Irreversibility and transformation randomness of thermoelastic martensitic transformation in Ni–Ti alloys

In this work,the in situ optical observation was carried out in complete and incomplete transformation cycles of Ni-Ti alloys.In complete transformation cycles,initial martensite plates nucleate randomly in austenite.However,in a partial transformation cycle,the existing martensite plates have an influence on guiding the formation of subsequent martensite plates.And the randomness decreases with the decrease in transformation volume involved in the partial cycle.It is suggested that the randomness of transformations contributes to the introduction of defects,and the irreversibility associates with transformation randomness of martensite plates.For instance,a higher randomness in transformations could introduce more defects and more obvious irreversibility.On the other hand,defects generated in thermoelastic martensitic transformation are responsible for the hysteresis of transformations.Therefore,the randomness of transformations also contributes to the transformation hysteresis.These results could help further understanding on some martensitic transformation phenomena of shape memory alloys,such as the nonlinear and history-dependent characteristic.     

Effect of Cu on the evolution of precipitation in an Fe–Cr–Ni–Al–Ti maraging steel

The evolution of precipitates in an Fe–Cr–Ni–Al–Ti stainless maraging steel alloyed with Cu was investigated during aging at 525 °C. Atom probe tomography was used to reveal the development of precipitates and to determine their chemical composition. Two types of precipitates were observed to form during the aging process. Based on their chemical composition these are assumed to be NiAl B2 and Ni₃(Ti,Al) (η-phase). The two phases of precipitates were found to develop independently of each other and the addition of Cu was found to accelerate precipitation. However, the effect of Cu on the nucleation of these phases is different: on the one hand, in the case of NiAl, Cu is incorporated and thus reduces the activation energy by reducing the lattice misfit; on the other hand, Cu acts as a nucleation site for the precipitation of Ni₃(Ti,Al) by forming independent Cu clusters.     

Effect of electrospark dispersion on the martensitic transformation in Co—Fe—Ni—Ti—(Cu) alloys

The effect of electrospark dispersion on the γ ai α matensitic transformation in Co-Fe-Ni-Ti-(Cu) alloyshas been studied. A relationship between the lattice parameters of the initial γ phase (austenite), the structure of the arising α martensite, the temperature of the onset of the martensitic transformation M _s , and the temperature hysteresis of the martensitic transformation ΔT has been established.     

Powder metallurgical processing of Ni–Ti–Zr alloys undergoing martensitic transformation: part I

Ni–Ti–Zr materials with Zr 12–25 at.% and Ni 42–50 at.% have been produced by powder metallurgy. Suitable temperatures for sintering in Ar-atmosphere Ni–Ti–Zr compacts are within the range 900–1000 °C. Sintering at temperatures above 1000 °C causes melting of the compacts with high Zr content. The presence of ZrC, ZrO₂, Zr₃O, TiO₂ and TiO of different modifications, complex oxides such as Ni₅TiO₇, Ti_0.5Zr_0.5O_0.2 and equilibrium phases after sintering at temperatures above 1000 °C in alloys with low Zr-content was derived from X-ray diffractometry. During sintering at temperatures below 1000 °C the phases belonging to the binary Ti–Ni and Ti–Zr systems were formed. Long-term sintering and slow furnace cooling allowed the precipitation of Ni₄Ti₃ and Ni₂Ti. The process of sintering is controlled by the diffusion of Ni in Ti and Zr particles during the early stages of sintering. Slow diffusion of Zr atoms in Ti₂Ni, Ti–Ni and diffusion of Ti atoms in Zr₂Ni, Ni–Zr controls the later stages of sintering.     

Effect of alloying additions on β′ precipitation in NiAl–Ti base alloys

The precipitation of β′–Ni₂AlTi is investigated in NiAl–3Ti–X (X=Cu, Hf) alloys to determine the effects of alloying additions. Hf enhanced the precipitation of β′ and both Ti and Hf were found to partition to the β′ phase. Atom Location by Channeling Enhanced Microanalysis (ALCHEMI) results indicate that Hf has a strong preference to reside on Al sites. Cu was found to enhance β′ precipitation, but only when added to replace Ni in the composition. In this case, the β′ phase was enriched in Ti but not Cu. By relating the stability of the phases with nearest neighbor pair interactions, it is shown that Hf increases the stability of β′ phase, while Cu decreases the stability of the matrix β phase relative to β′.     

Effect of Co addition on the martensitic transformation and magnetocaloric effect of Ni–Mn–Al ferromagnetic shape memory alloys

A series of Ni_50−xCo_xMn₃₂Al₁₈ (x = 3, 4, 5, 6, 7, and 8) alloys were prepared by the arc melting method. The martensitic transformation (MT) shifts to a lower temperature with increasing Co concentration and can be tuned to occur from a ferromagnetic austenite to a weak-magnetic martensite in the range of 6 ≤ x ≤ 8. The field-induced metamagnetic behavior was realized in Ni₄₂Co₈Mn₃₂Al₁₈ sample in which a large magnetic entropy change of 7.7 J/kg K and an effective refrigerant capacity value of 112 J/kg were obtained under the field of 60 kOe. The large magnetocaloric effect and adjustable MT temperature suggest that Ni–Co–Mn–Al alloys should have promising potential as magnetic refrigerants.     

