Formation Of Martensite Phase Research Articles

Heterogeneous nucleation of martensite at dislocations and the martensitic-transformation kinetics in shape memory alloys

The martensite phase formation in elastic fields of isolated screw and edge dislocations, as well as in planar clusters of like-sign dislocations and in a two-dimensional network of opposite-sign edge dislocations, is quantitatively analyzed within the theory of smeared martensitic transitions. The heterogeneous nucleation of martensite at dislocations is shown to increase the characteristic temperature of the martensitic transition and its temperature smearing.

Transformation Textures in Unstable Austenitic Steel

During the martensitic transformation in elastic-plastic materials, the local transformation strain as well as the plastic flow inside austenite are strongly related with the crystallographic orientation of the austenitic lattice. Two mechanisms involved in these materials, i.e., plasticity by dislocation motion and martensitic phase formation are coupled through kinematical constraints so that the lattice spin of the austenitic grains is different from the one due to classical slip. In this work, the lattice spin ω˙eA of the austenitic grains is related with the slip rate on the slip systems of the two phases, γ˙A and γ˙M, the evolution of the martensite volume fraction f˙ and the overall rotation rate Ω˙ of the grains. This new relation is integrated in a micromechanical model developed for unstable austenite in order to predict the evolution of the austenite texture during TRansformation Induced Plasticity (TRIP). Results for the evolution of the lattice orientation during martensitic transformation are compared with experimental data obtained by X-ray diffraction on a 304 AISI steel.

Residual Stress of TiNi Shape Memory Alloy Thin Films with (111) Single-crystal Silicon Wafer

As micro-actuator materials, TiNi shape memory alloy thin films with substrate are used more and more in MEMS field. The residual stress in Ti-rich TiNi thin films with silicon substrate prepared by the magnetron-sputtering technique is measured by both X-ray glancing and contour method. The influence of crystallization annealing temperature and film thickness on the residual stress is tested. It is shown that the tensile residual stress decreases from 106 to 37 MPa with increasing annealing temperature from 723 to 923 K. However, it increases again above 1023 K. While increasing the film thickness from 2.4 to 6.5 \\micron, the residual stress reduces from 323 to 80 MPa. A lot of beautiful sunflower-like structures in crystal grains are observed by OM and TEM. The origin of the residual stress and its affecting factors are discussed. The thermal stress during cooling after annealing is the main factor resulting in tensile stress and the formation of martensite phase will release a part of tensile stress.

Magnetostriction in ferromagnetic shape memory alloys

There is intense interest in the development of new magnetostrictive materials having large strains. Recent experiments [1,2] showed that the ferromagnetic shape memory alloys (SMA) have some perspectives to generate significant magnetic-field-induced deformations. Taking into account the high mobility of the martensite crystal lattice variants, the corresponding magnetic measurements were performed by using the SMA Ni–Mn–Ga and Fe–Pd. It is concluded that the condensation of the soft acoustic phonon mode TA2 q= 1 3 〈 1 1 0 〉 takes place at T= T i> M s in Ni–Mn–Ga alloy ( M s—martensite start temperature). Magnetic property anomalies accompany this phase transition which can be interpreted as a transition to an intermediate phase. The characteristic temperatures M s and T i are close in some alloys. The simultaneous action of the lattice instabilities connected with the formation of martensite and intermediate phases produces the essential decreasing of the elastic constants and as a result the strong growth of the high temperature phase magnetostriction nearly at T= M s. The behavior of the martensitic phases in magnetic field is also discussed.

