Constrained phase-transformation of a TiNi shape-memory alloy

The phase-transformation behavior of a TiNi shape-memory alloy (SMA) under constraint of a constant strain is experimentally investigated by means of mechanical testing and differential scanning calorimetry (DSC) measurements. It is indicated that the reverse-transformation-temperature span under constraint is much larger than that of the unconstrained state. After an incomplete constrained transformation cycle, a two-stage recovery-stress in the constrained state as well as a two-stage recovery-strain in the unconstrained state emerges upon subsequent heating. This is rationalized on the basis of a mechanism which takes into account the influence of stress on the formation of the austenite and the plastic deformation of martensite and austenite during constrained heating. Most importantly, the constrained reverse and forward transformations corresponding to the redeformation of the oriented martensite and the formation of the stress-induced martensite and thermal martensite, respectively, lead to the subsequent two-stage recovery-strain and recovery-stress characteristics. Both the prestrain level and the constrained heating temperature play important roles in the phase transformations and thermomechanical characteristics of the TiNi SMA.     

Hot rolling texture development in CMnCrSi dual-phase steels

The amount of strain below the temperature of nonrecrystallization, T _nr , has an important influence on the phase fractions and the final crystallographic texture of a hot-rolled dual-phase ferrite+martensite CMnCrSi steel. The final texture is influenced by three main microstructural processes: the recrystallization of the austenite, the austenite deformation, and the austenite-to-ferrite transformation. The amount of strain below T _nr plays a major role in the relative amounts of deformed and recrystallized austenite after rolling. Recrystallized and deformed austenite have clearly different texture components and, due to the specific lattice correspondence relations between the parent austenite phase and its transformation products, the resulting ferrite textures are different as well. In addition, austenite deformation textures result from either dislocation glide or the combination of dislocation glide and mechanical twinning, depending on the stacking fault energy (SFE). The texture components in hot-rolled dual-phase steels were studied by means of X-ray diffraction (XRD) measurements and orientation imaging microscopy (OIM). A clear crystallographic orientation difference was observed between the ferrite phase, transformed at temperatures near A _r3 , and the ferritic bainite and martensite phases, formed at lower temperatures. The results suggest that the primary ferrite, nucleated at temperatures close to A _r3 , transformed from the deformed austenite. The low-temperature constituents, bainite and martensite, form in the recrystallized austenite.     

Cyclic deformation behavior of a transformation-induced plasticity-aided dual-phase steel

Cyclic hardening-softening behavior of a TRIP-aided dual-phase (TDP) steel composed of a ferrite matrix and retained austenite plus bainite second phase was examined at temperatures ranging from 20 °C to 200 °C. An increment of the cyclic hardening was related to (1) a long-range internal stress due to the second phase and (2) the strain-induced transformation (SIT) behavior of the retained austenite, as follows. Large cyclic hardening, similar to a conventional ferrite-martensite dual-phase steel, appeared in the TDP steel deformed at 20 °C, where the SIT of the retained austenite occurred at an early stage. This was mainly caused by a large increase in strain-induced martensite content or strain-induced martensite hardening, with a small contribution of the internal stress. In this case, shear and expansion strains on the SIT considerably decreased the internal stress in the matrix. With increasing deformation temperature or retained austenite stability, the amount of cyclic hardening decreased with a significant decrease in plastic strain amplitude. This interesting cyclic behavior was principally ascribed to the internal stress, which was enhanced by stable and strain-hardened retained austenite particles.     

Effect of retained austenite on the yielding and deformation behavior of a dual phase steel

Tensile tests were conducted on a vanadium containing dual phase steel at temperatures between −53 and + 187°C to determine the effect of retained austenite stability on tensile properties. The transformation of retained austenite to martensite with stress/strain was shown to be a contributing factor in the yielding and strain hardening behavior of the dual phase steel. Increasing the stability of the austenite, by increasing the test temperature, caused the expected shift in the austenite to martensite strain transformation to higher strains. This led to a lower initial strain hardening exponent which increased with strain, compared to the ambient and sub-zero temperature deformation where the initial strain hardening exponent was higher but decreased with strain. The former behavior, which accentuated the strain hardening ability at higher strains, led to an increase in the uniform and total elongations, suggesting that the ductility of dual phase steels can be further improved by optimizing the stability of the retained austenite.     

