A rapid magnetometric technique to plot isothermal transformation diagrams

A sensitive magnetic permeability method for rapid determination of isothermal transformation diagrams in steels and iron base alloys is described. The method consists of quenching the sample from an austenitizing temperature to a subcritical temperature in an isothermal bath, and holding it within the magnetic field of an inductor coil. The increase in permeability accompanying austenite decomposition increases the inductance of the coil, and this changes the resonant frequency of the circuit. An automatic continuous recording of the corresponding period provides a convenient and accurate method for following the austenite decomposition of AISI 4340 in the bainite and martensite temperature ranges. This method provides quantitative information on austenite decomposition kinetics within two seconds after the start of quenching. Formerly Graduate Student, University of California, Berkeley     

Isothermal martensitic transformation in Fe-Ni and Fe-Ni-C alloys at subzero temperatures

A survey of experimental work on the isothermal martensitic transformation in the presence of prior martensite is given in Fe-Ni and Fe-Ni-C alloys having subzero Ms (Mb) temperatures. Low temperature dilatometric measurements are correlated with internal friction measurements in the 5 to 200 K temperature range. This study will show that this transformation can be discussed in terms ofC-curve behavior as in Fe-Ni-Mn alloys. Nevertheless, in Fe-Ni and Fe-Ni-C alloys internal stresses created by the austenite-to-martensite transformation during cooling play an important part in the development of the isothermal transformation. Furthermore, internal friction is shown to be proportional to expansivity thus indicating that the internal friction technique can be applied successfully for the study of phase transformations.     

Development of Multiphase Microstructure with Bainite, Martensite, and Retained Austenite in a Co-Containing Steel Through Quenching and Partitioning (Q&P) Treatment

The quenching and partitioning (Q&P) treatment of steel aims to produce a higher fraction of retained austenite by carbon partitioning from supersaturated martensite. Q&P studies done so far, relies on the basic concept of suppression of carbide formation by the addition of Si and/or Al. In the present study Q&P treatment is performed on a steel containing 0.32 C, 1.78 Mn, 0.64 Si, 1.75 Al, and 1.20 Co (all wt pct). A combination of 0.64 Si and 1.75 Al is chosen to suppress the carbide precipitation and therefore, to achieve carbon partitioning after quenching. Addition of Co along with Al is expected to accelerate the bainite transformation during Q&P treatment by increasing the driving force for transformation. The final aim is to develop a multiphase microstructure containing bainite, martensite, and the retained austenite and to study the effect of processing parameters (especially, quenching temperature and homogenization time) on the fraction and stability of retained austenite. A higher fraction of retained austenite (~13 pct) has indeed been achieved by Q&P treatment, compared to that obtained after direct-quenching (2.7 pct) or isothermal bainitic transformation (9.7 pct). Carbon partitioning during martensitic and bainitic transformations increased the stability of retained austenite.     

Correlation of isothermal bainite transformation and austenite stability in quenching and partitioning steels

The possible decomposition of metastable austenite during the partitioning process in the highend quenching and partitioning(QP)steels is somewhat neglected by most researchers.The effects of primary martensite and alloying elements including manganese,cobalt and aluminum on the isothermal decomposition of austenite during typical QP process were studied by dilatometry.The transformation kinetics was studied systematically and resulting microstructures were discussed in details.The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries.This effect can be eliminated by a flash heating which recovered dislocations.Co addition significantly promoted the bainite transformation during partitioning while Al and Mn suppressed the isothermal bainite transformation.The bainite transformation played an important role in carbon distribution during partitioning,and hence the amount and stability of austenite upon final quenching.The bainite transformation during partitioning is an important factor in optimizing the microstructure in QP steels.     

Formation of hcp martensite during the isothermal aging of an fcc Co-27Cr-5Mo-0.05C orthopedic implant alloy

The evolution of the microstructure of a Co-27Cr-5Mo-0.05C alloy was investigated during isothermal aging between 650 °C and 950 °C. The main structural change observed as a result of aging was an fcc (metastable)→hcp isothermal martensitic transformation. The relationships between transformation, temperature, and time for this phase transition were determined using two different techniques: (1) room-temperature X-ray diffraction on samples aged after quenching from 1150 °C to 25 °C and (2) high-temperature in situ X-ray diffraction on samples cooled at 50 °C/min from 1150 °C to the aging temperature. The results show that the intermediate water quench significantly retards the kinetics of the phase transition by up to one order of magnitude in time. In addition, it was found that the grain size of the metastable fcc phase prior to aging does not affect the kinetics of the transformation. Age hardening resulting from this transformation varies linearly with the amount of hcp phase formed during the isothermal treatment and does not depend on the aging temperature. It is suggested that local plastic deformation, due to thermal and transformation stresses induced by quenching, reduces the number of hcp martensite embryos formed in the metastable fcc phase. This effect decreases the number of nucleation sites available for the fcc→hcp transformation during isothermal aging and leads to the slower transformation rates observed in water-quenched material.     

