A survey of eutectoid decomposition in ten Ti-X systems

The mechanism of eutectoid decomposition in ten Ti-X binary systems, wherein X was successively Bi, Co, Cr, Cu, Fe, Mn, Ni, Pb, Pd and Pt, has been investigated. In hypo-eutectoid alloys, the bainite reaction (defined in the present context as a nonlamellar dispersion of intermetallic compound particles amongst proeutectoid a) predominated in all alloys studied but Ti-Cu, where both bainite and pear lite were formed. In alloys of near eutectoid composition pearlite was the dominant product in some systems and bainite was in others. These results are essentially independent of reaction temperature. They are quite different from analogous ones in Fe-C alloys, where pearlite is the principal eutectoid structure formed at high temperatures and bainite plays this role at low tem-peratures. The difference between the Ti-X and Fe-C behaviors was explained in terms of the much more pronounced tendency for proeutectoid α plate formation in Ti-X than in Fe-C alloys and, on a theory due to Hillert, of the need for disordered interphase boundaries in order to develop the cooperative growth mode that leads to the evolution of pearlite.     

On bainite formation

Driving force calculations for Fe-C, Fe-X-C, and Cu-Zn alloys show that the formation of bainite by a shear mechanism is thermodynamically impossible. There exist superledges on the broad faces of bainite in steels, revealing that the thickening of bainite probably proceeds by a ledge mechanism. In some Fe-Ni-C alloys and commercial steels, no simple relationship was found between the strength of austenite and theBs temperature; however, there is a linear relationship betweenBs and the diffusion coefficient of carbon and iron in austenite, as well as between the incubation period and the function containing D _Fe ^γ . The bainite reaction seems to be controlled by diffusional processes. In a low-carbon Ni-Cr steel, the morphology of the bainite/matrix interface boundary is different from that of the martensite, and the habit plane of the bainite (1 7 11)_α deviates 13.3 deg from that of the martensite (1 1 0)_α, indicating that the mechanism of the bainite reaction is not necessarily analogous to that of the martensitic transformation. At temperatures nearMs, as the driving force will be large enough, the growth of bainite by shear may be able to occur, and evidence is given by the morphology of bainite showing shear characteristic. X-ray diffraction study in Ag-Cd and internal friction measurements in Ag-Cd and Cu-Zn-Al alloys, 18CrNiWA steel, and its decarburized specimen reveal that the nucleation process occurs within the incubation period of bainite formation. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Effect of Mo Addition on the Transformation Stasis Phenomenon During the Isothermal Formation of Bainitic Ferrite

The effect of Mo addition on the transformation stasis (TS) phenomenon during bainitic transformation has been investigated in Fe-0.2C-1.5Mn-1.5Si-(0.2, 1.5, 3)Mo (wt pct) alloys. Both diffusionless and diffusional models failed in predicting the TS that is strongly affected by Mo concentration in experiment. The Gibbs energy balance approach could capture TS in alloys with 0.2 and 1.5 wt pct Mo, but significantly underestimate bainite fraction at stasis for alloy with 3 wt pct Mo, which is deduced to arise from the so-called anti-coupled solute drag effect.     

Application of the Thermodynamic Extremal Principle to Massive Transformations in Fe-C Alloys

The thermodynamic extremal principle was applied to propose a model in which trans-interface diffusion from the product phase to the interface, from the interface to the parent phase and interface migration are integrated for diffusion-controlled phase transformations in Fe-C alloys. Compared with the classical models with either a local-equilibrium condition or a constrained carbon equilibrium condition, the current model is able to predict massive transformations in the two-phase region. Application to isothermal phase transformations showed that the phase transformation mode is independent of (dependent on) trans-interface diffusion when the initial composition is close to the T₀ line (close to the α/α?+?γ boundary). Ascribed to the large solute diffusivity of C, the thickness of the spike upon massive transformations could be much larger than the atomic spacing and the diffusion-controlled phase transformations could be faster than the interface-controlled phase transformations. Three stages, i.e., the diffusive transformation, massive transformation and the soft impingement stages, were predicted for phase transformations upon continuous cooling, according to which the critical limit between diffusive and massive transformations was determined to be within the two-phase region, being consistent with the experimental results in ultra-low-carbon Fe-C alloys. The current work could be very useful to control diffusion-controlled phase transformations and modulate the mechanical properties of steels.     

New Equation for Prediction of Martensite Start Temperature in High Carbon Ferrous Alloys

Since previous equations fail to predict M _S temperature of high carbon ferrous alloys, we first propose an equation for prediction of M _S temperature of ferrous alloys containing > 2 wt pct C. The presence of carbides (Fe₃C and Cr-rich M ₇C₃) is thermodynamically considered to estimate the C concentration in austenite. Especially, equations individually specialized for lean and high Cr alloys very accurately reproduce experimental results. The chemical driving force for martensitic transformation is quantitatively analyzed based on the calculation of T ₀ temperature.     