The effect of austenite prestrain above the Md temperature on the martensitic transformation in Fe-Ni-Cr-C alloys

The effect of austenite prestrain above theM _d temperature on the structure and transformation kinetics of the martensitic transformation observed on cooling was determined for a series of Fe-Ni-Cr-C alloys. The alloys exhibited a shift in martensite morphology in the nondeformed state from twinned plate to lath while theM _s temperature, carbon content, and austenite grain size were constant. The transformation behavior was observed over the temperature range 0 to -196°C as a function of tensile prestrains performed above theM _d temperature. A range of prestrains from 5 pct to 45 pct was investigated. It is concluded that the response of a given alloy to austenite prestrain above theM _d temperature can be correlated with the morphology of the martensite observed in the nondeformed, as-quenched state. For the range of prestrains investigated, the transformation of austenite to lath martensite is much more susceptible to stabilization by austenite prestrain above theM _d temperature than is the transformation of austenite to plate martensite.     

Martensitic Transformation Behavior of B‐Bearing Steel During Isothermal Deformation

The purpose of the present research is to study the martensitic transformation in 22MnB5 steel under thermomechanical conditions by means of dilatation data. To reach this aim, the effects of deformation temperature and strain rate on the martensitic dilatation as well as martensite start temperature (M_s) were investigated. Thermomechanical treatments were performed in a deformation dilatometer including the isothermal deformation of samples in the temperature range of 550–900°C up to the final strain of 0.5 in three strain rates of 0.1, 1, and 10 s^?1. Finally, deformation temperatures were divided into two regimes of lower and higher than 800°C. In the former, strain‐induced phase transformations, while in the latter, occurrence of dynamic recovery against mechanical stabilization of austenite influenced martensitic transformation.     

Elementary Transformation and Deformation Processes and the Cyclic Stability of NiTi and NiTiCu Shape Memory Spring Actuators

The present work addresses functional fatigue of binary NiTi and ternary NiTiCu (with 5, 7.5, and 10 at. pct Cu) shape memory (SM) spring actuators. We study how the alloy composition and processing affect the actuator stability during thermomechanical cycling. Spring lengths and temperatures were monitored and it was found that functional fatigue results in an accumulation of irreversible strain (in austenite and martensite) and in increasing martensite start temperatures. We present phenomenological equations that quantify both phenomena. We show that cyclic actuator stability can be improved by using precycling, subjecting the material to cold work, and adding copper. Adding copper is more attractive than cold work, because it improves cyclic stability without sacrificing the exploitable actuator stroke. Copper reduces the width of the thermal hysteresis and improves geometrical and thermal actuator stability, because it results in a better crystallographic compatibility between the parent and the product phase. There is a good correlation between the width of the thermal hysteresis and the intensity of irrecoverable deformation associated with thermomechanical cycling. We interpret this finding on the basis of a scenario in which dislocations are created during the phase transformations that remain in the microstructure during subsequent cycling. These dislocations facilitate the formation of martensite (increasing martensite start (M _S ) temperatures) and account for the accumulation of irreversible strain in martensite and austenite.     

Structure of martensites in Fe-Ni-Ti alloys

The morphology, internal structure and crystal structure of martensites in three Fe-Ni-6 pct Ti alloys containing 10, 20, and 25 pct by weight of nickel have been studied. The 10 and 20 pct nickel alloys transform to lath martensite with a dislocated sub-structure, and are not tetragonal. The 25 pct nickel alloy forms martensite plates on transformation, with a substructure of dislocations and twins. This alloy is tetragonal in both twinned and dislocated regions with ac/a ratio of 1.017. Electron diffraction evidence for a tetragonal phase produced by the Bain strain of Ni₃Ti has also been obtained. The observations support the hypothesis that the tatragonality results from the formation of an ordered Ni₃Ti phase in austenite prior to transformation to martensite.     