New modelling of the B2 phase and its associated martensitic transformation in the Ti–Ni system

A symmetric two-sublattice model (Ni, Ti, Va)_0.5(Ni, Ti, Va)_0.5 is applied to describe the intermediate B2 compound in order to cope with the order–disorder transition in the Ti–Ni system. Using this model, the ordered B2 and the disordered Ti-rich b.c.c. are described by a single Gibbs free energy function. The B2 phase is the parent phase of the martensitic transformation in the TiNi shape memory alloys (SMAs), and its thermodynamic properties are then reassessed with emphasis on its composition range that is critical for SMAs. The low temperature B19′ phase is also evaluated on the basis of the selected experimental data from the martensitic transformation. Properties related to the transformation are studied in comparison with experimental data. The magnetic contribution is examined for the martensitic transformation. All calculations are in satisfactory agreement with experimental phenomena.     

Effect of grain boundary character on the martensitic transformation in Fe–32at.%Ni bicrystals

The effect of grain boundary character on the martensitic transformation was examined in two types of Fe–32at.%Ni bicrystals containing a 90°<211> tilt or a 90°{211} twist boundary focusing on the martensite-start temperature (Ms) during cooling, the morphology of the martensite and the variant selection. The Ms of bicrystals with a tilt boundary was higher than that of single crystals, while bicrystals with a twist boundary showed no significant difference from that of single crystals. Coarse lenticular martensites formed symmetrically in neighbouring grains around the tilt boundary. In contrast, tiny martensites were uniformly distributed in bicrystals with a twist boundary, and in single crystals. In the vicinity of the tilt and twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants. Moreover, since the equivalent variants in neighbouring grains at the tilt boundary are selected, strain compatibility of the shape strain of martensite across the tilt boundary is satisfied, resulting in an increased Ms. The effect of pre-strain on the heterogeneous nucleation of martensite at the boundaries was also investigated.     

Prediction of crystallographic characteristics of martensitic transformation in a Ni–Mn–Ga based Heusler alloy

The energy minimization theory was applied to analyze the crystallographic features of the martensitic transformation in the alloy Ni_52.5Mn_23.5Ga₂₄ and the results obtained were compared with those determined by experiments. It is revealed that the index of the boundary between martensite variants belongs to the lattice plan family of {112}_M and the orientation relationship between the martensite variant 1 and 2 is characterized by a rotation of 82.1° about [1 0 0]_M1/[0 1 0]_M2. The orientation relationship between the parent phase and the martensite features a rotation of approximately 4° about the c axis of the parent lattice from the position of the conventional Bain relationship.     

Martensitic transformation behavior of Ti–Ni–Ag alloys

Microstructures and martensitic transformation behavior of Ti–Ni–Ag alloys prepared by arc melting were investigated by means of scanning electron microscopy (SEM), electron probe micro analysis (EPMA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermal cycling tests under constant load. Ti–Ni–Ag alloys consisted of Ti–Ni–Ag matrices, Ti₂Ni and TiAg phases. Ti–Ni–Ag matrices contained 0.27–0.52 at.% of solute Ag atoms depending on alloy compositions. The B2–B19′ transformation occurred in Ti–50.1Ni–0.7Ag, Ti–49.2Ni–0.9Ag, Ti–49.2Ni–0.6Ag and Ti–49.0Ni–0.7Ag alloys, while the B2-R-B19′ transformation did in Ti–47.5Ni–1.3Ag and Ti–44.4Ni–1.1Ag alloys. Thermo-mechanical treatment separated the B2-R from the R–B19′ transformation clearly and improved shape recovery by increasing the critical stress for slip deformation in a Ti–50.0Ni–0.7Ag alloy.     

Composition-dependent crystal structure and martensitic transformation in Heusler Ni–Mn–Sn alloys

In the present work, modulated four- and five-layered orthorhombic, seven-layered monoclinic (4O, 10M and 14M) and unmodulated double tetragonal (L1₀) martensites are characterized in Heusler Ni–Mn–Sn alloys using X-ray diffraction, high-resolution transmission electron microscopy, electron diffraction techniques and thermal analysis. All modulated layered martensites exhibit twins and stacking faults, while the L1₀ martensite shows fewer structural defects. The substitution of Sn with Mn in Ni₅₀Mn_37+xSn_13?x (x = 0, 2, 4) enhances the martensitic transition temperatures, while the transition temperatures decrease with increasing Mn content for constant Sn levels in Ni_50?yMn_37+ySn₁₃ (y = 0, 2, 4). The compositional dependence of the martensitic transition temperatures is mainly attributed to the valence electron concentration (e/a) and the unit-cell volume of the high-temperature phase. With increasing transition temperatures (or e/a), the resultant martensitic crystal structure evolves in a sequence of 4O → 10M → 14M → L1₀ in bulk Ni–Mn–Sn alloys.     

