Solidification of undercooled Fe-Cr-Ni alloys: Part I. Thermal behavior

Solidification of undercooled Fe-Cr-Ni alloys was studied by high-speed pyrometry during and after recalescence of levitated, gas-cooled droplets. Alloys were of 70 wt pct Fe, with Cr varying from 15 to 19.7 wt pct, balance was Ni. Undercoolings were up to about 300 K. Alloys of Cr content less than that of the eutectic (18.1 wt pct) have face-centered cubic (fee) (austenite) as their equilibrium primary phase, and alloys of higher Cr content have body-centered cubic (bcc) (ferrite) as their equilibrium primary phase. However, except at low undercoolings in the hypoeutectic alloys, all samples solidified with bcc as the primary phase; the bcc then transformed to fcc during initial recalescence for the lower Cr contents or during subsequent cooling for the higher Cr contents. The bcc-to-fcc transformation, whether in the semisolid or solid state, was detected by a second recalescence. In the hypoeutectic alloys, the growth of primary metastable bcc apparently results from preferred nucleation of bcc. The subsequent nucleation of fcc may occur at bcc/bcc grain boundaries. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10 and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystallization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part I. Theoretical analysis of nucleation behavior

The selection of the primary solidifying phase in undercooled stainless steel melts is theoretically analyzed in terms of nucleation theory. Nucleation phenomena are considered using different models for the solid-liquid interface energy. The classical nucleation theory for sharp interfaces and an improved modification, the diffuse interface theory, are applied. The influence of deviations of the nucleus composition from the overall alloy composition is also revealed. A preferred nucleation of the metastable bcc phase in fcc equilibrium solidification-type alloys is predicted. The critical undercooling of metastable crystallization as a function of alloy composition is calculated for an isoplethal section at 69 at. pct Fe of Fe₆₉Cr_31-x Ni _x alloys. The results are summarized in a phase selection diagram predicting the primary solidification mode as a function of undercooling and melt composition.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part II. Containerless solidification experiments

The solidification behavior of undercooled Fe-Cr-Ni melts of different compositions is investigated with respect to the competitive formation of δ-bcc (ferrite) and γ-fcc phase (austenite). Containerless solidification experiments, electromagnetic levitation melting and drop tube experiments of atomized particles, show that δ (bcc) solidification is preferred in the highly undercooled melt even at compositions where δ is metastable. Time-resolved detection of the recalescence events during crystallization at different undercooling levels enable the determination of a critical undercooling for the transition to metastable bcc phase solidifcation in equilibrium fcc-type alloys. Measurements of the growth velocities of stable and metastable phases, as functions of melt undercooling prior to solidification, reveal that phase selection is controlled by nucleation. Phase selection diagrams for solidification processes as functions of alloy composition and melt undercooling are derived from two types of experiments: X-ray phase analysis of quenched samples and in situ observations of the recalescence events of undercooled melts. The experimental results fit well with the theoretical predictions of the metastable phase diagram and the improved nucleation theory presented in an earlier article. In particular, the tendency of metastable δ phase formation in a wide composition range is confirmed.     

Incipient chemical instabilities of nanophase Fe-Cu alloys prepared by mechanical alloying

We used Mössbauer spectrometry, X-ray diffractometry, a novel imaging method of electron energy loss spectrometry, and small-angle neutron scattering (SANS) to study early stage thermal instabilities of nanophase Fe-Cu alloys prepared by mechanical attrition. Mössbauer spectrometry confirmed previous reports of an extended Cu solubility in the body-centered cubic (bcc) phase of the as-milled material. Mössbauer spectrometry also provided evidence that in the compositional range of bcc-face-centered-cubic (fcc) two-phase coexistence, the bcc phase had a Cu concentration nearly the same as the overall composition of the alloy. After the as-milled powders were annealed at temperatures as low as 200 °C, however, Mössbauer spectrometry showed significant chemical unmixing of the Cu and Fe atoms. In annealed bcc Fe-20 pet Cu alloys, SANS measurements indicated that Cu segregated to grain boundaries. This segregation of Cu atoms to bcc grain boundaries did not alter significantly the tendency for grain growth, however. X-ray diffractometry showed that grain growth during thermal annealing was similar for all alloys, although grain growth was small at temperatures below 300 °C. The two-phase (bcc plus fcc) alloy of Fe-30 pc Cu was more unstable against chemical segregation than were the single-phase (bcc or fcc) alloys. Energy-filtered imaging indicated that the Cu atoms segregated to regions around the bcc grains, perhaps to the adjacent fcc crystallites.     

Nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in Fe-C-X alloys

The nucleation kinetics of proeutectoid ferrite allotriomorphs at austenite grain boundaries in Fe-0.5 at. Pct C-3 at. Pct X alloys, where X is successively Mn, Ni, Co, and Si and in an Fe-0.8 at. Pct C-2.5 at. Pct Mo alloy have been measured using previously developed experimental techniques. The results were analyzed in terms of the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries. Classical nucleation theory was employed in conjunction with the pillbox model of the critical nucleus applied during the predecessor study of ferrite nucleation kinetics at grain boundaries in Fe-C alloys. The free energy change associated with nucleation was evaluated from both the Hillert-Staffanson and the Central Atoms Models of interstitial-substitutional solid solutions. The grain boundary concentrations of X determined with a Scanning Auger Microprobe were utilized to calculate the reduction in the austenite grain boundary energy produced by the segregation of alloying elements. Analysis of these data in terms of nucleation theory indicates that much of the influence of X upon ferrite nucleation rate derives from effects upon the volume-free energy change,i.e., upon alterations in the path of theγ/(α + γ) phase boundary. Additional effects arise from reductions in austenite grain boundary energy, with austenite-forming alloying elements being more effective in this regard than ferrite-formers. By difference, the remaining influence of the alloy elements studied evidently results from their ability to diminish the energies of the austenite: ferrite boundaries enclosing the critical nucleus. The role of nucleation kinetics in the formation of a bay in the TTT diagram of Fe-C-Mo alloys is also considered. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University     

Formation of a new phase after high-temperature annealing and air cooling of an Fe-Mn-Al alloy

The observation of a new phase precipitated in the bcc matrix of a Fe-Mn-Al alloy heated at 1573 K and air cooled to room temperature is reported. The composition of the alloy was Fe-24.1 wt pct Mn-7.6 wt pct Al-0.03 wt pct C. These precipitates had a morphology of Widmanstätten side plates distributed uniformly in bcc grains, and some precipitates resided along the bcc grain boundaries. From transmission electron microscopy (TEM) and X-ray analyses, the crystal structure of the new phase was identified as a simple cubic (SC) Bravais lattice and was related to an ordered fcc phase. The new phase is similar to the L1₂ crystal structure. The orientation relationships between the SC Widmanstätten side plate and the ferrite matrix are [011]_SC//[111]_bcc and \((1\bar 11)\) _SC//\((10\bar 1)\) _bcc, which correspond to the well-know Kurdjumov-Sachs (K-S) orientation relationship. The formation of SC annealing twins from the ferrite phase at the initial stage of the nucleation during air cooling was observed for the first time in the Fe-Mn-Al alloys. It is noted that both twin grains grow within the ferrite matrix and maintain both the K-S orientation relationship with the parent ferrite phase and the twinning orientation relationship between them.     

