The stability of precipitated austenite and the toughness of 9Ni steel

A correlation was confirmed between the good low temperature Charpy toughness of 9Ni steel and the stability of its precipitated austenite against the martensitic transformation. Changes in the microstructure during isothermal tempering were studied in detail. The austenite/martensite interface is originally quite coherent over ∼100 A distances. With further tempering, however, the dislocation structure at the austenite/martensite interface changes, and this change may be related to the increased instability of the austenite particles. The reduction in austenite carbon concentration does not seem large enough to account for the large reduction in austenite stability with tempering time. The strains inherent to the transformation of austenite particles create dislocation structures in the tempered martensite. The large deterioration of the Charpy toughness of overtempered material is attributed, in part, to these dislocation structures.     

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     

Fatigue deformation-induced response in a superduplex stainless steel

The cyclic deformation behavior of SAF 2507 superduplex stainless steel (SDSS) was studied under constant plastic-strain amplitudes. The cyclic hardening/softening curves show initial hardening, followed by softening and, finally, saturation behavior. Two regimes can be differentiated in the cyclic stress-strain curve (CSSC) of SDSS. The transition point at which the cyclic strain-hardening rate changes is identified to be ɛ _p/2=7 × 10⁻³. Transmission electron microscopy (TEM) results on dislocation structures suggested that there is a close relationship between the CSSC, hardening/softening curves, and the dislocation substructure evolution. In the low-plastic-strain-amplitude regime of the CSSC, the dislocation activity in the austenite grains is found to be higher than that in the ferrite grains. At higher plastic strain amplitudes, low-energy dislocation structures are found in the ferrite grains, while clusters and bundles of dislocations can be observed in the austenite grains. Strain localization due to formation of these structures resulted in a decrease in the cyclic strain-hardening rate within the high-plastic-strain-amplitude regime. Dislocation substructure evolution is also used to explain the shape of the hardening/softening curve.     

The reversible martensite transformation in iron-platinum alloys near Fe3Pt

A series of iron-platinum alloys containing 25 or 27 at. pct platinum and with ordering of the γ-phase varying from substantial disorder to nearly complete order have been thermally cycled between 25°C and - 196°C. The kinetics of the γ⇌α transformations, the hysteresis revealed by electrical resistanceJtemperature plots, the thermoelastic growth and the reappearance of an identical microstructure after thermal cycling (the microstructural memory effect) were studied as a function of the ordering of the γ-phase. Thermoelastic growth does not appear to be affected by changes in the degree of order of partially ordered specimens but the microstructural memory was imperfect in the most highly ordered specimen examined. In agreement with earlier observations by Dunne and Wayman,³ the difference between the A_s and M_s temperatures and the hysteresis decrease markedly as the order is increased. It is shown by transmission electron microscopy that in all but the most highly-ordered specimens the α- γ transformation produces plates of austenite with a high density of dislocations. These plates are separated from the surrounding untransformed parent austenite by arrays of dislocation loops lying in the interfaces between untransformed parent austenite and the original martensite plates. All the dislocations have a Burgers vector direction which is the same as that of the usual slip dislocation in austenite. Such dislocations lying in a habit plane must be sessile. In the well-ordered specimen dislocation pairs, typical of glide dislocations in a crystal with long-range order, were formed in the austenite formed by the reverse transformation. These dislocations were segregated into roughly plate-like clusters, but the number of clusters in unit volume was appreciably less than the number of original plates of martensite. In this case, no arrays of sessile loops of dislocations mark the locations of the original martensite-austenite interface. It is deduced from the microscopic and kinetic results that the inherited nuclei responsible for the microstructural memory effect are located in localized volumes of highly dislocated austenite formed by the α- γ transformation. No unique dislocation configurations which could be associated with specific nuclei were found. The effects of ordering on the various kinetic effects and the microstructural memory are discussed in terms of the concept of inherited nuclei, the change of the flow stress of the γ-phase with ordering and temperature and the variation of T_o and the transformation driving-force with ordering. Formerly at Northwestern University     

Relationship between austenite dislocation density introduced during thermal cycling and M s temperature in an Fe-17 wt pct Mn alloy