Residual Stress Investigations in Austenitic Steel Samples With Different Degree of Low Cycle Fatigue

Austenitic stainless steels are widely used because of their high corrosion resistance and toughness. The influence of the applied cyclic load on the mechanical properties of the material is of great current interest. In order to investigate residual stress evolution a series of the austenitic steel samples (X6CrNiTi1810) with different degrees of low cycle fatigue was studied. Martensitic phase formation was observed at different degrees of low cycle fatigue. Residual stresses, microstresses in both phases, as well as martensitic precipitation volume fractions, were estimated. Usually after plastic deformation the martensite phase produces a structure of oriented plates or laths. Therefore martensite texture formation during phase transition can be expected. Indeed the registered neutron diffraction spectra from austenitic fatigued samples show texture presence and its variation in dependence of the fatigue degree. The attempt to consider the texture evolution in dependence of fatigue degree and its influence on the residual stresses was made.

Compositional dependence of γ-α transition induced by plastic deformation of nitrified Fe-Cr-Ni alloys

Nitrified carbonless Fe-Ni-Cr stainless steels possess high tensile strengths and ductility, corrosion resistance and stability, hence they represent an attractive material for practical application. The nitrogen improves the mechanical parameters of austenitic steels but the problem of their corrosion resistance, hardening and thermal stability is connected with the composition as well as with the formation of martensite under external stress and plastic deformation. The authors reached the conclusion that the formation of martensite in some nitrified austenite-ferrite stainless steels leads to effective strengthening and conservation of high ductility. However for machine elements which operate at low and high temperature the appearance of deformation martensite decreases the alloy stability. The study of cyclic deformation of an Fe-18Cr-6.5Ni-0.19C alloy demonstrates the existence of a critical value of the strain amplitude necessary to provoke the formation of martensitic phase. The height of this energy barrier for {gamma}-{alpha} transition at fixed temperature is strongly dependent on the characteristics of substitutional and interstitial atoms. When this barrier is overcome, if the free energy values satisfy the relation F{sup {alpha}} < F{sup {gamma}}, the transition is realized at high velocity. This work is a continuation of the earlier study of nitrified alloys, and its aim ismore » to correlate the composition of austenitic Fe-Cr-Ni-N alloys with the formation of {alpha}-phase in the process of plastic deformation using X-ray and neutron diffraction methods.« less

Acoustic emission studies on welded and thermally treated AISI 304 stainless steel during tensile deformation

The present investigations are planned to study the influence of prior martensites formed due to cold treatment as 77K in AISI 304 SS welded specimens, on strain-induced martensites occurred during tensile deformation using AE technique. AE parameters like count rate and root mean square (r.m.s.) voltage have been used to characterize AE activities generated during tensile deformation process in as-welded and welded-treated samples. Frequency spectrum analysis of AE signals captured from the samples has been done to understand the dynamic behavior of the martensite phase formation. Tensile properties of these samples have also been reported. Volume fraction of the magnetic phase (martensite and delta ferrite) formed in these samples are measured before and after straining. X-ray diffraction (XRD) technique has been used to support the presence of delta ferrite (formed during welding) and martensite in the weld region.

Neutron diffraction investigations of stresses in austenitic steel

High-resolution neutron diffraction was applied to investigate the mechanical properties of austenitic stainless steel. The neutron elastic constants for the given material were determined in situ under uniaxial load. The strain was measured for different crystal planes ( hkl), parallel and perpendicular to the loading direction. Martensitic phase formation was studied at different degrees of low cycle fatigue for a series of same-type steel samples. Residual stresses, microstresses in both phases, as well as martensitic precipitation volume fractions, were estimated.

Martensitic Transformation in Cathodically Hydrogen Charged Type 316 Austenitic Stainless Steels.

Cathodically hydrogen charged type 316 austenitic stainless steel has been studied by x-ray diffraction. Both e and α' martensites are formed by catholic charging in solution-treated samples, however very small amount of additional martensitic phase formation was observed in cold-worked samples. The volume fraction of e phase decreases with increasing ageing time. e and α' phases revert to γ phase below 723K and 923K, respectively. The incorporated hydrogen induces the metastable e and α' phases. The transformation is suppressed by the tangled dislocation networks introduced by cold work. e phase is more unstable, thereby it converts to α' and γ phases by hydrogen release. The hydrogen concentration in e phase is higher han that in γ phase and that in α' phase is very low. The change of the x-ray profiles during ageing comes from the change of the spatial distribution of hydrogen atoms. By fitting the observed profiles to those calculated by Fick's law, the diffusion coefficent of hydrogen has been evaluated as 1.0×10-11cm2s-1, for both e and γ phases.

