been observed. This shows that the deformation mechanism of martensitic polycrystalline NiTi SMAs under tension is different from that under compression.

Microstructural analysis of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy : Part II: Transformation reversibility

The shape memory effect exhibited by some Fe–Mn–Si based alloys is related to the γ(f.c.c.)- ε(h.c.p.) martensitic transformation. In these alloys, the shape memory effect, incomplete even for low deformation rates, drastically decreases when the deformation rate increases. The evolution of the microstructure of the martensite, and particularly the interaction of martensite plates and grain boundaries, is studied in a Fe–Mn–Si–Cr–Ni alloy to determine the origin of the loss of shape memory. The influence of the back-stress created by the dislocations located at the tip of the martensite bands is pointed out. The reduction of the shape memory is attributed to the accommodation of this back-stress when the deformation rate, i.e. the martensite band width, increases. The same kind of analysis is used to analyse the problem of superelasticity.

Characterization of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy: microstructural observation at different scales, mechanism of formation and growth

The martensitic transformation induced by traction at room temperature in a Fe–16Mn–9Cr–5Si–4Ni (%mass) has been studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. The samples were previously submitted to a thermomechanical treatment which increases the shape memory properties. The martensitic microstructure and the fine structure of the stacking faults are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out; these bands correspond to a mixture of thin martensite plates and extremely thin austenitic zones. Inside a grain, the monopartial nature of the martensite has been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation.

In situ observation of stress-induced martensitic transformation and plastic deformation in TiNi alloy

Stress-induced martensitic transformation and plastic deformation in TiNi alloy have been investigated using transmission electron microscope with an in situ deformation technique. It was found that martensite bands were formed and grew with increasing strain and shrank with decreasing strain in tension. In the second deformation stage, a high density of looped dislocations was formed in the reverse-transformed matrix, while in the first deformation stage dislocations were not formed in the reverse-transformed matrix. The formation of dislocation in the second deformation stage is considered as bearing a relation to reverse martensitic transformation and twinning dislocations lying inside the stress-induced martensite.