The reason why thermal cycling decreases the martensite start (M _s ) temperature of an Fe-17 wt pct Mn alloy was quantitatively investigated, based on the nucleation model of ε martensite and a thermodynamic model for a martensitic transformation. The M _s temperature decreased by about 22 K after nine cycles between 303 and 573 K, due to the increase in shear-strain energy (ΔG _sh ) required to advance the transformation dislocations through dislocation forests formed in austenite during thermal cycling. The ΔG _sh value increased from 19.3 to 28.8 MJ/m³ due to the increase in austenite dislocation density from 1.5 × 10¹² to 3.8 × 10¹³/m² with the number of thermal cycles (in this case, up to nine cycles). The austenite dislocation density increased rapidly for up to five thermal cycles and then increased gradually with further thermal cycles, showing a good agreement with the increase in austenite hardness with the number of thermal cycles.     

Influence of alloying elements on the kinetics of strain-induced martensitic nucleation in low-alloy, multiphase high-strength steels

Low-alloy multiphase transformation-induced-plasticity (TRIP) steels offer excellent mechanical properties in terms of elongation and strength. This results from the complex synergy between the different phases, i.e., ferrite, bainite, and retained austenite. The precise knowledge of the austenite-to-martensite transformation kinetics is required to understand the behavior of TRIP steels in a wide array of applications. The parameters determining the stability of the metastable austenite were reviewed and investigated experimentally, with special attention paid to the effect of the chemical composition, the temperature, and the size of the austenite particles. The results show that the stability and rate of transformation of the austenite particles in TRIP steels have a pronounced composition dependence: austenite particles transform at a faster rate in CMnSi TRIP steel than in TRIP steels in which Si is fully or partially replaced by Al and P. The results clearly support the view that (1) both a high C content and a submicron size are required for the room-temperature stability of the austenite particles and (2) the effect of the chemical composition on the transformation is due to its influence on the intrinsic stacking-fault energy. In addition, the composition dependence of the Md ₃₀ temperature was derived by regression analysis of experimental data. The influence of the size of the retained austenite particles on their Ms ^σ temperature was studied by means of a thermodynamic model. Both the analysis of the transformation-kinetics data and the microstructural analysis by transmission electron microscopy revealed the very limited role of autocatalysis in the transformation.     

Modelling of austenite stability in low-alloy triple-phase steels

A model for the stability of dispersed austenite in low alloy triple-phase steels has been developed. The model was based on the dislocation dissociation model for classical heterogeneous martensitic nucleation by considering stress effects on the nucleation site potency distribution. The driving force for martensitic transformation has been calculated with the aid of computational thermodynamics. The model allows for the effects of chemical composition of austenite, mean austenite particle size, yield strength of the steel and stress state on austenite stability. Chemical enrichment in C and Mn, as well as size refinement of the austenite particles lead to stabilization. On the contrary, the increase in the yield strength of the steel and triaxiality of the stress state lead to destabilization. The model can be used to determine the microstructural characteristics of the austenite dispersion, i.e. chemical composition and size, for optimum transformation plasticity interactions at the particular stress state of interest and can then be useful in the design of low-alloy triple-phase steels.     

Austenite decomposition structures in the gibeon meteorite

A sample of the Gibeon meteorite has been examined, using several modern metallographic techniques, in particular, electron backscattering diffraction (EBSD). The original single crystalline austenite structure had transformed during slow cooling to a mixture of Widmanstätten ferrite and a two-phase structure known as plessite, which consists of ferrite grains and residual austenite. All the ferrite orientations bear relationships to the austenite structures that are spread between the Nishiyama-Wassermann (N-W) and Young-Kurdjmov-Sachs (Y-K-S) conditions, with close matching of their respective close-packed planes. The plessite consists of a structure of ferrite grains and subgrains, with particles of austenite at the boundaries. However, the scale of the structures in the plessite varies by some two orders of magnitude between different regions, despite these regions having the same chemical composition. It is concluded that this diversity of structures is due to the independent nucleation of ferrite at different temperatures in the different volumes of austenite, depending on the availability of nucleants. The transformation shows evidence of both diffusional and displacive mechanisms. Abnormal grain growth is evident in some plessite regions; this leads to small islands of austenite being trapped inside larger grains of ferrite. Such particles may adopt platelike morphologies, when their crystal lattices rotate to different variants of the Y-K-S relationship.     

