The dislocation and martensite substructures of a fatigued,polycrystalline, pseudoelastic Cu-AI-Ni alloy

Polycrystalline Cu-AI-Ni specimens, subjected to pulsating compression fatigue, while capable of pseudo-elastic deformation, nevertheless exhibit cyclic hardening and fatigue fracture, as reported inMet. Trans. A, 1977, vol. 8A, p. 955. In order to interpret this behavior, transmission electron microscopy has been used to study the microstructure. Increasing amounts of martensite and deformation substructure result from decreasing the test temperature and raising the stress level. Martensite of different morphologies has been observed and identified. Large plates ofβí, common to all samples in varying amounts, were determined to be of the 18R structure, with lattice parameters ofa = 4.382Å,b = 5.356Å, andc = 38.0Å. Samples deformed at low temperature or high stresscontain not only β₁, but two distinct forms of γ both with lattice parameters ofa = 4.41Å,b = 5.31Å, andc = 4.222Å, and of the 2H crystal structure. The largeβí and γ plates seem to be characterized by interfacial dislocations between the matrix and plates. In all samples, antiphase boundaries (APB’s) can be imaged, even in heavily dislocated areas, indicating that the deformation has not destroyed the matrix order. It is concluded that hardening results from interactions between matrix dislocations and the stress-induced martensite, as well as by collisions between groups of martensite plates having different habits.     

Ordering kinetics of Fe3Pt austenites and reversed austenites

Fe₃Pt austenites undergo an ordering reaction belowT _c in addition to the martensite transformation at a much lower temperature. It is to be expected that theM _s temperature will be affected by the degree of austenite order and the progress of austenite ordering can be conveniently represented by the variation ofM _s temperature. In this study it was found that heat treating the annealed austenites belowT _c , a single-stage ordering reaction which followed first order kinetics was observed. The ordering rate or the rate ofM _s temperature change was found to be composition dependent, faster for alloy closer to the stoichiometry. The reversed austenite contains transformation induced defect structure. Consequently, heat treating the reversed austenites belowT _c a two-stage reaction was observed. Annealing of defect structure was found to precede the ordering reaction. Both annealing and ordering followed first order kinetics. The salient feature during annealing was that the initial rate ofM _s temperature change appeared to be independent of alloy composition.     

Correlation of coercive force to microstructure in cyclic martensite → austenite transformations in an fe-ni-co alloy

Microstructural changes which occur in an Fe-29 Ni-18 Co alloy during cyclic martensite austenite transformations have been monitored by coercive force measurements and transmission electron microscopy. Cycling was performed between liquid nitrogen and temperatures between 400°C (just aboveAs) and 800°C (about 100°C above A_f). For all temperatures belowAf, there was no measurable change in the value ofHc following immersion in liquid nitrogen; changes inHc occurred only in a subsequent high temperature cycle. This stabilization of the reversed austenite was attributed to transformation-induced strains. On the other hand, for temperatures of 700° and 800°C and any given cycle,Hc following liquid nitrogen immersion indicated formation of new martensite. At 800°C, transformation-induced strains were sufficient to promote recrystallization of the reversed austenite. An irrational martensitic habit plane for the nucleation of the reversed austenite was identified. Formerly Senior Engineer, Westinghouse Research and Development Center     

Magnetic investigation of the effect of small additions of cobalt and manganese on the martensite reversal in an Fe-Ni alloy

Data on the temperature and composition dependence of the magnetic moment and Curie temperature of several Fe-Ni-Co and Fe-Ni-Mn alloys have been obtained. The temperature dependence of the magnetization was obtained for each alloy from 298 to 873 K, following the magnetization change through the transformation from martensite to austenite. The effect of cobalt and manganese additions to an Fe-29.9 at. pct Ni alloy on the reverse transition temperature,A _s , the Curie temperature,T _c , and the saturation magnetization at absolute zero, ρ_so, has been determined, Values forA _s , T _c , and ρ_so were obtained by fitting a Brillouin function to the respective contributions of austenite and martensite to the total magnetization. This technique represents a very sensitive method of obtaining transition temperatures and the respective amounts of each phase present in the alloys. A theoretical prediction of ρ_so andT _c was in agreement with the experimentally determined values.     