Bainite in the light of rapid continuous cooling information

Rapid continuous cooling of pure iron can produce three different transformations yielding acicular structures: Widmanstätten α, lath martensite, and lenticular martensite. The information on their extensions into binary systems with carbon, nickel, and chromium has been reviewed, and admittedly rough methods have been used for estimating growth rates in order to examine the role of diffusion. The effect of alloying elements on their plateau temperatures and growth rates indicates that Widmanstätten α in Fe-C alloys grows under conditions close to local equilibrium for carbon, and it is suggested that the same should hold for edgewise growth of bainite. In Fe-Ni alloys, there are indications that Widmanstätten α grows under a considerable solute drag, an effect which may also occur for bainite. In Fe-Cr alloys, the solute drag effect seems to be weaker but may increase with the carbon content.     

Determination of Optimal Process Parameters of Friction Stir Welding to Join Dissimilar Aluminum Alloys Using Artificial Bee Colony Algorithm

This paper presents the efforts of joining dissimilar aluminum alloys (AA6351-T6 and AA6061-T6) by friction stir welding (FSW) process. FSW experiments are conducted according to the three factors five level central composite rotatable design method, and the response surface methodology was used to establish the empirical relationship between FSW process parameters such as tool rotational speed (N), tool traverse speed (S) and axial force (F), and the response variables such as ultimate tensile strength, yield strength, and percentage of elongation. The developed empirical models’ adequacies are estimated using the analysis of variance technique. This paper also presents the application of the artificial bee colony algorithm to estimate the optimal process parameters to achieve good mechanical properties of FS weld joints. Results suggest that the estimations of the algorithm are in good agreement with the experimental findings.     

Bainite formation in low carbon Cr-Ni steels

A low carbon Cr-Ni steel has been used to investigate the formation of upper bainite. Experimental results indicate that the start temperatures of the three morphologies of upper bainite in this steel,i.e., carbide-free bainite, bainite with carbide between and within ferrite laths, are about 600°, 500δ, and 425 °C, respectively; the habit plane of bainitic ferrite in this steel is close to (1 7 11)_α, which is 13.3 deg away from (0 ll)_α; and the orientation relationship between cementite and ferrite is consistent with Bagaryatskii’s. By means of the superelement approach, a thermodynamic treatment which applies to Fe-C alloys is extended into that suitable for low alloy steels, and calculation shows that the driving force for bainite formation at B_S temperatures is insufficient to compensate for shear strain energy. Formerly Graduate Student, Department of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China.     

Morphological Stability of <Emphasis Type="Italic">δ</Emphasis>-Ferrite/<Emphasis Type="Italic">γ</Emphasis> Interphase Boundary in Carbon Steel

The morphological changes of the δ-ferrite/γ interphase boundary have been observed in situ with a high-temperature confocal scanning laser microscope (HTCSLM) during δ/γ transformations (δ → γ and γ → δ) of Fe-0.06 wt pct C-0.6 wt pct Mn alloy, and a kinetic equation of morphological stability of δ-ferrite/γ interphase boundary has been established. Thereafter, the criterion expression for morphological stability of δ-ferrite/γ interphase boundary was established and discussed, and the critical migration speeds of δ-ferrite/γ interphase boundaries are calculated in Fe-C, Fe-Ni, and Fe-Cr alloys. The results indicate that the δ-ferrite/γ interphase boundary is very stable and nearly remains absolute planar all the time during γ → δ transformation in Fe-C alloy. The δ-ferrite/γ interphase boundary remains basically planar during δ → γ transformation when the migration speed is lower than 0.88 μm/s, and the interphase boundary will be unstable and exhibit a finger-like morphology when the migration speed is higher than 0.88 μm/s. The morphological stability of δ-ferrite/γ interphase boundary is primarily controlled by the interface energy and the solute concentration gradient at the front of the boundary. During the constant temperature phase transformation, an opposite temperature gradient on both sides of δ-ferrite/γ interphase boundary weakens the steady effect of the temperature gradient on the boundary. The theoretical analysis of the morphological stability of the δ-ferrite/γ interphase boundary is coincident with the observed experimental results utilizing the HTCSLM. There is a good agreement between the theoretical calculation of the critical moving velocities of δ-ferrite/γ interphase boundaries and the experimental results.     