The enhancement of strengthening dislocated martensite

The basis of this work was the investigation of improving the tensile properties of dislocated martensites by dispersion of precipitates in the austeniteprior to the martensite transformation. Two types of precipitation-hardenable austenitic alloys were used. One is based on Fe-22 Ni-4 Mo-0.28 C where the precipitates are Mo₂C and are obtained by ausforming and aging, and the other is Fe-28 Ni-2 Ti where the precipitates are the coherent fccγ’ (Ni₃Ti) ordered phase obtained by ausaging. After the austenitic dispersion treatment both alloys were transformed to martensite by quenching to liquid nitrogen and the properties measured and compared to martensites obtained by conventional heat treatment (i.e. no precipitates in austenite). The results show that prior dispersions increase the strength of martensite and this is interpreted as being due to an increase in dislocation density resulting from dislocation multiplication at the particles during the γ →M _s transformation. In addition, the stabilities of the austenitic alloys are such that upon certain aging treatments, the alloys transform partially to martensite (due to precipitation) and “composite” materials are obtained whose strength depends on the volume fraction and yield strengths of the phases present. Formerly Graduate Student, Department of Materials Science and Engineering, University of California, Berkeley, Calif.     

Effect of Prior Cold Work on the Martensite Transformation in SAE 52100

Numerous publications refer to the phase transformations and properties of SAE 52100 steel, and this paper concerns itself with the effect of prior cold deformation on the martensitic hardening response. TheA _c1 and A_c3 temperatures are lowered due to cold work as is theM _s with a resultant increase in the retained austenite content for a given hardening cycle. Significantly, the prior cold deformation results in a refinement of the austenite grain size. The low angle dislocation cells produced by the cold deformation recover during the heating to the austenitizing temperature to form fine ferrite subgrains. The intersections of the fine ferrite subgrains with the spheroidal carbides in the soft annealed microstructures are preferential sites for nucleation of austenite. This results in finer austėnite grains, which produces accelerated carbide dissolution and austenite alloy enrichment compared to un worked, soft annealed structures. The mechanism for the accelerated austenitization is significant in predicting heat treatment response from published phase transformation data for SAE 52100 steel.     

The mean size of plate martensite: influence of austenite grain size,partitioning, and transformation heterogeneity

The evolution of transformation microstructure during martensite transformation is revisited. The influence of “spread” heterogeneity on current partitioning concepts is assessed. Following that, a method to infer details of martensite transformation in an average austenite grain is described. The results indicate that the mean martensite plate size may remain constant up to a significant extent of grain transformation. Moreover, it is found that the mean martensite plate volume in fine grain austenite is proportionally larger than that observed with coarse grain austenite, and these effects are ascribed to autocatalysis. Finally, a transformation model is derived which accurately describes the data up to V_v = 0.65, irrespective of austenite grain size.     

A dislocation-based model for variant selection during the γ-to-α′ transformation

A phase transformation model is described for variant selection during the austenite-to-martensite transformation. The model depends entirely on the presence of glide dislocations in the deformed austenite. The direct correlation between the 24 slip systems of the Bishop and Hill (B-H) crystal plasticity model and the 24 〈112〉 rotation axes of the Kurdjumov-Sachs (K-S) orientation relationship is employed. Two selection criteria, based on slip activity and permissible dislocation reactions, govern the variants that are chosen to represent the final transformation texture. The development of the model via analysis of the experimental results of Liu and Bunge is described. The model is applied to the four distinct strain paths: (1) plane strain rolling, (2) axisymmetric extension, (3) axisymmetric compression, and (4) simple shear. Experimental deformation and transformation textures were produced for comparison purposes via appropriate deformation and quenching procedures. In each case, the transformation texture predicted using the dislocation reaction model is in excellent agreement with the experimental findings.     

Effect of Warm-Rolled Pearlite Microstructural Features on Austenitic Transformation

The austenite transformation characteristics for various warm-rolled pearlite during rapid heating were investigated. The results indicate that the start temperature (Ts) is sensitive to the microstructural feature of pearlite,whereas the dislocation plays an important role in the transformation rate; at the same time, the uniformity of austenite grains is more or less affected by the amount of spheroidized pearlite. A critical effect on the state of austenite grain is created through the influence of initial microstructures on the start temperature of transformation.     

