Martensitic Transformation Characteristics Research Articles

Effect of Microstructure on Martensitic Transformation and Cyclic Deformation Characteristics in a Cu-Al-Ni Based Alloy with Unidirectionally Solidified Structure

The effect of the betatizing conditions on microstructure, transformation characteristics and pseudoelastic behavior has been studied in an unidirectionally solidified Cu-Al-Ni based alloy. The coarse γ2 phase is precipitated in the cast-rod betatized at 650 and the volume fraction of it is decreased by increasing the betatizing temperature above 700 . However, a small amount of γ2 phase is left behind despite betatizing at 800 . No martensitic transformation takes place in the cast-rod betatized at 650, because the precipitation of γ2 phase leads to stabilization of β1 parent phase. However, β1→β1′ martensitic transformation occurs in the cast-rod betatized at 700 and both β1→β1′ and β1→γ1′ martensitic transformations concurrently undergoes in the cast-rod betatized above 750 because of the slope of Al concentration nearby γ2 phase. The loaddisplacement loop of the as-cast rod betatized at 750 is linear, but those of the as-cast rods betatized at 800 and 850 are non-linear showing the typical pseudoelastic loop and the deformation is completely recovered upon unloading.

Martensitic transformation behavior of hot-deformed Fe–32% Ni alloy

The martensitic transformation behavior of hot-deformed Fe–32% Ni alloy was investigated by measurement of electrical resistance and X-ray diffraction. As the strain increases, the austenite goes through from the work-hardened to the partial dynamically recrystallized then to the completed dynamically recrystallized. The martensitic transformation characteristics depend on the austenitic states. The work-hardening in small strain is helpful to martensitic transformation due to the low dislocation density and little lattice distortion, while the high dislocation density and severe lattice distortion by the increase in strain will hinder the martensitic nucleation. Once dynamic recrystallization (DRX) takes place, the martensitic transformation will be enhanced again, which is related to the heterogeneous dynamic substructures. The growing DRX grain can enhance the martensitic nucleation due to the low dislocation density near its grain boundary.

Influence of stress on the temperature characteristics of martensitic transformation and on strain variation in Ti–Ni shape memory alloys

The aim of the present study is the investigation of the influence of the preliminary thermo-mechanical treatment and stress on the difference of the temperature intervals of the martensitic transformation and of the strain variation in deformed TiNi alloys. Several techniques were used: differential scanning calorimetry, simultaneous measurement of strain and electrical resistance, and measurement of strain and differential thermal analysis. The obtained data show that a discrepancy between the martensitic transformation temperatures and the strain variation temperatures occurs during the realization of the basic shape memory effects and depends on the stress applied to the sample. There are two main reasons for the observed phenomena: (1) strain variation prior to the transformation can be due to mechanisms of recoverable unelastic deformation other than the transformation; (2) deformation does not occur during the appearance and disappearance of completely twinned B19′ martensite.

Crystallography of carbide-free bainite in a hard bainitic steel

The convergent beam Kikuchi line diffraction technique has been used to accurately determine the orientation relationships between bainitic ferrite and retained austenite in a hard bainitic steel. A reproducible orientation relationship has been uniquely observed for both the upper and lower bainite. It is ( 1 1 1 ) A 0.70 ± 0.45 ° from ( 1 0 1 ) B , [ 1 ¯ 0 1 ] A 2.5 ± 1.5 ° from [ 1 ¯ 1 1 ] B However, the habit plane of upper bainite is different from that of lower bainite. The former has habit plane that is either within 5° of (2 2 1) A or of (2 5 9) A. The latter only corresponds with a habit plane that is within 5° of (2 5 9) A. The determined orientation relationship is completely consistent with reported results determined using the same technique with an accuracy of ±0.5° in lath martensite in an Fe–20 wt.% Ni–6 wt.% Mn alloy and in a low carbon low alloy steel. It also agrees well with the orientation relationship between granular bainite and austenite in an Fe–19 wt.% Ni–3.5 wt.% Mn–0.15 wt.% C steel. Hence it is believed that, at least from a crystallographic point view, the bainite transformation has the characteristics of martensitic transformation.