Phase distributions in rapidly solidified stainless steels

Phase distributions and the internal magnetic fields have been determined in rapidly solidified stainless steels (Fe-nCr-8Ni-0.05C, Fe-nCr-5Ni, and Fe-nCr withn in the range of 10 to 24) by transmission and conversion electron Mössbauer spectroscopy (TMS and CEMS). Based on these results, a modification of the phase boundaries in the Schaeffler diagram is suggested to account, in particular, for rapidly solidified stainless steels. The suggested modification is primarily an expansion of the austenite field toward higher Cr and lower Ni equivalent contents. Combining CEMS and TMS makes it possible to determine the phase distributions both in the near surface region (outmost 300 nm) and in the bulk of the ribbons. For the low-Cr alloys, the content of the bcc phase (martensite) in the surface region is higher than in the sample as a whole. In the high-Cr alloys, the content of the bcc phase (ferrite) is lower in the surface than in the bulk. This disparity is ascribed to the different mechanisms of formation of martensite (diffusionless) and ferrite (nucleation and growth) in relation to the higher cooling rates of the surface layers. The determinations of the internal magnetic field are in good agreement with earlier investigations on conventionally processed Fe-Cr steels, where it was found that the internal magnetic field decreases with increasing Cr content.     

合金元素Ni对Fe-Cu合金析出过程的影响

以添加Ni前后的Fe-Cu合金和Fe-Cu-Ni合金为研究对象,采用金相显微镜(OM)观察了Fe-Cu合金与Fe-Cu-Ni合金时效过程中的显微组织形貌变化,借助硬度测试分析了两种合金在等温时效过程中的硬度变化规律,并利用透射电子显微技术(TEM)进行了两种合金的析出相精细结构观察与衍射分析,在此基础上,通过实验与理论...     

Structural Characterization of Sputter-Deposited 304 Stainless Steel+10 wt pct Al Coatings

An SS304?+?10?wt pct Al (with a nominal composition of Fe-18Cr-8Ni-10Al by wt pct and corresponding to Fe-17Cr-6Ni-17Al by at. pct) coating was deposited on a 304-type austenitic stainless steel (Fe-18Cr-8Ni by wt pct) substrate by the magnetron sputter-deposition technique using two targets: 304-type stainless steel (SS304) and Al. The as-deposited coatings were characterized by X-ray diffraction, transmission electron microscopy, and three-dimensional (3-D) atom probe techniques. The coating consists of columnar grains with ?? ferrite with the body-centered cubic (bcc) (A2) structure and precipitates with a B2 structure. It also has a deposition-induced layered structure with two alternative layers (of 3.2 nm wavelength): one rich in Fe and Cr, and the other enriched with Al and Ni. The layer with high Ni and Al contents has a B2 structure. Direct confirmation of the presence of B2 phase in the coating was obtained by electron diffraction and 3-D atom probe techniques.     

Solidification of undercooled Fe-Cr-Ni alloys: Part III. Phase selection in chill casting

In Parts I and II of this series of articles, it was shown that a range of levitation-melted Fe-Cr-Ni alloys, both hypoeutectic and hypereutectic, all solidified with the hypereutectic phase (bcc) as their primary phase, except for the hypoeutectic alloys at low undercoolings. In this article, the effect of heat extraction on phase formation is studied by chill casting the undercooled alloys before nucleation. Two of the previously studied alloys are examined; one hypoeutectic and the other hypereutectic. Chill substrates employed were copper, stainless steel, alumina, zirconia, and a liquid gallium-indium bath. Contrary to the case of levitation melting and solidification, it is found that the dominant primary phase to solidify in both alloys, independent of chill substrate, is the hypoeutectic phase (fcc). It is concluded that chilling the undercooled melt results in nearly concurrent nucleation of bcc and fcc. Two different mechanisms are considered as possible explanations of the subsequent fcc phase selection during growth. These are termed “growth velocity” and “phase stability” mechanisms. Evidence favors a phase stability mechanism, in which the bcc phase massively transforms to fcc early in solidification so that fcc then grows without competition. It is suggested that this mechanism may also explain structures observed in welds and other rapid solidification processes.     