The isothermal decomposition of austenite in a high purity Iron-Chromium binary alloy

The transformation characteristics of a series of high purity iron-chromium alloys, within the γ-loop composition range, have been studied using continuous-cooling dilatometry. An Fe-10 wt pct Cr alloy, which exhibited a relatively slow γ → α transformation, was chosen for detailed investigation by isothermal dilatometry, and by optical and transmission electron microscopy. The TTT diagram exhibited a high-temperature Ccurve in which the transformation products were equiaxed ferrite and Widmanstätten ferrite, the latter developing from perturbations on the α:γ interface. In this range, a ledge mechanism was the predominant mode of ferrite growth. A simple activation energy analysis suggests that the γ→ α reaction is interface controlled, and supports the existence of a “solute-drag” effect by carbon even at low concentrations. Direct quenching of the alloy produced martensite. In the intermediate temperature range, it is proposed that the γ→ α reaction is bainitic in character.     

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.     

Shape memory behavior of FeNiCoTi single and polycrystals

We present experimental and theoretical evidence of thermoelastic martensites in Fe29Ni18Co4Ti alloys. In this class of alloys, the high strength in the austenite domains limits the slip deformation as verified with transmission electron microscopy. The restriction of slip permits a higher degree of recoverability of the transformation. Using both single crystals with [123] orientation and polycrystals, the appearance of martensite plates upon deformation, and their reversion back to austenite upon heating (the shape memory effect), is revealed with in-situ optical microscopy. Theoretical results for the transformation strains and the detwinning of martensite are presented, which demonstrate convincingly the potential of these classes of alloys. Electrical resistance measurements identified the stress and temperature levels at the onset of forward and reverse transformations in isothermal deformation and thermal cycling experiments, respectively. The return of the electrical resistance to its reference value, upon austenite to martensite followed by martensite to austenite transformation, verified the recovery in the transformation strains measured in the experiments.     

The synthetic isothermal martensitic transformation in high carbon Fe-Ni-C steel

For the analysing of the results from earlier hydrostatic isothermal tests a new athermal test series was conducted. Suitable amounts of martensite were brought into the specimens by quenching. From these specimens the amount of martensite, number of martensitic plates per unit area of random section and hardness were measured. Using the measured data, the effects of the mechanical surroundings around and inside the specimen were taken up for consideration. It seems that the mechanical surroundings inside the specimen have an important effect on martensitic transformation. It also seems that there are temperature dependent components in the transformation. It may be that the temperature dependence of nucleation needed plastic deformation and the temperature dependence of relaxations.     

Martensitic transformation cycling effects in Cu-Zn-Al shape memory alloys

The effects of transformation cycling between the parent (P) and martensite (M) phases of selected Cu-Zn-Al: alloys were studied. Using differential scanning calorimetry it was observed that the martensite start temperature, M_s, progressively shifts upward with cycling. Also, the proportion of the microstructure actually undergoing P↔M transformation decreases. Transmission electron microscope observations reveal dislocation substructures generated by martensite transformation and reversion.     

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     

AM对低碳贝氏体钢等温相变和显微组织的影响

摘要：设计了马氏体起始相变温度（Ms）以上和以下5个不同温度等温淬火实验,研究了Ms以上和以下温度等温淬火对低碳贝氏体钢组织和相变动力学的影响。结果表明,试样在Ms以下等温淬火时,保温前生成的先马氏体（AM）显著缩短了等温贝氏体相变孕育期,加速贝氏体形核,细化贝氏体组织。然而,Ms以下等温淬火时,总的等温贝氏体相变动力学与先马氏体的体积分数（fAM）有很大关系,当fAM较低时,AM的形成缩短了贝氏体相变孕育期,加速了贝氏体相变,当fAM过高时,又阻碍贝氏体相变,延长贝氏体总的相变时间。最后,采用Austin Rickett(AR)和Johnson Mehl Avrami Kolgomorov(JMAK)动力学模型对等温贝氏体相变动力学进行分析,结果表明,与AR模型相比,JMAK模型更适用于本研究的实验结果。     

Isothermal martensitic transformation under hydrostatic pressure in an FeNiC alloy at low temperatures

The isothermal martensitic transformation under applied hydrostatic pressure has been investigated with an Fe21.5Ni0.95C alloy single crystal by electrical resistivity, magnetic susceptibility, X-ray diffraction and optical microscopy. The martensitic transformation starting temperature, M_s, is lowered by applied hydrostatic pressure. The isothermal martensitic transformation occurs first as a burst under a critical pressure and it is continued by further transformation with decreasing hydrostatic pressure. The critical pressure under which the isothermal martesitic transformation starts changes curvilinearly with decreasing temperature. The morphology of the isothermal martensite formed under hydrostatic pressure is similar to that of athermal martensite. The temperature dependence of the critical pressure under which martensitic transformation starts has been calculated based on the homogeneous nucleation model and the heterogeneous nucleation model.     