Austenite reversion in 18 Ni Co-free maraging steel

The mechanism of austenite reversion in 18 Ni Co-free maraging steel (250 grade) has been established by conducting extensive X-ray diffraction (XRD) and transmission electron microscopy (TEM) under differently aged conditions. It has been proposed that contrary to the precipitate dissolution mechanism suggested for the initiation of austenite reversion in 18Ni-8Co-5Mo type maraging steels, the initiation of transformation of martensite to austenite in this type of maraging steel is due to the diffusion of Ni from matrix to the dislocations and other defect structures on prolonged/high temperature ageing. This results in local enrichment of Ni which lowers both A_S and M_S temperatures of the region. Lowering of these transformation temperatures is responsible for the early reversion of martensite to Ni-enriched stable austenite which, on subsequent cooling to room temperature, does not transform back to martensite.     

Fracture and fractography of metastable austenites

External test variables such as rate and temperature, and changes in alloy composition are shown to have a number of effects on the fracture of high-strength, metastable austenitic steels. One rate-dependent phenomenon is an unusual fracture mode transition wherein a flat mode changes to a shear mode when the amount of transformation product in the vicinity of the crack tip is reduced by adiabatic heating. The point at which this happens in any one test is dependent upon the velocity of the slowly growing crack which in turn is dependent upon the crosshead rate. Because of this rate effect, the plane stress fracture toughness decreases by as much as 30 pct at higher crosshead rates. Fractographically, it was ascertained that at room temperature, both phases failed in a ductile manner, but at ?196°C, martensite containing greater than about 0.27 wt pct C would cleave. This resulted in a “ductile-brittle” transition in metastable austenites at ?196°C as a function of carbon content. Other compositional variations change the austenite stability which controls the amount of strain-induced marteniste occurring at the crack tip. It is shown that a plane stress fracture toughness (K _C) approaching 500,000 psi-in.^1/2 may be achieved by decreasing the stability of the austenite. The variation ofK _c with austenite stability agrees qualitatively with a theoretical model for the invariant shear contribution to the fracture toughness of metastable austenites.     

Stabilization of retained austenite due to partial martensitic transformations

The stabilization effect of retained austenite has been studied using FeNiC alloys with M_s temperatures below 0°C via a two-step cooling procedure, i.e. the samples were first cooled to a temperature (T_a) below M_s temperature and then heated to room temperature (RT), after being held at RT for a while, the samples were recooled to low temperatures (23 or 82 K) and then heated to RT. It was found that, during the second step of cooling, the martensitic transformation occurred at a temperature of M_s′ which was lower than T_a. With increasing the amount of martensite formed during the first cooling, the difference in the martensitic transformation starting temperatures, ΔM_s = M_s − M_s′, increased. The mechanism of the stabilization of retained austenite during the second step of cooling is proposed to be mainly due to the inhibition effect produced by the previously formed martensite. The aging processes, which retard the growth of the previously formed martensite plates and reduce the number of the available nucleation sites, are the necessary conditions for the above mechanism to operate. By simplifying the internal resisting stress acting on the retained austenite due to the existence of martensite phase as a hydrostatic compressive stress, which increases with increasing the amount of martensite, the change in ΔM_s is discussed from a thermodynamic point of view.     

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.     

Martensite crystal lattice,mechanism of austenite-martensite transformation and behavior of carbon atoms in martensite

Results concerning the crystal lattice of freshly formed martensite with abnormally low and abnormally high axial ratioc/a as well as the results of neutron diffraction studies of the positions of carbon atoms are reviewed. Mechanisms of the austenite to martensite transformation are considered, which can explain the formation of martensite with differentc/a for the same carbon content in the initial austenite. Occurrence of (011)m transformation twins explains both a lowering ofc/a and an orthorhombic distortion of the martensite lattice. It follows from analysis of the experimental results that the well known dependence of martensite lattice parameters and axial ratio on the carbon content relate to a partly disordered distribution of carbon atoms between three sublattices of the octahedral interstitial sites (ois). Using new values of the concentration coefficients of linear expansion of the martensite lattice, calculated for the case of carbon atoms distribution only in a single sublattice of the ois, leads to some corrections of the previous temperature and concentration dependences of order-disorder processes in martensite. Phenomena such as the reversible change of axial ratio due to the redistribution of carbon atoms between their normal positions and “traps” in irradiated martensite are described.     