Stress-induced γ→ α martensitic transformation in two carbon stainless steels. Application to trip steels

The influence of the temperature θαof a prestraining of austenite above M_don the subsequent stress-induced γ→ α’ transformation in the(M _s, M_d) range is examined in two carbon stainless steels. It is shown that the yield stress, which is controlled by the transformation, increases with θαat given testing temperature and amount of prestraining. This behavior is related to the influence of θαon the nature and arrangement of the defects present in austenite after the prestraining: planar defects(i.e., stacking faults, twins, e platelets) predominate if θαis close to M_dwhereas undissociated dislocation cells are only to be observed if θif higher. This is consistent with the strong increase of the intrinsic stacking fault energy of the austenite, as inferred from measurements using the node method on a hot stage microscope. In addition, the ability of plane defects to propagate under stress is shown to be lower after a prestraining at higher θα, which is attributed to a segregation of impurity atoms on dislocations. It is concluded that the nucleation stress of the γ→ α’ transformation is the stress necessary to allow planar defects to propagate in the prestrained austenite. This work is part of a thesis prepared at the Centre des Matériaux de l’Ecole des Mines, Corbeil, France, and submitted at the University of Nancy, June 1972.     

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.     

Influence of austenite predeformation on mechanical properties and strain-induced martensitic transformations of a high manganese steel

A thermomechanical treatment such as used in TRIP steels has been applied to a high Manganese steel. Tensile predeformation was carried out at 373 and 773 K. At these temperatures austenite deforms by twinning and slip, respectively. The mechanical behavior and the strain-induced ε and α martensitic transformations have been examined. Austenite predeformation increases tensile strength and low temperature ductility. The beneficial influence of predeformation on fracture has been emphasized. It has been shown that twins or dislocation cells introduced by predeformation give rise to a higher austenite stability. The role of these defects on the growth and nucleation of ε platelets has been discussed.     

Evolution of the structure and the phase composition of a bainitic structural steel during plastic deformation

The evolution of the phase composition and the imperfect substructure of the 30Kh2N2MFA bainitic structural steel subjected to compressive deformation by 36% is quantitatively analyzed. It is shown that deformation is accompanied by an increase in the scalar dislocation density, a decrease in the longitudinal fragment sizes, an increase in the number of stress concentrators, the dissolution of cementite particles, and the transformation of retained austenite.     

The chemical composition of precipitated austenite in 9Ni steel

Analytical scanning transmission electron microscopy and a novel Mössbauer spectrometry technique were used to measure the chemical composition of austenite particles which precipitate during intercritical tempering of 9Ni steel. Both techniques showed an enrichment of Ni, Mn, Cr, and Si in the austenite. A straightforward analysis involving data on both austenite composition and austenite formation kinetics suggests that the growth of austenite particles is controlled by a 3-dimensional diffusion process. The segregation of solutes to the austenite accounts for much of its stability against the martensitic transformation at low temperatures. Composition inhomogeneities develop in austenite particles after long temperings; the central regions of the particles are lean in solutes and are first to undergo the martensitic transformation. However, changes in solute concentrations of the austenite during long temperings seem too small to account for the large changes in austenite stability. It appears that some of the stability of precipitated austenite must be microstructural in origin.     

An acoustic emission study of martensitic transformation of retained austenite in intercritically annealed HSLA steels

The martensitic transformation of retained austenite particles in an intercritically annealed high-strength low-alloy steel has been studied using the acoustic emission technique. It was found that the stability of austenite particles was mainly dependent on the particle size and to a lesser extent on the enrichment of austenite by carbon and manganese. We have demonstrated that a substantial fraction of the retained austenite particles in the as-annealed condition already contains martensite embryos which can initiate martensitic transformation during cooling below 300 K. The martensite embryos are believed to form from a suitable group of dislocations. Those austenite particles which do not contain such dislocations will not transform by supercooling alone. The particle size stabilization effect is viewed as a decreasing probability of finding dislocations in a particle with decreasing particle volume. Plastic deformation can bring about the transformation by introducing dislocations in the austenite particles. The transformation of the retained austenite particles is shown to enhance the ductility of the HSLA steel.