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.     

Observations of aging effects in a Cu-Sn shape memory alloy

A Cu-15.0 at. pct Sn alloy has been chosen as a model alloy for the study of aging effects in copper-based shape memory alloys. Different thermal aging treatments were carried out to determine the effects of both parent phase and martensite aging on the amount of shape recovery and the characteristic transformation temperaturesM _s ,A _s , andA _f . Aging of the martensite reduces both the amount of shape recovery and the extent of the reverse martensite → parent transformation. High martensite heating rates promote complete shape recovery and reverse transformation while the aging occurring during slow heating can inhibit or prohibit both. But irrespective of the martensite heating rate the transformation temperature hysteresis as given by (M _s -A _s ) is large for the Cu-15 pct Sn alloy compared to other shape memory alloys exhibiting thermoelastic behavior. On the other hand, some beneficial effects were noted when the Cu-15 pct Sn alloy was aged in the parent phase condition prior to subsequent transformation to martensite. TheM _s ,A _s , andA _f were lowered following prior parent phase aging, possibly because of a change in long range order, but prior parent phase aging was found to diminish the deleterious effect of martensite aging. Both shape recovery and the extent of the reverse martensite → parent transformation are enhanced by prior parent phase aging. The enhancement is greater the higher the aging temperature or the longer the aging time at a given temperature. J. D. STICE, formerly Research Assistant at the University of Illinois     

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.     

Study of the Ni<Subscript>41.3</Subscript>Ti<Subscript>38.7</Subscript>Nb<Subscript>20</Subscript> wide transformation hysteresis shape-memory alloy

The shape-memory characteristics in the Ni_41.3Ti_38.7Nb₂₀ alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature M _s could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature A′ _s rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (M _s +30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni_41.3Ti_38.7Nb₂₀ alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced 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.     

On Various Aspects of Decomposition of Austenite in a High-Silicon Steel During Quenching and Partitioning

Using a Gleeble thermomechanical simulator, a high-silicon steel (Fe-0.2C-1.5Si-2.0Mn-0.6Cr) was laboratory hot-rolled, re-austenitized, quenched into the M _s–M _f range, retaining 15 to 40 pct austenite at the quench stop temperature (T _Q), and annealed for 10 to 1000 seconds at or above T _Q in order to better understand the mechanisms operating during partitioning. Dilatometer measurements, transmission electron microscopy, and calculations showed that besides carbon partitioning, isothermal martensite and bainite form at the partitioning temperature. While isothermal martensite formation starts almost immediately after quenching with the rate of volume expansion dropping all the time, the beginning of bainite formation is marked by a sudden increase in the rate of expansion. The extent of its formation depends on the partitioning temperature following TTT diagram predictions. At the highest partitioning temperatures martensite tempering competes with partitioning. Small fractions of bainite and high-carbon martensite formed on cooling from the partitioning temperature. The average carbon content of the austenite retained at room temperature as determined from XRD measurements was close to the carbon content estimated from the M _s temperature of the martensite formed during the final cooling.     

Stress-induced martensite formation in Cu−Al−Ni alloys

A study has been made of superelasticity and the strain-memory effect in Cu?Al?Ni alloys in the composition range 14 wt pct Al and 2 to 3 wt pct Ni. These alloys have a bcc structure on quenching and show a low temperature martensitic transformation which is responsible for both the superelastic and strain-memory effects. Tests on both single and polycrystalline specimens showed that the maximum superelasticity occurred close toA _s. At higher temperatures the effect gradually decreased, whilst at lower temperatures it decreased very quickly. The magnitude of the effect was large in single crystal specimens (>5.8 pct), but small in polycrystal specimens (<1.5 pct). The superelastic effect was caused by stress-induced martensite (SIM). Two types of SIM were observed; thin plates of thermoelastic martensite which were always reversible, and wide plates of burst-type martensite. This burst-type martensite was responsible for the major portion of SIM, and whether it was reversible or not on removal of the stress controlled the amount of superelasticity observed. The strain-memory effect occurred on deformation either in the martensitic state (temperature <M _f) or in the temperature range where the martensite once formed was stable (temperature close toM _s). Deformation caused reorientation of the martensite plates and when the specimen was heated, the martensite disappeared and the specimen reverted back to its original shape. This effect was explained on the basis of development of martensite plates of favorable orientation on stressing.     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

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     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

Influence of strain rate and temperature on the deformation behavior of a metastable high carbon iron-nickel austenite

Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10⁻⁴ s⁻¹ and 10⁻¹ s⁻¹ over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred, the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation. Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms that the substructure following deformation will depend on the proximity of the deformation temperature to theM _s ^σ temperature. At temperatures much greater thanM _s ^σ , austenite twinning will occur, while at temperatures close toM _s ^σ , bcc martensite will form.     

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM _s toM _d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM _s toM _d range in these alloys. This paper is based upon a thesis submitted by F. LECROISEY in partial fulfillment of the degree of Doctor of Philosophy at the University of Nancy.     

