The effect of austenite ordering on the martensite transformation in Fe-Pt alloys near the composition Fe3Pt: I. Morphology and transformation characteristics

Fe-Pt alloys near the composition Fe₃Pt transform from fee austenite to bcc martensite at near ambient temperatures. The effect of austenite ordering in depressing theM _s temperature has been reported previously, but more importantly the present work shows that ordering leads to a reversible martensitic transformation. The characteristics of this reversible transformation have been investigated by optical metallography, cinematography, and electrical resistivity measurements. It is concluded that in austenite ordered to an appropriate degree, the transformation to martensite possesses all of the characteristics of a thermoelastic martensite transformation. This transformation in ordered Fe~25 at. pct Pt alloys is the first thermoelastic martensite transformation reported for an iron-base alloy. The present experiments indicate that martensite “nuclei” are not destroyed by the transformation, and are reactivated on each cooling cycle at approximately the same temperature. D. P. DUNNE, formerly with the University of Illinois at Urbana-Champaign, Urbana, 111. 61801     

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     

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 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.     

Obtaining Fe—Ni—Co—Ti alloys having a thermoelastic martensite transformation

Using powder metallurgy methods, we have produced Fe—Ni—Co—Ti alloys that have a thermoelastic martensive transformation, which is the basis of the shape-memory effect manifested by such materials. Since pores are believed to improve the shape memory, specifically the reversible nature of the strain, attention was focused on development of the technology and investigation of the characteristics of porous Fe—Ni—Co—Ti alloys. The problems that arise during sintering of such alloys from sputtered powders are due to the chemical inhomogeneity of the initial structure and of the structure formed when the liquid phase appears. Various forms of activation, such as cyclic sintering and a stepped increase in temperature, were used to prevent the liquid phase from appearing. The properties of Fe—Ni—Co—Ti alloys with a shape memory effect can be improved if the porosity is increased by obtaining larger powder grains with a more complicated shape. Materials Science Institute, Ukrainian Academy of Sciences Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 79–85, January–February. 1997.     

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.     

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.     

The morphology of martensite in iron alloys

Light and electron microscopy have been used to determine the main structural differences between the two major types of martensite in ferrous alloys. In the martensite that forms in dilute alloys of iron, the basic transformation unit takes the shape of a lath, and hence the term lath martensite is appropriate for identifying this morphology. Each lath is the result of a homogeneous shear, and successive shears produce a packet of parallel laths containing a high density of tangled dislocations. The other type, plate martensite, differs in the shape taken by a transformation unit and its transformation sequence is characterized by nonparallel plate formation. Investigation of a large number of binary ferrous systems shows that alloy composition and the transformation temperature influence the transition from lath to plate martensite. These two factors are discussed in terms of their possible effects on the plastic deformation mechanisms which must occur in the parent austenite and product martensite during transformation.     

The stabilization of martensite in Cu-Zn-AI shape memory alloys

The martensitic transformation temperature in shape memory alloys can be affected differently by aging above and below the transformation temperature. Under such circumstances the normally reversible transformation can be prevented and the martensite structure “stabilized”. This effect has been studied using electron microscopy, differential scanning calorimetry, and mechanical testing. Evidence is given of an apparently martensitic high temperature transformation, and a careful comparison is made of the stabilized and unstabilized states of the alloy. Three possible models for stabilization are considered in the light of the results obtained, and it is concluded that no single mechanism can be responsible for all the phenomena observed. Formerly with the Department of Metallurgy and Materials Science, University of Cambridge, England     

The average volume of martensite plates during transformation

Using quantitative metallography the average volumes of the martensite plates have been measured in Fe-23.8Ni-0.42C and Fe-28.5Ni-0.4C after varying degrees of transformation. Over a range of volume fraction transformed from 0.07 to 0.55 (the highest volume fraction sample upon which measurements could be made), the average volume of the martensite plates is constant. Fisher’s equation, which predicts a decrease in the average volume by two orders of magnitude over the experimental range of transformation, is not substantiated by the present experiments chiefly because the influence of autocatalysis is neglected in its derivation.     

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.     

Effects of specimen thickness on the temperature of nucleation of martensite in Fe−Ni alloys

M _S (M_b) temperatures of thin Fe?Ni foils were measured as a function of thickness. It is found that two opposing effects yield a scatter ofM _s (M_b) to lower as well as to higher temperatures. These effects can be related to the distribution of nuclei per unit volume and to the effects of image forces on martensite nuclei in thin foils.     

The compctition between martensite and omega in quenched Ti-Nb alloys

A careful experimental study of the phase transformations which occur in annealed β phase Ti-Nb alloys during quenching has been completed. The compctition of the α″ and ω phases to form in alloys of 20 to 70 at. pct Nb was investigated as a function of quench rate and alloy composition. Particular attention was paid to the interstitial content and chemical homogeneity of the alloys. The martensitic α″ phase was found only in 20 and 25 at. pct Nb alloys, and then only using fast water quenches of ~300 °C/sec. Under slower quench conditions,e.g., ~0.3 to 3 °C/sec, ω phase precipitates were found in these alloys and in 30 and 35 at. pct Nb alloys. Evidence of “diffuse” ω phase precipitation was observed in alloys up to 50 at. pct Nb. Only alloys of 60 and 70 at. pct Nb were found to retain the single phaseβ structure upon quenching. These results constitute the first part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison.     

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 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