Changes in transformation behaviour and microstructure of a CrV-spring steel by thermomechanical treatment

The effect of austenite deformation on the transformation behaviour was investigated on a CrV-spring steel with the major attention put on the martensitic transformation. In the first part, a small review is given on the relation between the state of austenite after hot deformation and its influence on the formation of martensite. In the laboratory tests, the second part of the paper, a conventional heat treatment (CHT) was compared with two types of austenite conditioning by thermomechanical treatment (TMT): TMT_R - with deformation above the recrystallization temperature ?_R leading to a fully recrystallized austenite and TMT_N- with deformation below ?_R with a not recrystallized but possibly polygonized austenite. For the laboratory tests, the hot deformation simulator Wumsi was employed. After quenching In oil, the martensite after TMT consisted of associations of many fine fragments with a smaller number of large acicular martensitic units than observed after CHT. In both TMT-variants small ferritic areas (< 1 μ m) could be revealed. Different behaviour of martensite during tempering at low temperatures was observed after CHT and TMT. It can be explained by reduced inherent stresses generated during martensitic transformation after TMT, presumably as a result of a better ability of deformed austenite to withstand the accommodation strain during martensitic transformation. This may have considerable consequences for the toughness properties of tempered martensite.     

The influence of preceding cryoforming and testing temperature on the mechanical properties of austenitic stainless steels

The TRIP effect in austenitic stainless steels leads to temperature dependent mechanical properties. As this is caused by stress or strain induced austenite/martensite transformation a predeformation at low temperatures (cryoforming) will change the microstructure and the transformation behaviour of the remaining austenite constituent. The mechanical properties in tensile tests and the J‐integral of the chromium and nickel alloyed steels 1.4301 and 1.4571 have been tested in the temperature range from 123 to 323 K in the as‐industrially supplied condition and after 10 % cryoforming at 77 K. The temperature dependence of the elongation values and the strain hardening behaviour of the undeformed steels is much more pronounced than of the yield and tensile strength. The mechanical behaviour can be explained by differences in response to the ?‐, the α_e'‐ and the α_g'‐martensite transformation. A cryoforming changes the mechanical properties of the examined austenitic stainless steels.     

Retained Austenite Transformation during Heat Treatment of a 5 Wt Pct Cr Cold Work Tool Steel

Retained austenite transformation was studied for a 5 wt pct Cr cold work tool steel tempered at 798 K and 873 K (525 °C and 600 °C) followed by cooling to room temperature. Tempering cycles with variations in holding times were conducted to observe the mechanisms involved. Phase transformations were studied with dilatometry, and the resulting microstructures were characterized with X-ray diffraction and scanning electron microscopy. Tempering treatments at 798 K (525 °C) resulted in retained austenite transformation to martensite on cooling. The martensite start (M _s ) and martensite finish (M _f ) temperatures increased with longer holding times at tempering temperature. At the same time, the lattice parameter of retained austenite decreased. Calculations from the M _s temperatures and lattice parameters suggested that there was a decrease in carbon content of retained austenite as a result of precipitation of carbides prior to transformation. This was in agreement with the resulting microstructure and the contraction of the specimen during tempering, as observed by dilatometry. Tempering at 873 K (600 °C) resulted in precipitation of carbides in retained austenite followed by transformation to ferrite and carbides. This was further supported by the initial contraction and later expansion of the dilatometry specimen, the resulting microstructure, and the absence of any phase transformation on cooling from the tempering treatment. It was concluded that there are two mechanisms of retained austenite transformation occurring depending on tempering temperature and time. This was found useful in understanding the standard tempering treatment, and suggestions regarding alternative tempering treatments are discussed.     