Lengthening kinetics of ferrite and bainite sideplates

The rate of lengthening of ferrite and bainite sideplates and the radius of curvature of the plate edges were measured as a function of reaction temperature in three Fe-C alloys. These data were analyzed on the basis of an equation due to Trivedi. The analysis proved that the mobility of the sideplate edges is limited. The interfacial energy of these edges is of the order of 200 erg/cm². Most of the supersaturation is used to drive the diffusion of carbon in austenite; comparatively little is accounted for by capillarity and by the finite mobility of the interface. On the basis of both the present results and of published micro-structural observations, it was concluded that ferrite and bainite sideplates lengthen by a ledge mechanism.     

Computer simulation of the crystal morphology and growth rate during ultrarapid cooling of an Fe-B Melt

The solidification of a binary Fe-B melt is studied by computer simulation with regard for the temperature dependence of the diffusion coefficient and the possibility of nonequilibrium alloying-component entrapment (i.e., the dependence of the distribution coefficient on the ratio of the solidification rate V to the diffusion rate V _D). The effect of high cooling rates of the melt on the dendritic morphology is analyzed. The dependence of the dendrite-tip growth rate on the melt supercooling is obtained. At large supercoolings, a morphological transition is shown to occur; it is related to the change from a diffusion to a diffusionless dendrite growth mode.     

Thermodynamic properties of liquid alloys of the Ni-Hf system

By the method of calorimetry in isoperibolic conditions are determined integral and partial mixing enthalpies of liquid alloys of the Ni-Hf system at 1770 ± 5 K. Defined that liquid Ni-Hf alloys are formed with allocated large quantity of heat. The analysis of own and literary data has allowed to establish for mixing enthalpies of binary Ni-Hf liquid alloys dependence on temperature. With use of the Schröder equation are calculated the activities of studied alloys components from co-ordinates of liquidus line of the phase diagram of this system. Between calculated and experimentally established values of melts components activities of the Ni-Hf system is observed only qualitative consent. Are also calculated ΔG and ΔS of liquid Ni-Hf alloys.     

Role of Internal Stresses on the Incubation Times during Stress Corrosion Cracking

We have examined the incubation times in two alloys, 7075-T651 aluminum alloy and 4140 steel, as a function of applied K, using the published data in aqueous environment. The role of overloads was compared with the results from those without overloads, for a given environment. Effect of environment (NaCl vs deionized water) was also examined. The results show that in a constant K test, the incubation time increases with decreasing K. When a single overload cycle was applied, the time increased with percent overload for a constant background K, indicating that overload cycle affected the crack tip driving forces. These effects varied with the environment. The changes in the incubation times are analyzed considering one-to-one correspondence between the crack tip driving force and the times. Overloads contributed to compressive residual or internal stresses, thereby affecting the crack tip driving force. The stresses are related to changes in the plastic zone (PZ) sizes formed before and after the overloads. The effective stress intensity due to internal stress, K _int, is defined and is shown to be a function of PZ size. Similarly, condition for crack initiation is expressed as K _total = K _app ± K _int ≥ K _Iscc. A detailed methodology for the determination of K _int is outlined.     

On the Stability of Reversely Formed Austenite and Related Mechanism of Transformation in an Fe-Ni-Mn Martensitic Steel Aided by Electron Backscattering Diffraction and Atom Probe Tomography

The stability of reversely formed austenite and related mechanism of transformation were investigated against temperature and time in an Fe-9.6Ni-7.1Mn (at. pct) martensitic steel during intercritical annealing at a dual-phase (α + γ) region. Dilatometry, electron backscattering diffraction (EBSD), atom probe tomography (APT), and X-ray diffraction (XRD) were used to characterize the mechanism of reverse transformation. It was found that under intercritical annealing at 853 K (580 °C), when the heating rate is 20 K/s (20 °C/s), reverse transformation takes place through a mixed diffusion control mechanism, i.e., controlled by bulk diffusion and diffusion along the interface, where Ni controls the diffusion as its diffusivity is lower than that of Mn in the martensite and austenite. Increasing the intercritical annealing to 873 K (600 °C) at an identical heating rate of 20 K/s (20 °C/s) showed that reverse transformation occurs through a sequential combination of both martensitic and diffusional mechanisms. The transition temperature from diffusional to martensitic transformation was obtained close to 858 K (585 °C). Experimental results revealed that the austenite formed by the diffusional mechanism at 853 K (580 °C) mainly remains untransformed after cooling to ambient temperature due to the enrichment with Ni and Mn. It was also found that the stability of the reversely formed austenite by martensitic mechanism at 873 K (600 °C) is related to grain refinement.     