Effect of external loading on the martensitic transformation – A phase field study

In this work, the effect of external loading on the martensitic transformation is analyzed using an elasto-plastic phase field model. The phase field microelasticity theory, incorporating a non-linear strain tensor and the effect of grain boundaries, is used to study the impact of applied stresses on an Fe–0.3%C polycrystalline alloy, both in two and three dimensions. The evolution of plasticity is computed using a time-dependent equation that solves for the minimization of the shear strain energy. Crystallographic orientation of the grains in the polycrystal is chosen randomly and it is verified that the said assumption does not have a significant effect on the final volume fraction of martensite. Two-dimensional (2-D) and three-dimensional (3-D) simulations are performed at a temperature significantly higher than the martensitic start temperature of the alloy with uniaxial tensile, compressive and shear loading, along with hydrostatic stresses. It is found that the 3-D simulations are necessary to investigate the effect of external loading on the martensitic transformation using the phase field method since the 2-D numerical simulations produce results that are physically incorrect, while the results obtained from the 3-D simulations are in good agreement with the empirical observations found in the literature. Finally, it is concluded that the given model can be used to predict the volume fraction of martensite in a material with any kind of external loading.     

Isothermal and athermal martensitic transformations in Ni–Ti shape memory alloys

Ni–Ti shape memory alloys are known to demonstrate three possible transformation paths between B2 and B19′ phases, B2–R–B19′, B2–B19′ and B2–B19–B19′, depending on their composition and thermo-mechanical treatment. In this work the isothermal kinetics of accumulation of martensite/austenite for all types of martensitic transformations in Ni–Ti and Ni–Ti–X (X = Fe or Cu) has been studied by means of resistance measurements during interruption of cooling/heating scans. Experimental results show that all transformations to the B19′ phase (B2–B19′, R–B19′, B19–B19′) demonstrate a substantial isothermal accumulation of martensite during isothermal dwelling between the martensitic transformation start and finish temperatures. The reverse transformations B19′–R and B19′–B19 are also classified as isothermal. The isothermal accumulation of austenite detected during the reverse B19′–B2 transformation is much less intense, at least partially due to the low sensitivity of resistance to the martensite fraction variation during the reverse transformation, and remains comparable with the resolution of the experimental set-up. The transformations between the B2 and R as well as between the B2 and B19 phases are athermal. Analysis of the entire set of possible transformations in β Ni–Ti systems allows one to conclude that isothermal transformations possess a much broader hysteresis and transformation range compared with athermal ones. Since the hysteresis of the transformation is related to the friction forces acting on interfaces this fact, and also observation of the isothermal effects during reverse martensitic and intermartensitic transformations, strongly support the interpretation of the observed isothermal effects in Ni–Ti as due to the diffusionless but thermally activated motion of interfaces during transformation. The difference between the transformation to B19′ martensite (isothermal) and all others (athermal) is attributed to a distinction in strain accommodation.     

Twin structure in Mn−Ni−Ti intermetallic alloys with a thermoelastic martensitic transformation

The formation of a twin structure in an alloy can occur in plastic deformation, in subsequent annealing, and as a result of a martensitic transformation. Martensitic transformations that occur in cooling of alloys whose structure includes Hume-Rothery phases are often of a thermoelastic nature and finish with formation of martensite of a twin type. It is of interest to determine the parameter of the twin structure, in particular, the mean width of the first-order twins, and establish its relationship with the properties of the martensite phase in Mn−Ni−Ti alloys. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 17–20, April, 1998.     

Effect of ultra-fine powders on the microstructure of Ti(CN)–xWC–Ni cermets

Microstructural and compositional analyses of Ti(CN)–xWC–20Ni cermets were done in order to better understand the dissolution behavior of ultra-fine sized Ti(CN) and WC in comparison with a micro-sized system. The WC content was varied from 5 to 75 wt%. A discrete composition (∼50 wt%WC addition) at which WC does not dissolve any longer in the ultra-fine system was found. At this composition, the W concentrations, not only in the rim, (Ti,W)(CN), but in the binder as well were at a maximum. Depending on the powder size and morphology, these values are strongly affected due to the surface areas of Ti(CN) and WC for dissolution and precipitation sites. Furthermore, a significant improvement in the mechanical properties of simple ultra-fine systems was found, compared to that of corresponding micron-sized systems (Hv 14–15 GPa, K_IC 8–10 MPa m^1/2), which are comparable to commercial cermets. The refined microstructure and high fraction of rim phase in the ultra fine systems are likely responsible for this difference.