Texture, strain, and phase-fraction measurements during mechanical cycling in superelastic NiTi

Superelastic NiTi was subjected to simultaneous neutron diffraction and uniaxial compressive cycling between 10 and 980 MPa. The objective was an in-situ investigation of the evolution of the stress-induced, reversible transformation between austenite and martensite, to determine the cause of the changes in the macroscopic stress-strain response with cycling. Rietveld refinement was used to analyze the neutron spectra and quantify the phase fraction, texture, and elastic strain. The average phase strain in the mechanically loaded austenite (at a given stress) remained unaltered during the 100 load-unload cycles. However, differences in both the volume fraction and texture of austenite and martensite were noted as cycling progressed, suggesting that these factors are responsible for the changes in the macroscopic stress-strain response of NiTi with mechanical cycling.     

Macromodelling of Transformation Induced Plasticity combined with Viscoplasticity for Low‐Alloy Steels

Metal forming processes are important technologies for production of engineering metal components. In order to optimize the resulting material properties, it becomes necessary to simulate the entire forming process by taking into account physical effects such as phase transformations. In this work we concentrate on the phase change from austenite to martensite and present a macroscopic material model, which combines the effects of viscoplasticity with the effect of transformation induced plasticity (TRIP). An extensive experimental data basis for a low‐alloy steel is used for parameter identification, thus taking into account the effects of uniaxial compressive and tensile stress on the kinetics of phase transformation at different temperatures. In a finite element simulation the austenite to martensite phase transformation within a shaft subjected to thermal loading is investigated.     

Magnetic investigation of martensite ⇌ austenite transformations in Fe-Ni-Co alloys

The martensite ⇌ austenite transformations were investigated in Fe-Ni-Co alloys containing about 65 wt pct Fe and up to 15 wt pct Co. A change in morphology of martensite from plate-like to lath-type occurred with increasing cobalt content; this change in morphology correlates with the disappearance of the Invar anomaly in the austenite. The martensite-to-austenite reverse transformation differed depending on martensite morphology. Reversion of plate-like martensite was found to occur by simple disintegration of the martensite platelets. Reverse austenite formed from lath-type martensite was not retained when quenched from much aboveA _s, with microcracks forming during theM→γ→M transformation.     

Grain Refinement by Cyclic Displacive Forward/Reverse Transformation in Fe-High-Ni Alloys

A novel method for grain refinement of martensite structures was proposed, in which transformation strain is accumulated by cyclic displacive forward and reverse transformations. This method can refine martensite structures in an Fe-18Ni alloy because a high density of austenite dislocations is introduced by a displacive reverse transformation in addition to an inheritance of dislocations in body-centered cubic martensite into austenite during cyclic transformation. The addition of a small amount of carbon accelerates structure refinement significantly, which results in the formation of ultra-fine-grained structures after ten cycles.

     

Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105?C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -170?C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory.     

The effect of severe marforming on shape memory characteristics of a Ti-rich NiTi alloy processed using equal channel angular extrusion

A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE). Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2 austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism. Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation conditions led to an increase in the strength levels and some deterioration of shape-memory characteristics. However, a subsequent low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior. However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than 90 deg should be used to reduce the amount of strain applied in one pass.     

Analysis of lattice parameter changes following deformation of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity sheet steel

Austenite and ferrite lattice parameters were monitored using X-ray diffraction subsequent to deformation in uniaxial and biaxial tension and plane straining of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity (TRIP) sheet steel. Details from peak position results suggest the presence of stacking faults in the austenite phase, especially after deformation in uniaxial tension. The results also indicate residual stress or composition effects (through changes in the average carbon concentration due to selective transformation of lower carbon regions of austenite). Compressive residual stresses in the ferrite matrix were measured, and found to increase with increasing effective strain in specimens tested in biaxial tension and plane strain. Strain partitioning between softer ferrite and harder austenite (and possibly bainite or martensite) may be responsible for these residual compressive stresses in the ferrite, although volume expansion from the γ → α′ transformation and texture gradients through the sheet thickness are also possible contributors.     

Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105‡C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -170‡C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory. T. MAKI, Formerly with the University of Illinois