Wide hysteresis NiTi(Nb) shape memory alloys with low Nb content (4.5 at.%)

The microstructure, phase transformation behavior and the effect of predeformation on the hysteresis of NiTi(Nb) (4.5 at.% Nb) shape memory alloys were examined in comparison with the classical Ni 47Ti 44Nb 9 wide hysteresis alloy. It is the Nb dissolved in the NiTi matrix that plays the key role in controlling the martensitic transformation characteristics of NiTi(Nb) shape memory alloys, resulting in the wide hysteresis upon predeformation of the alloys in martensitic state. The isolated β-Nb phase in the matrix contributes little to the hysteresis expansion.

Phase transformations in rapidly solidified (Ti-Zr) 50(Ni-Cu-Sn) 50 alloys

Rapidly solidified samples of Ti 45Zr 5Ni 20Cu 25Sn 5 (at.%) alloy have been produced using the planar flow casting, twin roll casting and injection casting techniques. Depending on the cooling rate, different structural states are formed: amorphous, crystalline or mixed. The glass transition and crystallization behaviour of the amorphous phase as well as the martensitic transformation in the crystallized material have been studied by differential scanning calorimetry and X-ray diffraction. The effect of crystallization conditions and resulting microstructure on the martensitic transformation characteristics has been investigated.

Phase transformation behavior and microstructures of rapidly solidified Co–Ni–Ga alloys

Martensitic transformation behavior, magnetic properties and microstructures of Co–Ni–Ga alloy ribbons are investigated and contrasted with those of Co–Ni–Ga bulk alloy. The rapidly solidified 40–60 μm-thickness ribbons of the alloy were prepared by an electro-magnetically controlled single-rolled melt-spinning apparatus. Martensitic transformation characteristics of the Co–Ni–Ga ribbons resemble those of bulk samples. However, transformation temperatures in the ribbons are higher than those of the bulk, suggesting the formation of stress-induced martensites. Coercive force of the Co–Ni–Ga ribbons strongly depend on the direction of the applied magnetic field. Maximum coercive force is obtained at θ = 70° ( θ is the angle between the magnetic field and the ribbon plane). This anisotropy of the magnetic properties is considered to be characteristics of the rapidly solidified ribbons. Microstructures of the martensitic phase in the ribbons are similar to those in the bulk samples, that is, optical and scanning electron microscopies reveal a typical lenticular shape and transmission electron microscopy shows internal twins and specific streaks on selected area electron diffraction patterns.

High temperature atomic rearrangements in melt-spun Ni–Mn–Ga ribbons

The characteristic martensitic transformation (MT) temperatures and the Curie point of two melt-spun Ni 52.5Mn 24.5Ga 23 (at.%) and Ni 49.5Mn 30.5Ga 20 ribbons (10 mm wide, 50 μm thick) were analyzed by differential scanning calorimetry, susceptibility and thermogravimetric analysis (TGA). The temperature dependence of the TGA magnetic signal reflects the parallel changes in the magnetic susceptibility (i.e. an increase associated with the martensite to austenite transformation and a sharp decrease around the Curie point during the heating process). High temperature anneals performed on the as-cast melt-spun ribbons promotes an increase in the MT and Curie transition temperatures and in the enthalpy associated with the MT. This shift towards higher values is interpreted as a consequence of the atomic rearrangements (ordering process) that take place in the as-cast ribbons under the performed thermal treatment.

Ni–Mn–Ga high-temperature shape memory alloys

The relationship among the composition, structure and martensitic transformation temperatures have been studied in detail for Ni 50− x Mn 25Ga 25− x (at.%) alloys by optical and transmission election microscopy, X-ray diffraction and differential scanning calorimeter measurements. The martensitic transformation temperatures almost linearly increase with increasing Ni content up to 670 K showing their potentials as high-temperature shape memory alloys. The results show that the tetragonality ratio ( c/ a) increases with the Ni substitution for Ga, but the unit-cell volume reduces simultaneously. The martensitic transformation characteristics of Ni 54Mn 25Ga 21 alloy with single martensite phase show excellent stability during thermal cycles or aging. The polycrystalline Ni 54Mn 25Ga 21 alloy exhibited a shape-memory effect of 4.2% and compressive strain larger than 10% by using grain refining technique. Addition of Fe as a fourth element is an effective method to improve the mechanical properties, especially plasticity of polycrystalline Ni–Mn–Ga alloy due to the precipitation of γ-phase. A compressive strain of 30% and a shape-memory effect of 2.8% are obtained for Ni 56Mn 17Fe 8Ga 19 alloy.

