The effect of austenite ordering on the martensite transformation in Fe-Pt alloys near the composition Fe3Pt: II. Crystallography and general features

Ordering of austenitic Fe ~ 25 at. pct Pt alloys results in a progressive change from a "normal" burst type transformation to a thermoelastic one. Experimental measurements are presented which show that despite marked changes in transformation behavior and kinetics, the transformation crystallography remains essentially unaffected by ordering. It is proposed that the thermoelastic transformation results from the effect of ordering in D. P. DUNNE, formerly with the University of Illinois at Urbana-Champaign, Urbana, 111. 61801     

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     

The effect of austenite ordering on the transformation temperature,transformation hysteresis,and thermoelastic behavior in Fe- Pt alloys

The change and transition process in transformation kinetics from a nonthermoelastic to a thermoelastic type accompanying an increase in parent phase order in Fe-Pt alloys near the stoichiometric composition Fe₃Pt has been investigated, using Fe-23, 24 and 25 at. pct Pt alloys. The thermal hysteresis,M _s temperature, martensite tetragonality and transformation volume change have been measured for specimens with various degrees of order, and correlations among these factors are discussed. The results indicate that the martensite tetragonality, or equivalently the.degree of order of the parent phase, is not the dominant factor which dictates a thermoelastic transformation. TheM _s tempera-ture appears to play an important role in the transformation kinetics, and must be lower than a certain value to obtain a thermoelastic transformation in Fe-Pt alloys. formerly Research Assistant at the University of Illinois at Urbana-Champaign, Urbana, IL     

On the austenite ordering and martensite tetragonality in off-stoichiometric Fe3Pt alloys

Ohne Zusammenfassung     

Crystallography of the martensite transformation in Fe−Al−C alloys

Crystallographic measurements on highly tetragonal bct martensite in two Fe?Al?C steels have been made and the results have been interpreted in terms of the phenomenological crystallographic theory of martensite formation. Agreement between theory and experiment was found to be excellent. Of particular interest is the high martensite tetragonality (c/a→1.14) in aluminum-steels compared to others. Because of this the principal distortions of the Bain deformation are substantially smaller and consequently the magnitudes of the shape and lattice invariant deformations are reduced considerably. Measurements of the dilatation parameter δ indicate that the shape deformation is not significantly different from an invariant plane strain.     

The relationship between austenite strength and the transformation to martensite in Fe-10 pct Ni-0.6 pct C alloys

The effect of austenite yield strength on the transformation to martensite was investigated in Fe-10 pct Ni-0.6 pct C alloys. The strength of the austenite was varied by 1) additions of yttrium oxide particles to the base alloy and 2) changing the austenitizing temperature. The austenite strength was measured at three temperatures above theM _s temperature and the data extrapolated to the experimentally determinedM _s temperature. It is shown that the austenite yield strength is determined primarily by the austenite grain size and that the yttrium oxide additions influence the effect of austenitizing temperature on grain size. As the austenite yield strength increases, both theM _s temperature and the amount of transformation product at room temperature decrease. The effect of austenitizing temperature on the transformation is to determine the austenite grain size. The results are consistent with the proposal¹ that the energy required to overcome the resistance of the austenite to plastic deformation is a substantial portion of the non-chemical free energy or restraining force opposing the transformation to martensite.     

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.     

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.     

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.     

Kinetics of the martensite transformation in athermal Fe-C-Ni-Cr alloys

An investigation was conducted to study the martensite transformation kinetics of athermal Fe-C-Ni-Cr alloys. The transformation rate was found to be dependent on alloy composition and was a function of i) the rate of change of chemical free energy with temperature, and ii) the energy expended in deforming the austenite surrounding a growing martensite embryo in order to accommodate the transformational volume and shear strains. Increasing either of the two factors leads to an increase in transformation rate. Also, it was found that the magnitude of the contribution of the deformation energy to the overall nonchemical free energy associated with the transformation depends on the alloy system and specific composition. Formerly at Rensselaer Polytechnic Institute, Troy, N.Y.     

The effect of quench rate on the martensitic transformation in Fe−C alloys

The effect of very high quench rates on the transformation kinetics of a series of Fe?C and low-alloy steels and the morphology of an Fe?14Ni-0.76C alloy was investigated. TheM _S temperatures of the Fe?C and Fe?C?X alloys increased between 90° and 122°C in a sigmoidal fashion over a quench rate range from 2,750° to 24,800°C per sec. The sensitivity of theM _s temperature to the quench rate from the austenitizing temperature to 315°C was shown to be related to the influence of the third alloying element on the diffusivity of carbon in austenite. Transmission electron microscopy and optical metallography showed that the morphology of an Fe?14Ni?0.76C martensite is changed from a lath structure in slow quenched samples to a plate structure in fast quenched samples. The substructure of the untransformed austenite adjacent to the martensite plates changed from planar dislocation arrays to dislocation tangles with increased quench rate. These results were explained using a model for ferrous martensite strengthening based upon the extent of carbon segregation to imperfections in the austenite during cooling.     

Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of low- and ultralow-carbon microalloyed steels containing additions of boron and/or molybdenum to enhance hardenability. Each alloy was subjected to simulated recrystallization and nonrecrystallization rolling schedules, followed by controlled cooling at rates from 0.1 °C/s to about 100 °C/s, and the corresponding continuous-cooling-transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element contents, through to bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. Combined additions of boron and molybdenum were found to be most effective in increasing steel hardenability, while boron was significantly more effective than molybdenum as a single addition, especially at the ultralow carbon content. Severe plastic deformation of the parent austenite (>0.45) markedly enhanced PF formation in those steels in which this microstructural constituent was formed, indicating a significant effective decrease in their hardenability. In contrast, in those steels in which only nonequilibrium ferrite microstructures were formed, the decreases in hardenability were relatively small, reflecting the lack of sensitivity to strain in the austenite of those microstructural constituents forming in the absence of PF.     

An investigation of the effects of austenite strength and austenite stacking fault energy on the morphology of martensite in Fe-Ni-Cr-0.3C alloys

The relative effects of austenite stacking fault energy and austenite yield strength on martensite morphology have been investigated in a series of three Fe-Ni-Cr-C alloys. Carbon content (0.3 wt pct) andM ₆ temperature (− 15°) were held constant within the series. Austenite yield strength atM _s was measured by extrapolating elevated temperature tensile data. Austenite stacking fault energy was measured by the dislocation node technique. Martensite morphologies were characterized by transmission electron microscopy and electron diffraction techniques. A transition from plate to lath martensite occurred with decreasing austenite stacking fault energy. The austenite yield strength atM _s for the low SFE, lath-forming alloy was found to be higher than previously reported for lath-forming alloys. The relative effects of these variables on martensite morphologies in these alloys is discussed.     

The effect of Ni on the decomposition of austenite in Fe-Cu alloys

This paper is concerned with the effect of ternary additions of Ni on the decomposition of austenite in Fe-Cu alloys. It is shown that the transformation kinetics, transformation structures and associated precipitate morphologies are sensitive functions of the Ni content. Specifically the results show that the addition of nickel increases the transformation times and also leads to a change in resultant ferrite morphology from equiaxed to Widmanstätten, with a concomitant increase in the partially coherent interphase boundary growth mode. The increase in transformation time allows precipitation of Cu to occur on the interphase boundary and in the matrix. It is also shown that the exact details of the ∈-Cu dispersion is controlled both by the nature of the austenite/ferrite interface and the kinetics of the transformation.