On the Selection of the Optimal Intercritical Annealing Temperature for Medium Mn TRIP Steel

A model is proposed to predict the room temperature austenite volume fraction as a function of the intercritical annealing temperature for medium Mn transformation-induced plasticity steel. The model takes into account the influence of the austenite composition on the martensite transformation kinetics and the influence of the intercritical annealing temperature dependence of the austenite grain size on the martensite start temperature. A maximum room temperature austenite volume fraction was obtained at a specific intercritical annealing temperature T _M. Ultrafine-grained ferrite and austenite were observed in samples intercritically annealed below the T _M temperature. The microstructure contained a large volume fraction of athermal martensite in samples annealed at an intercritical temperature higher than the T _M temperature.     

The volume expansion accompanying the martensite transformation in iron-carbon alloys

A dilatometric investigation was conducted to determine the effect of carbon on the volume expansion accompanying the martensite transformation in iron-carbon alloys. It was found that the volume expansion at theM _s temperature varies from 2.0 pct at 0.19 wt pct carbon to 3.1 pet at 1.01 pct carbon, largely due to the effect of carbon on lowering the temperature at which the transformation occurs. Also of importance is the solid solution effect of carbon on altering the lattice parameters of both the austenite and martensite phases at theM_s.     

Phase Transformation and Shape Memory Effect of Ti(Pt, Ir)

The martensitic transformation and shape memory effect of Ti₅₀(Pt, Ir)₅₀ with 5?C37.5 at. pct Ir were investigated using differential thermal analysis (DTA), high-temperature X-ray diffraction (HT-XRD), and compression tests. The austenite finish temperature, A _f, increased with increasing Ir content from 1331?K (1058?°C) in Ti-50 at. pct Pt to 1491?K (1218?°C) in Ti-12.5Pt-37.5Ir. The structure of the parent and martensite phases was identified as B2 and B19 in all tested alloys. A large strain recovery rate was obtained in Ti₅₀(Pt, Ir)₅₀ with 10 to 30 at. pct Ir. The highest shape recovery ratio was 57?pct in Ti-25Pt-25Ir after deformation at 1123?K (850?°C), followed by heating to above A _f. Using HT-XRD, the dependence of lattice parameter change on Ir content and temperature was investigated. The volume change during phase transformation from B2 to B19 was estimated using the lattice parameter of the B2 and B19 phases. Strain recovery is discussed along with volume change and lattice parameter change.     

Kinetics of Martensite Formation in Substitutional Fe-Al Alloys: Dilatometric Analysis

High-resolution differential dilatometry was employed to study the kinetics of the martensite formation upon isochronal cooling/quenching of substitutional Fe-(0.5, 0.7, and 1.0) at. pct Al alloys at fast cooling/quenching rates in the range of 17 K (17 °C) through 100 K (100 °C) s^?1, with an emphasis on the as-yet unexpected influence of cooling/quenching rate. The martensite transformation initiated at nearly the same temperature (i.e., the $ M_{\text{S}} $ temperature) in the ferrite-phase region for all cooling/quenching rates applied, which indicates athermal nucleation: the chemical driving force governs the initiation of the nucleation of the martensite plates. Variation of the cooling/quenching rates revealed two principal kinetic features: both the temperature ranges passed during transformation and the grain size of the product martensite increase with the increase of cooling/quenching rates. A modular phase-transformation model, incorporating a classic partitioning analysis for nucleation and anisotropic growth for impingement, has been employed to extract the velocity of the migrating martensite/austenite interface from the dilatometric data. The thus obtained velocity of the martensite/austenite interface as function of temperature indicates a thermally activated growth governed by relatively lower activation energy, as determined by evaluation of the martensite-formation-rate maximum as function of cooling/quenching rate.     

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.     

Nucleation and growth in martensitic transformations of ordered Fe3Pt alloys

Behavior of nucleation and growth of thermoelastic martensitic transformations in Fe₃Pt was examined by electrical resistance measurements, X-ray diffractometry, optical microscopy, andin situ TEM observation. The results suggest that fcc-bct and fcc-fct martensitic transformations of the alloy are independent from each other. The nucleation behavior of bct martensite varies with the decrease ofM _s (bct) temperature. The soft mode of the elastic constant,C′, is considered to exert an influence on the nucleation behavior of fcc-bct transformation, such that the tweed contrast is observed in localized narrow regions in the fcc matrix where bct martensite successively nucleates. This localized tweed structure was induced by the transformation shear associated with the fcc-bct transformation at theM _s temperature and enhanced by the lattice softening.     