Microstructual Evolution of a Nb‐Microalloyed Advanced High Strength Steel Treated by Quenching‐Partitioning‐Tempering Process

The recently developed “quenching and partitioning” heat treatment and “quenching‐partitioning‐tempering” heat treatment are novel processing technologies, which are designed for achieving advanced high strength steels (AHSS) with combination of high strength and adequate ductility. Containing adequate amount of austenite phase is an important characteristic of the above steel, and the partitioning treatment is a key step in Q&P or Q‐P‐T process during which the austenite phase is enriched with carbon and achieves thermal stability. However, the microstructural evolution of the steel during the partitioning process is rather complicated. In present study, evolution of complex microstructure in a low carbon steel containing Nb during the Q‐P‐T process has been studied in detail. The microstructural evolution of the steel was investigated in terms of X‐ray diffraction, scanning electron microscope and transmission electron microscope. The experimental results show that the Nb‐microalloyed steel demonstrates a complex multiphase microstructure which consists of lath martensite with high dislocation density, retained austenite, alloy carbide, transition carbide, and a few twin martensite after the Q‐P‐T process. The experimental results can be helpful for the design of Q‐P‐T heat treatment and for the control of mechanical properties of Q‐P‐T steel.     

The Effects of Composition and Aging on the Martensite and Magnetic Transformations in Ni-Fe-Ga Ferromagnetic Shape Memory Alloys

The martensite and magnetic transformations in Ni-Fe-Ga ferromagnetic shape memory alloys are very sensitive to both alloy chemistry and thermal history. A series of Ni-Fe-Ga alloys near the prototype Heusler composition (X₂YZ) were used to investigate how the martensite and magnetic transitions change with alloy composition and isothermal aging above and below the B2/L2₁ ordering temperature. Calorimetry and magnetometry were employed to measure the martensite transformation temperatures and Curie temperatures. Compositional variations of only a few atomic percent result in martensite start temperatures and Curie temperatures that differ by about 230 and 35 K, respectively. Aging a Ni₅₃Fe₁₉Ga₂₈ alloy for 3600 seconds at various temperatures shifts the martensite start temperature and the Curie temperature by almost 70 K. Transmission electron microscopy investigations were conducted on the aged Ni₅₃Fe₁₉Ga₂₈ alloy. The considerable variations in the martensite and magnetic transformations in these alloys are discussed in terms of microstructural differences resulting from alloy chemistry and aging treatments. This article is based on a presentation made in the symposium “Phase Transformations in Magnetic Materials: Magnetic Shape Memory Alloys which occurred March 14, 2006, during the TMS Spring Meeting in San Antonio, TX, under the auspices of the ASMI/MPMD-Phase Transformations, EMPMD/SMD-Chemistry & Physics of Materials, and EMPMDNanomaterials Committees."     

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.     

Influence of aging on transformation characteristics in shape memory CuZnAl alloys

Shape memory alloys exhibit superelasticity when they are deformed in a temperature range where the thermoelastic martensite forms on application of a strain. The martensite persists upon removal of the applied strain, and the alloy recovers the original shape on heating over the reverse-transformation temperature after removing the strain. The β-phase CuZnAl alloys have β-type superlattice in the parent case, and M9R or M18R martensites occur on quenching the alloys from the homogenization temperature. The basal plane of martensite is exposed to hexagonal distortion with martensitic transformation as well as the monoclinic distortion in the crystal structure, and splittings are observed in some selected diffraction-peak pairs due to the differences in atom sizes in lattice points. These pairs have a great importance as ordering criteria and satisfy a special relation between Miller indices. The present text reports the variation of the differences in interplane spacings (Δd) between some selected planes upon the further aging at room temperature at which alloys are fully martensitic. The decrease of Δd during the aging implies that the monoclinic distortion decreases. The mass increases are caused by the oxidation upon heating the alloys at high temperatures close to the betatizing temperature at free atmosphere.     

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.     

Microstructure of Low C Steel Isothermally Transformed in the M S to M f Temperature Range

An isothermal transformation was observed when a fully austenitized lean-alloyed, low C steel was quenched to a temperature in the M _S to M _f temperature range and held at the quenching temperature. The dilatometric analysis revealed that the isothermal transformation was distinct from the bainitic transformation. Internal friction (IF) measurements and X-ray diffraction (XRD) analysis showed that the dislocation density in the isothermal transformation product was larger than in lower bainite, and lower than in athermal martensite. Microstructural analysis by transmission electron microscopy (TEM) revealed that the isothermal transformation product had a specific microstructure consisting of large lath-type constituent units with wavy boundaries, with a Nishiyama?CWassermann orientation relationship (NW OR) with respect to the parent austenite. The isothermal transformation below M _S proceeds by the thickening of athermally formed laths.