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     

Pseudoelasticity and the strain-memory effect in Cu-Zn-Sn alloys

Pseudoelasticity and the strain-memory effect have been studied in alloys with compositions in the range Cu-33 to 35 wt pct Zn-3 to 3.5 wt pct Sn, having a retainedβ structure and a martensitic transformation below room temperature. The alloys show maximum pseudoelasticities of 8.5 pct for single crystals and 4.5 pct for polycrystals at temperatures close toA _f . In single crystals high elasticity is retained to at least 100°C aboveA _f but in polycrystals it decreases rapidly aboveA _f . The strain-memory effect occurs on deformation belowM _s with subsequent heating betweenA _s andA _f . The two effects are complementary, such that when one is large the other is small and vice versa. The total pseudoelastic and strain-memory recoveries are normally close to 100 pct. Both effects can be explained on the basis of the formation of a particular variant of the martensite giving significant elongations to the specimens. For pseudoelasticity, the initial structure is theβ phase and the oriented martensite reverts to theβ phase on removal of the stress. In the strain-memory effect the initial structure is oriented thermal martensite and the oriented martensite disappears only on heating to betweenA _s andA _f so that the martensite reverts to theβ matrix. L. C. BROWN, currently on leave from the Department of Metallurgy, University of Melbourne, Victoria, Australia     

The development of martensitic microstructure and microcracking in an Fe-1.86C alloy

The development of the martensitic microstructure in a 1.86 wt pct C steel has been followed by quantitative metallographic measurements over the transformation range of 0.12 to 0.50 fraction transformed (f). The transformation kinetics are described by the equationf = 1 − exp [−0.008 (M _s − T_q)] where M_s and T_q are the martensite start and the quenching temperatures respectively. Fullman’s analysis shows that the average volume per martensite plate decreases by almost an order of magnitude over the transformation range studied, but this decrease is less than that predicted by the Fisher analysis for partitioning of austenite by successive generations of martensite. Microcracking increases with increasingf up to 0.3, but does not increase forf above 0.3 where transformation proceeds by the nucleation of large numbers of small martensite plates. These observations indicate that a critical size of martensite plate is necessary to cause microcracking. Formerly Postdoctoral Fellow at Lehigh University     

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.     

Effects of carbon content and ausaging on γ ↔ α′ transformation behavior and reverse-transformed structure in Fe-Ni-Co-Al-C alloys

The effects of carbon content and ausaging on austenite γ ↔ martensite (α′) transformation behavior and reverse-transformed structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co-4Al-0.4C (wt pct) alloys. TheM _s temperature, the hardness of γ phase, and the tetragonality of α′ increase with increasing ausaging time, and these values are higher in the carbon-bearing alloys in most cases. The γ → α′ transformation behavior is similar to that of thermoelastic martensite; that is, the width of α′ plate increases with decreasing temperature in all alloys. The αt’ → γ reverse transformation temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the addition of carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4Al-0.4C alloy when the ausaged specimen is deformed at theM _s temperature and heated to 1120 K. There are two types of reverse-transformed austenites in the carbon-bearing alloy. One type is the reversed y containing many dislocations which were formed when the γ/α′ interface moved reversibly. The plane on which dislocations lie is (01 l)_γ if the twin plane is (112)_α′. The other type of reverse-transformed austenite exhibits γ islands nucleated within the α′ plates.     

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.     

Microstructure and martensitic transformations in a dual-phase α/β Cu-Zn alloy

The martensitic transformations in a dual-phase α/β Cu-Zn shape-memory alloy, containing 15 pct by volume of α particles, were studied during subcooling and deformation. The crystal structure and characteristics of the martensitic transformation of a dual-phase Cu-Zn alloy were found to be similar to those of a single-phase alloy. Both the thermal martensite formed by subcooling and the stress-induced martensite (SIM) formed by loading possessed an M9R long-period stacking-order (LPSO) structure, with internal stacking faults on the (001) basal plane. Upon subcooling, the α particles were deformed in order to accommodate the shape strain accompanying the martensitic transformation. Although most of them are deformed by slip, deformation twins have, nevertheless, been found in a few α particles. Upon loading, the SIM with an M9R structure nucleates and grows at a given temperature; subsequently, another martensite phase (α _s ) possessing an fct structure is formed, with a shear developing on the basal plane of the initial M9R SIM during further loading. However, during unloading, both the α _s and SIM are transformed and follow the reverse sequence back to the parent phase. However, some residual SIM and α _s were found at zero load, due to a constraint effect of the deformed α particles and grain boundaries. The α _s martensite may be formed by two intersecting plates of SIM or by advanced deformation on a single plate of SIM. In addition to the residual SIM and α _s martensite, an SIM/α _s lamellar martensite was found in the deformed specimen.     

Structure and mechanical properties of Fe−Cr−C−Co steels

As part of a continuing program concerning the microstructures and mechanical properties of steels in which particular attention is given to transformation substructures, the present work is concerned with martensite and bainite in Fe−Cr−C steels with and without cobalt. Although cobalt raises theM _s temperature it does not affect the extent of twinning for the same carbon level and so M _s temperature alone does not control transformation substructure. Thus cobalt is not effective in retaining dislocated martensite as carbon is increased and in this regard cobalt is not beneficial to toughness. TheM _s temperatures of the steels were relatively high and hence isothermal transformation yielded mixtures of bainites and tempered martensite depending on the temperature of transformation. The mechanical properties of the isothermally transformed steels were inferior to those of the tempered steels due to the interference of upper bainite or (tempered) martensite during the isothermal transformation. Thus, in the steels having highM _s temperatures the twinning tempered martensitic structure had relatively better mechanical properties compared to the isothermally transformed steels. Attempts to produce desirable autotempered structures by air cooling (single heat treatments) were not successful and did not improve the mechanical properties since the structure consisted of a mixture of bainite and martensite. This paper is based upon a thesis submitted by M. RAGHAVAN in partial fulfillment of the requirements of the degree of Master of Science at the University of California.