Martensitic transformations and thermal expansion behaviors of structural heterogeneous NiTi alloys

Martensitic transformation characteristics of NiTi shape memory alloys after severe cold deformation were studied. By designing the shape of the original surface curve, distinct macroscopic domains with different dislocation densities were introduced into the NiTi alloys, and materials possessing the characteristics of composites were obtained. Due to the interactions between the deformation structure and martensite variants, the reverse transformation temperature range was significantly enlarged. One of the possible applications for these materials is the so-called temperature memory effect in a wide temperature range.

Structure and functional properties of thermally and thermomechanically treated Ti-Ni-Nb based alloys with a wide martensitic hysteresis: II. Ti-Ni-Nb alloys with zirconium

The effect of zirconium on the structure, characteristics of thermoelastic martensitic transformation, and functional properties of Ti-Ni-Nb shape-memory alloys has been studied. The possibility of using this material for the manufacture of thermomechanical couplings intended for the design of extra reliable thermomechanical joints operating under extreme temperature conditions is considered.

Effect of alloying on the plasticity of martensitic transformation in Cu-Al alloys

Structural specific features and characteristics of martensitic transformation in Cu-Al alloys with manganese, cobalt, and nickel (14–15 wt % Al and 4–10 wt % Mn + Co + Ni) have been studied. These studies showed that the alloying elements can change the degree of plastic deformation upon martensitic transformation. After quenching and aging of the Cu-Al-Co-Mn alloys, the martensitic deformation is larger by a factor of two in comparison with the Cu-Al-Co, Cu-Al-Co-Ni, and Cu-Al-Mn alloys. This effect can be due to a change in the type of crystal structure of martensite.

Structure and functional properties of heat-and thermomechanically treated Ti-Ni-Nb-based alloys with a wide martensitic hysteresis. I. Ti-Ni-Nb ternary alloys

The effect of heat-and thermomechanical treatments on the structure, characteristics of martensitic transformation, and functional properties of ternary Ti-Ni-Nb alloys has been investigated. The parameters of martensitic transformations whose realization makes it possible to recommend the alloys as promising materials for technologically high-efficient thermomechanical joints have been determined. The temperature-deformation conditions have been revealed under which optimum characteristics of shape recovery and force characteristics are realized in the alloys.

HOT DEFORMATION AND MARTENSITIC TRANSFORMATION BEHAVIORS OF Fe-32%Ni ALLOY

The Hot deformation and martensitic transformation behaviors of Fe-32%Ni alloy was investigated by measurements of electrical resistance and X-ray diffraction. With the increase in strain, the austenite goes through from the work-hardened to the partial dynamically re-crystallized and then to the completed dynamically re-crystallized. The martensitic transformation characteristics depend on the austenite states. The work-hardening in small strain is helpful to martensitic transformation due to the low dislocation density and little lattice distortion, while the high dislocation density and severe lattice distortion by the increase in strain will hinder the martensitic nucleation. Once dynamic re-crystallization (DRX) takes place, the martensitic transformation will be enhanced again, which is related to the heterogeneous dynamic substructures. The growing DRX grain can enhance the martensitic nucleation due to the low dislocation density near its grain boundary.