<Emphasis Type="Italic">In Situ</Emphasis> Investigation of the Evolution of Lattice Strain and Stresses in Austenite and Martensite During Quenching and Tempering of Steel

Energy dispersive synchrotron X-ray diffraction was applied to investigate in situ the evolution of lattice strains and stresses in austenite and martensite during quenching and tempering of a soft martensitic stainless steel. In one experiment, lattice strains in austenite and martensite were measured in situ in the direction perpendicular to the sample surface during an austenitization, quenching, and tempering cycle. In a second experiment, the sin² ψ method was applied in situ during the austenite-to-martensite transformation to distinguish between macro- and phase-specific micro-stresses and to follow the evolution of these stresses during transformation. Martensite formation evokes compressive stress in austenite that is balanced by tensile stress in martensite. Tempering to 748 K (475 °C) leads to partial relaxation of these stresses. Additionally, data reveal that (elastic) lattice strain in austenite is not hydrostatic but hkl dependent, which is ascribed to plastic deformation of this phase during martensite formation and is considered responsible for anomalous behavior of the 200 _γ reflection.     

The precipitation sequence of Ni3Ti in Co-free maraging steel

TEM, microdiffraction, and X-ray microanalysis were used to study the precipitation processes in Co-free maraging steel. Austenite crystals were found to nucleate in the martensite matrix as the first step in the precipitation sequence of Ni₃Ti. The austenite reversion is a result of Ni segregation. Ni₃Ti nucleates in the austenite. The shape and orientation of Ni₃Ti is determined by the austenite precursor. The same sequence occurs for heterogeneous nucleation on dislocations and grain boundaries. At the later stages of growth Mo is incorporated in the Ni₃Ti lattice.     

Influence of austenite and martensite strength on martensite morphology

The martensite morphology and austenite flow strength have been determined in a variety of ferrous alloys chosen so that the austenites were paramagnetic, ferromagnetic, substitutional strengthened, and interstitial strengthened. It is demonstrated that two of the most important variables in determining the habit plane (and thus morphology) of martensite in a given alloy are the resistances to dislocation motion in austenite and in ferrite (i. e., martensite). In the wide variety of alloys where martensite with a {259}_γ habit plane was observed, the austenite flow strength atM _s is greater than 30,000 psi. At lower austenite strengths, either {225}_γ or {111}_γ habit planes are found depending on the resistance to dislocation motion in ferrite. Thus, {225} martensites are not always found as part of the spectrum between {111} and {259} martensites but only in the cases (e. g., interstitial strengthening) where ferrite is preferentially strengthened relative to austenite. All of the observations are consistent with the idea that the habit plane observed in a given alloy is the one involving the minimum plastic work for the lattice invariant shear.     

The influence of austenite strength upon the austenite-martensite transformation in alloy steels

The resistance of austenite to plastic deformation (austenite flow stress) was measured using a high temperature tensile apparatus. The flow stress was then correlated with the M_s temperature as determined magnetically during subsequent cooling. In one part of the study, the flow stress of the austenite was varied only by work hardening the austenite, allowing the austenite composition, which is known to affect M_s, to be held constant. A decrease in M_s temperature with increasing austenite flow stress was observed. This observation was supported by the observation of a decrease in the amount of austenite transformed at 25°C. In the other part of the study, a series of alloy steels of different chemical compositions was tested. A decrease in M_s temperature with increasing austenite flow stress was again observed. Strengthening of austenite by plastic deformation was shown not to change the chemical driving force for transformation. The effect of deformation on M_s temperature thus results from its influence on either the nucleation or the growth process. While the effect of austenite deformation on martensite nucleation is uncertain, specific nucleation models can account for only approximately one-third of the nonchemical free energy change which accompanies transformation. A proposal, consistent with the observations, was made that the energy expended for the deformation of austenite during martensite plate growth could reasonably account for a substantial part of the nonchemical free energy change.