Improvement of the strength of a metastable austenitic stainless steel by cyclic deformation-induced martensitic transformation at 103 K

Specimens of a metastable austenitic stainless steel were cyclically deformed at 103 K under constant plastic strain control with constant plastic strain ranges Δε pl = 1% and 2%. The tests were performed both with a symmetrical plastic strain amplitude (mean plastic strain ε pl = 0 ) and with an asymetrical plastic strain amplitude ( ε pl > 0 ) with the minimum plastic strain equal to zero. The cyclic deformation behaviour was investigated, especially in the stage of cyclic hardening, and correlated with the deformation-induced formation of martensitic phases which could be detected by transmission electron microscopy. The effect of the transformed martensite on the mechanical properties of the steel was quantified by subsequent tensile tests at room temperature with the cyclically deformed specimens. In contrast to results of earlier studies at room temperature, no cumulative plastic “incubation” strain was necessary at 103 K to trigger the deformation-induced martensitic transformation. At Δε pl = 2% and ε pl = 0 the maximum cyclic range was attained after only 18 cycles. In the reference test with an asymmetrical plastic strain amplitude ( ε pl = 1% ) 23 cycles were required to reach the maximum cyclic stress range. Tension tests revealed an increase of up to 200% in the 1% offset yield stress and a 75% increase in the tensile strength at room temperature after cycling the specimens to the maximum stress range with either symmetrical or asymmetrical plastic strain amplitudes. The ductility and the toughness of the steel remained surprisingly high with an elongation after failure of 45%.

Structural observations during amorphization of chromium-iron nitride powders by mechanical alloying : T. Fukunaga et al. (Nagoya University, Nagoya, Japan.) J. Japan Soc. Powder and Powder Metallurgy, Vol 38, No 7, 1991, 940–943. (In Japanese.)

Cr-alloyed PM steels attract great interest in industry, especially in the automotive industry, because, due to the mechanical requirements for the parts, the parts are often subjected to hardening thermal treatments after sintering.In the present work, a sinter-hardening process was carried out in a single stage for pieces based on PM Astaloy CrA steel using concentrated solar energy. The influence of the sintering cycle and atmosphere used, as well as the content of the alloy, were studied.Among the main results obtained, it has been demonstrated that alloys based on Astaloy CrA have been successfully sintered using solar energy. Almost complete densities have been achieved, but these densities are not achieved by conventional techniques, and much shorter sintering times and temperatures (∼ 15 min of holding time at 1070 ± 10 °C) are required compared to those used in the usual processing techniques. In addition, the use of concentrated solar energy in metal treatment leads to high heating and cooling speeds that have encouraged the sinter-hardening process through the formation of bainitic and martensitic phases, which are characterized by being harder and more resistant than ferritic and pearlitic phases. Homogeneous microstructures have been obtained without the need to apply subsequent heat treatments such as tempering that it uses in the processed by conventional techniques.This research work is also intended to promote the use of concentrated solar energy in metallurgical processes that require high temperatures because it is a cheap, nonpolluting and renewable technology. Additionally, high heating and cooling speeds that often result in an improvement of the final properties of the pieces can be obtained without the need to use any external cooling medium.