Solidification of undercooled Fe-Cr-Ni alloys: Part II. Microstructural evolution

Results are reported on microstructures of Fe-Cr-Ni alloys, solidified over a range of undercoolings and quenched during or after recalescence. Alloys studied contained 70 wt pct Fe and with Cr varying from approximately 15 to 20 wt pct. The three lower Cr alloys were hypoeutectic (with fee as primary phase in equilibrium solidification); the two higher Cr alloys were hypereutectic (with bcc as primary phase in equilibrium solidification). Results obtained are in agreement with predictions based on thermal analyses previously presented; they confirm and extend the understanding gained in that work. The primary phase to solidify in the hypoeutectic alloys is bec when undercooling is greater than an amount which decreases with increasing Cr content. At the lower Cr contents, the stable fcc phase then forms by solid-state transformation of the metastable phase and its subsequent engulfment by additional fcc. At the higher Cr content, transformation is by a peritectic-like reaction in the semisolid state, except near the surface at higher undercoolings where the transformation is massive. In the hypereutectic alloys, primary solidification at all undercoolings is the stable bcc phase. Partial transformation to fcc occurs in the semisolid or solid state, depending on composition and undercooling. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

The decomposition of austenite in a high purity iron-chromium-nickel alloy

This paper presents the results of a kinetic and microstructural study into the formation of ferrite from austenite in a high purity Fe-7 pct Cr-2 pct Ni alloy. Electron microscopy has been used extensively to obtain detailed information concerning the fine scale microstructure of the reaction products, and the morphology of the interphase boundary present during the transformation. The results of this investigation have shown that two ferrite reactions, dissimilar in both kinetics and resultant microstructure, exist in this alloy, and that the ferrite morphologies obtained are essentially similar to those found in many transformable alloy steels.     

Microstructural transitions during containerless processing of undercooled Fe-Ni alloys

The microstructural development associated with solidification in undercooled Fe-Ni alloys has been reported in different studies to follow various pathways, with apparent dissimilarities existing as a function of sample size and processing conditions. In order to identify the possible hierarchy of microstructural pathways and transitions, a systematic evaluation of the microstructural evolution in undercooled Fe-Ni alloys was performed on uniformly processed samples covering seven orders of magnitude in volume. At appropriate undercooling levels, alternate solidification pathways become thermodynamically possible and metastable product structures can result from the operation of competitive solidification kinetics. For thermal history evaluation, a heat flow analysis was applied and tested with large Fe-Ni alloy particles (1 to 3 mm) to assess undercooling potential. Alloy powders (10 to 150 μm), with large liquid undercoolings, were studied under the same composition and processing conditions to evaluate the solidification kinetics and microstructural evolution, including face-centered cubic (fcc)/body centered cubic (bcc) phase selection and the thermal stability of a retained metastable bcc phase. The identification of microstructural transitions with controlled variations in sample size and composition during containerless solidification processing was used to develop a microstructure map which delineates regimes of structural evolutions and provides a unified analysis of experimental observations in the Fe-Ni system.     

Microstructural Evolution of Gas Atomized Fe-25Cr-3.2C Alloy Powders

 The microstructural evolution of the gas atomized Fe-25Cr-32C powders was investigated by using optical microscope, scanning electron microscope, and X-ray diffraction. The experimental results showed that the atomized Fe-25Cr-32C powders were mainly composed of austenite and (Fe,Cr)7C3 carbide. Eutectic microstructure was developed in the larger particles, whereas dendritic microstructure was obtained in the particles with diameter less than 38 μm. The reason for microstructure change should be the difference of nucleation undercooling for particles.     