Characterization of the martensitic transformation in melt-spun NiMnGa ribbons by magnetoinductive effect

The magnetoinductance effect, in particular, the temperature dependence of the AC complex impedance, Z ( T ) , has been employed in the characterization of the martensitic transformation (MT) in a melt-spun Ni 52.5Mn 24.5Ga 23 ribbon. While the resistive component shows the characteristic decrease associated with the martensite-to-austenite transformation, the inductive component reflects the temperature dependence of the magnetic permeability of the sample. These structural and magnetic changes associated with the characteristic MT were experimentally checked by differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). The results indicate that the AC impedance technique can be employed as a very simple and versatile characterization technique in the analysis of MT in magnetic memory shape alloys.

Synthesis of NiTi particles by chemical reaction in molten salts

NiTi shape memory alloy particles were prepared in molten (NaCl+KCl) salts in the temperature range from 680 to 800 °C. NiTi particles of 100 nm to several microns in diameter were obtained and reversible martensitic transformation was confirmed in these particles. The synthesis temperature and the ingredients of (Ni+Ti) powder had no significant influence on the particle size, morphology or the martensitic transformation characteristics. The ball milling time of (Ni+Ti) powders prior to the chemical reaction in molten salts, and the holding time in the molten salts had a significant influence on the martensitic transformation characteristics of the obtained NiTi particles.

Thermodynamics of thermally induced martensitic transformations in Cu–Al–Ni shape memory alloys

The dependence of martensitic transformation characteristics on the type (β ′, γ ′ or β ′+γ ′) of thermally induced martensite has been studied in Cu–Al–Ni shape memory alloys. The energy balance of the martensitic transformation (MT) has been analyzed for the three possible transformations by calorimetric measurements. When a single MT, β→β ′ or β→γ ′ is present, the dissipative term linked to the irreversible entropy S i has been evaluated. This term that accounts for the hysteresis is higher for the β→γ ′ transformation than for the β→β ′ one, according to the hysteresis behavior. On the other hand, during mixed β→γ ′+β ′ transformation, a higher irreversible entropy and a negative total exchanged heat have been measured. In order to explain this behavior, a modification of the energy balance, which takes into account the presence of plastic deformation when martensite coexistence occurs, has been proposed.

Martensitic phase transformation in Ni-rich NiTi single crystals with one family of Ni4Ti3 precipitates

Abstract Compression-aging treatments tests were carried out with single crystals in order to nucleate precipitates under a compression stress in the [1 1 1] direction, conditions which favor the formation of a single family of Ni4Ti3 precipitates. The precipitation behavior under this type of stress-assisted aging is investigated. An attempt is made to study the effects of aging on the characteristics of the martensitic transformations. The objective of the present study is to lay the groundwork for investigating the characteristics of martensitic transformations in Ni-rich NiTi alloys by excluding effects associated with heterogeneous nucleation on grain boundaries in polycrystals.

Magnetic field induced strains caused by different martensites in Ni-Mn-Ga alloys

Strains induced by magnetic fields applied at room temperature were measured for three single crystalline Ni-Mn-Ga alloys with similar magnetic properties and martensitic transformation characteristics- one containing 5-layered tetragonal martensite (alloy A), the others containing 7-layered monoclinic martensite (alloys B,C). Strains of 1.8% have been achieved in alloy A while alloys B and C showed only strains comparable to the true magnetostriction.

Martensitic transformation and magnetic-field-induced strain in Ni-Mn-Ga magnetic shape memory alloys

The present paper deals with the thermal- and stress-induced martensitic transformations, effects of bias magnetic field and external compressive stress on transformation- and magnetic-field-induced strain in Ni-Mn-Ga alloys. The effect of the external magnetic field and compressive stress on the reorientation of martensitic variants is discussed. It is found that the stress-induced spear-like martensitic crystal forms at the beginning of loading, at the temperature a little higher than Ms. The interfaces between the parent phase and the stress-induced martensite migrate reversibly under the increasing/decreasing stress, showing the characteristics of thermoelastic martensitic transformation. When the Ni 52 Mn 24 Ga 24 single crystal is cooled down to 200K, an intermartensitic transformation occurs, however there is no appearance of this case for the powder sample of the same alloy with average particle size of about 50 μm. About 1.8% magnetic-field-induced strain is obtained in the [100] direction of Ni 2 MnGa single crystal when a 1.5T pulse magnetic field is applied at the Ms temperature.