A discussion on the formation of bainite and other precipitates in Cu-Zn and Ag-Zn alloys

Various transformation products appearing in Cu-Zn and Ag-Zn alloys upon aging at temperatures in the α + β and single α-phase range were examined by optical and electron microscopy. Upon aging at relatively low temperature, bainite plates with the 9R structure are first formed and then retransformed to face-centered cubic (fcc). At higher temperature, other transformation products with the fcc structure, such as rod-shaped α and massive α, are concurrently observed with the bainite. Measurements of solute concentration in bainite were performed by using analytical electron microscopy. Bainite plates were found to have lower solute concentration than that of stable α, not only in Cu-Zn but also in Ag-Zn alloys. These results were discussed in terms of the free-energy change in the transformation. The first appearance of the 9R bainite in place of the stable fcc phase in low-temperature aging should be attributed to the “shear” process which lowers the nonchemical energy and diminishes the necessary driving force for the transformation. The measured solute concentration of the bainite is well reproduced theoretically on the view that the bainite is a product of shear transformation combined with the concentration change. Formerly Graduate Student, Hokkaido University This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM International’s Phase Transformations Committee.     

Widening of Laths in Bainite

Units of bainite in Fe-C alloys from the upper temperature range inherit their shape from Widmanstätten plates of ferrite, which are lathlike. The thickness increases by long-range diffusion of carbon and the length by short-range diffusion of carbon from the advancing edge of the tip. Both have been studied extensively and are fairly well understood. Widening growth seems to have been much neglected, but a study of some aspects of widening is now presented. The present report is the last one in a series of four morphological studies of bainite, isothermally formed in Fe-C alloys with 0.3 or 0.7 mass pct carbon, mainly in the upper temperature range. It contains a number of morphological observations made on cross sections of packets of bainite, and it elucidated a number of interesting questions about bainite and resulted in some proposals. The ferrite plates in a packet are nucleated as a group on a grain boundary, not each one separately on the side of a prior plate. Lengthening occurs by advancement of a short edge that is formed in close contact to the grain boundary. Widening of laths does not start spontaneously. It is initiated by a modification of the structure of the long edge of the lath. When it then moves, the lattice of the new ferrite is rotated relative to the ferrite formed by lengthening and the habit plane is different. In a section through the length direction, it is difficult to recognize what part of ferrite has formed by widening growth. Furthermore, it is proposed that the individual plates in a microstructure, previously used to illustrate subunits formed by repeated nucleation, were nucleated on a hidden grain boundary.     

Fractality and reversibility of ferrous martensite

Martensite formation is characterized by a diffusionless structural phase transformation from austenite to martensite, associated with a considerable amount of lattice variant shear γ_γα ? 0.2. Ferrous martensite shows all possible features connected with the transformation. Different modes of initiation of the martensite formation are possible. The reasons for the burst phenomenon can be considered as an analogy to discontinuous yielding. The transformation only procedes if further thermodynamical driving force is provided by cooling or shear stress. In some cases fractal microstructures are formed in which several fragmentations can be recognized. In contrast to shape memory alloys, steels usually do not show reversibility of the reverse α → γ transformation. The factors which favour reversibility have been defined. Knowledge of these is necessary for the development of iron-base shape-memory alloys.     

Plasma Nitriding Behavior of Fe-C-M (M = Al, Cr, Mn, Si) Ternary Martensitic Steels

Change in surface hardness and nitrides precipitated in Fe-0.6C binary and Fe-0.6 mass pct C-1 mass pct M (M = Al, Cr, Mn, Si) ternary martensitic alloys during plasma nitriding were investigated. Surface hardness was hardly increased in the Fe-0.6C binary alloy and slightly increased in Fe-0.6C-1Mn and Fe-0.6C-1Si alloys. On the other hand, it was largely increased in Fe-0.6C-1Al and Fe-0.6C-1Cr alloys. In all the Fe-0.6C-1M alloys except for the Si-added alloy, fine platelet alloy nitrides precipitated inside martensite laths. In the Fe-0.6C-1Si alloy, Si-enriched film was observed mainly at a grain boundary and an interface between cementite and matrix. Crystal structure of nitrides observed in the martensitic alloys was similar to those in Fe-M binary ferritic alloys reported previously. However, there was a difference in hardening behavior between ferrite and martensite due to a high density of dislocations acting as a nucleation site of the nitrides and partitioning of an alloying element between martensite and cementite changing the driving force of precipitation of the nitrides.     

Orientation of Magnetized MnBi in a Strong Static Magnetic Field

Solidification of Bi-4.5 wt pct Mn alloy was investigated in the presence and absence of a strong static magnetic field (SSMF). A cooling rate (R) of 60 K/min caused MnBi to orient with the SSMF, owing to the force moment and attractive force. The attractive force and magnetic gradient force induced formation of multilayered MnBi when R was 5 K/min. The magnetic gradient force was damped when R was 60 K/min. Low cooling rates favored the aggregation process.