Microstructural characterization

Rapidly solidified microstructures of Fe-Cr-W-C quaternary alloy deposited on low-carbon steel by laser cladding were investigated. The clad-coating alloy, a powder mixture of Fe, Cr, W, and C with a weight ratio of 10:5:1:1, was processed with a high-power continuous wave CO₂ laser. The developed clad coatings possessed fine microstructures, uniform distributions of al- loying elements, and high microhardness. Analytical electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the crystal structures and microchemistries of the various phases in the clad coatings. The laser processed microstructure comprised fine primary dendrites of a face-centered cubic (fcc) austenitic y phase and interdendritic eutectic consisting of a network of pseudohexagonal M₇C₃ carbides rich in Cr in an fcc austenitic γ phase. The interlamellar spacing in the eutectic matrix was about 20 nm. The relatively high microhardness, about 900 kg_f/mm², of such fine microstructures is attributed to the formation of complex ter- nary carbides uniformly distributed in the eutectic matrix. In situ transmission electron micros- copy (TEM) of thermally treated clad coatings revealed that transformation of the primary γ phase to body-centered cubic (bcc) ferrite (α phase) commenced after heating at 843 K for about 7 minutes. The transformation initiated at the interface of the primary dendrites and the eutectic and propagated gradually into the primary phase. Phase change of the interdendritic γ austenite to a bcc α ferrite occurred after about 30 minutes of hold period at 843 K. Transformation of the M₇C₃ carbides did not occur even after heating at 843 K for about 3.2 hours. The growth of a thin M₂O₃ (M = Fe, Cr) oxide scale was detected after heating at 843 K for approximately 24 minutes. After cooling gradually to room temperature, the softened (723 kgy/mm²) micro- structure consisted of primary dendrites with a bcc α ferrite crystal structure and interdendritic ternary eutectic of untransformed M₇C₃ carbides in α ferrite.     

Effect of Manganese on Microstructures and Solidification Modes of Cast Fe-Mn-Si-Cr-Ni Shape Memory Alloys

We investigated microstructures and solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni shape memory alloys to clarify whether Mn was an austenite former during solidification. Furthermore, we examined whether the Cr_eq/Ni_eq equations (Delong, Hull, Hammer and WRC-1992 equations) and Thermo-Calc software^® together with database TCFE6 were valid to predict the solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni shape memory alloys. The results have shown that the solidification modes of Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni alloys changed from the F mode to the FA mode with increasing the Mn concentration. Mn is an austenite former during the solidification for the cast Fe-Mn-Si-Cr-Ni shape memory alloys. The Delong, Hull, Hammer, and WRC-1992 equations as well as Thermo-Calc software^® together with database TCFE6 are invalid to predict the solidification modes of cast Fe-(13-27)Mn-5.5Si-8.5Cr-5Ni SMAs. To predict the solidification modes of cast Fe-Mn-Si-Cr-Ni alloys, a new Cr_eq/Ni_eq equation should be developed or the thermodynamic database of Thermo-Calc software^® should be corrected.     

Cracking kinetics of two-phase stainless steel alloys in hydrogen gas

The kinetics of hydrogen-induced slow crack growth (SCG) under constant load was studied in two stainless steel alloys containing mixtures of bcc and fcc phases. FERRALIUM 255, a duplex stainless steel, consisting of ∼50 pct austenite in a ferrite matrix, was tested in hydrogen gas at 0 to 100 °C with the loading axis both perpendicular and parallel to the rolling direction. In addition, specimens of AISI 301 were deformed in air in different ways to produce various amounts of bcc phase in an austenite matrix prior to testing in H₂ gas at room temperature. The kinetics of subcritical slow crack growth (SCG) in these alloys was compared with that for austenitic and for ferritic stainless steels. The SCG rates were rationalized in terms of differences in hydrogen permeation in the two phases. The results confirm that a higher rate of supply and accumulation of hydrogen in the region ahead of the crack tip allows a higher cracking velocity.     

Growth of intragranular ferrite in Fe-Ni-P alloys

Analytical electron microscopy (AEM) techniques were used to study the growth of intragranular ferrite in Fe-Ni-P alloys. The spatial resolution of the AEM was exploited to gather microchemical information regarding elemental redistribution at ferrite/austenite interfaces in order to determine the growth mechanism for intragranular ferrite. In this alloy system, the growth kinetics are dictated by the bulk diffusion of Ni in austenite. Full equilibrium occurs during intragranular ferrite growth with full partitioning of Ni and P between austenite and ferrite, and chemical equilibrium occurs at the α/γ interface in both phases. A numerical model to simulate ferrite growth was developed based on equilibrium growth considerations. The Ni concentrations and precipitate sizes predicted by the model agree well with those measured by AEM techniques in the experimental alloys. The computer model has been extended to predict the thermal histories of iron meteorites and their parent asteroidal bodies.