CEMS study of strain induced phase transformation in manganese Hadfield steel

A Conversion Electron Mossbauer Spectroscopy, (CEMS), study of phase transformations in a Hadfield steel induced by high rate strains is reported. Hadfield steel samples were impact deformed and the ensuing changes in the magnetic properties at the deformed zone and its surroundings have been studied by CEMS. The CEMS results are compared with wear tests and optical microscopy and show a formation of martensite by impact deformation only at the surface. Martensite is not produced by compression or tensile stresses but appears after wear tests in proportions that depend on the load and velocity conditions of test. The understanding of martensite phase formation and its evolution during deformation processes is also addressed.

Fracture mechanism and martensite phase distribution in the zones of plastic deformation of N26Kh5T3 steel aged according to an interrupted mechanism

1. Impact fracture of steel is accompanied by formation of two forms of microrelief corresponding to areas of interrupted decomposition and to areas free of it. In the first case an acicular microrelief is formed at all test temperatures. Embrittlement of the areas in which interrupted decomposition did not occur with a reduction in temperature is apparently related to formation of martensite phase. 2. In movement from the surface of impact fractures into the depth of the specimen the quantity of α-martensite continuously decreases, reaching a minimum at the boundary of the micro- and macrozones of plastic deformation in the fractures of specimens tested at negative temperatures. In specimens tested at normal temperature α-martensite is formed only on the surface of the fractures. In the weakly deformed macrozone of low-temperature fractures a small quantity of α- and e-martensite is observed. 3. Local heating of the material has a significant influence on the phase transformations in the zones of plastic deformation at the tip of a propagating crack.

The damping power of unstable chrome-nickel-manganese base steels

1. Amplitude-dependent energy dissipation in unstable Cr-Ni -Mn steels is determined by the development of microplastic deformations. This finds substantially expression in low-amplitude energy dissipation; the character of its changes under the effect of stresses with increasing amplitudes depends on the carbon content and on the initial structure of the unstable steels. 2. The formation ofα-martensite in the low-carbon biphase steel OKhl3N3G8 begins at cyclic stresses with amplitudes lower than its yield strength. 3. The formation of martensitic phases in cyclic deformation of unstable steels ensures their high endurance. The fatigue limit of the investigated steels is higher than their yield strength. In these steels, cyclic deformation with stress amplitudes exceeding the yield strength is accompanied by great energy dissipation.

Pseudoelastic behaviour associated with thermoelastic martensitic transformation in an Au52.5xAgxCd47.5pseudobinary alloy

Abstract The relation between pseudoelastic behaviour and the thermoelastic martensitic transformation has been studied by means of the measurement of stress–strain curves in the temperature range above and below the martensitic transition temperatures in an Au52.5− x AgxCd47.5 pseudobinary β–phase alloy. It is of special interest that the so–called superelastie loops accompanying two–stage yieldings in stress–strain curves were observed just above M8 , (the martensite–start temperature on cooling). These are thought to be due to successive formation of martensite phases. In conjunction with the measurements of heat capacity and of electrical resistivity, the thermoelastic martensitic properties of the present alloys are discussed from a thermodynamic point of view.

Martensitic alpha and omega phases as deformation products in a titanium-15% molybdenum alloy

The structure of omega phase and its formation by heat treatments in certain titanium alloys are well established. Formation of this phase during cold deformation of β-phase alloys, although claimed by some investigators, has not been studied extensively and conflicting evidence has been published. The effect of compressive stresses on coarse and fine grained specimens of a Ti-15% Mo alloy in the retained beta phase condition has been examined. Specimens have plastically deformed initially by formation of martensitic alpha phase and subsequently by slip. In coarse grained specimens some alpha phase plates have been observed to restrict their own growth by generating slip across the growth direction. Analysis of slip line separation magnitudes in one of these instances indicated that a maximum stress, of the order of 10 10 dyn/cm 2, is present at a plate tip and that stress decreases linearly with distance from the point in the direction of propagation of the plate. X-ray diffraction results have established that omega phase in this alloy forms in greater quantities as cold deformation proceeds, at least until about 40 per cent strain has been effected. The phase originates on octahedral planes in the lattice of the beta matrix and appears to grow on these planes.