Microstructural investigation of the oxidation of an Fe-3 pct Cr alloy

The microstructure and microchemistry of the oxide scale on an Fe-3 wt pct Cr alloy have been investigated after oxidation in the temperature range 700 to 800 °C. Transmission and scanning electron microscopy along with energy dispersive X-ray analysis and Auger electron spectroscopy techniques were used for the investigation. Multilayered scales are observed which vary in composition and structure; the innermost oxide is an Fe-Cr spinel of the type Fe(Fe_2•xCr_x)O₄. The intermediate layer and the outer oxide layer are both α-Fe₂O₃ hematite. The outer hematite layer is nonadherent and wrinkling is observed. Spallation occurs readily at the inner hematiteJspinel interface and at the spinel oxideJalloy interface. The poor oxidation resistance of the alloy is discussed in terms of these observations.     

Aluminum deoxidation equilibrium in liquid Fe-16 pct Cr alloy

For thermodynamic prediction, the deoxidation equilibrium of aluminum in liquid Fe-16 pct Cr alloy was studied by employing the electromagnetic levitation technique with a cold crucible in an Ar gas atmosphere at 1923 K. The interaction parameters were determined to be e _Al(Fe) ^Cr =0.0001 (0.19/T, 1823 K≤T<1923 K) and r _Al(Fe) ^O,Cr =−0.001. The calculated relationship between aluminum and oxygen contents in Fe-16 pct Cr alloy by thermodynamic data obtained in this study is in good accordance with the experimental results of the present study and other research.     

Isothermal transformations in an Fe-9 pct Ni alloy

Isothermal transformation from austenite in an Fe-9.14 pct Ni alloy has been studied by optical metallography and examination by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range 565 °C and 545 °C, massive ferrite (α _q ) forms first at prior austenite grain boundaries, followed by Widmanst?tten ferrite (α _W ) growing from this grain boundary ferrite. Between 495 °C and 535 °C, Widmanst?tten ferrite is thought to grow directly from the austenite grain boundaries. Both these transformations do not go to completion and reasons for this are discussed. These composition invariant transformations occur below T ₀ in the two-phase field (α+γ). Previous work on the same alloy showed that transformation occurred to α _q > and α _W on furnace cooling, while analytical TEM showed an increase of Ni at the massive ferrite grain boundaries, indicating local partitioning of Ni at the transformation interface. An Fe-3.47 pct Ni alloy transformed to equiaxed ferrite at 707 °C ±5 °C inside the single-phase field on air cooling. This is in agreement with data from other sources, although equiaxed ferrite in Fe-C alloys forms in the two-phase region. The application of theories of growth of two types of massive transformation by Hillert and his colleagues are discussed. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

Embrittlement of a continuously cooled Fe-25 Cr alloy

A study has been carried out on the effects of isothermal heat treatment at 475 and 550‡C and of continuous cooling at different rates from 850°C on the brittleness (as assessed by the ductile-brittle impact transition temperature) of a vacuum melted Fe-25 Cr alloy. The ductile-brittle transition temperature was found to be the lowest for the water quenched condition and highest for the furnace cooled condition and for material aged at 475‡C for long times (~500 h). An increase of brittleness with decreased cooling rate in the continuously cooled samples is attributed to the formation of more continuous and larger amounts of chromium nitrides and carbonitrides at the grain boundaries. Very little or no body centered cubic chromium-rich phase (alpha prime), associated with 475°C embrittlement, was observed. On aging at 550°C, the increased brittleness with time is also accounted for by the formation of grain boundary nitrides and carbonitrides. Although a similar effect takes place in the alloy heat treated at 475°C, the precipitation of alpha prime after long aging times enhances the brittleness. The tendency towards a more brittle condition with aging treatment and slower cooling rate is explained in terms of the Cottrell theory for brittle fracture.     

Embrittlement of a continuously cooled Fe-25 Cr alloy

A study has been carried out on the effects of isothermal heat treatment at 475 and 550‡C and of continuous cooling at different rates from 850°C on the brittleness (as assessed by the ductile-brittle impact transition temperature) of a vacuum melted Fe-25 Cr alloy. The ductile-brittle transition temperature was found to be the lowest for the water quenched condition and highest for the furnace cooled condition and for material aged at 475‡C for long times (~500 h). An increase of brittleness with decreased cooling rate in the continuously cooled samples is attributed to the formation of more continuous and larger amounts of chromium nitrides and carbonitrides at the grain boundaries. Very little or no body centered cubic chromium-rich phase (alpha prime), associated with 475°C embrittlement, was observed. On aging at 550°C, the increased brittleness with time is also accounted for by the formation of grain boundary nitrides and carbonitrides. Although a similar effect takes place in the alloy heat treated at 475°C, the precipitation of alpha prime after long aging times enhances the brittleness. The tendency towards a more brittle condition with aging treatment and slower cooling rate is explained in terms of the Cottrell theory for brittle fracture.     

The tempering of martensite in an Fe-1.5 pct N alloy

The tempering of Fe 1.5 pct N martensite has been studied at temperatures up to 300°C using X-ray and electron microscope techniques. Stage 1 decomposition occurs below 270°C by the general precipitation, resembling spinodal morphology, of fine τa^" (Fe₁₆ N₂) lamellae on 001 habit planes in both matrix and twin crystals of the partially 112 twinned martensite plates. Yet, gaged by changes in the X-ray spectrum, the reaction is discontinuous, the tetragonal martensite doublets decaying in intensity without change in their Bragg positions. The anomaly and the failure to detect by electron microscopy regions exhibiting fractional stages of the fine scale α^’ → α + α^" reaction is attributed to its occurrence at different times in different martensite (or parts of martensite) plates. It is believed that transformation occurs in this manner because the nucleation of coherent α^" plates is controlled by the prevailing internal stress field. Thus the time exponent “n” for the reaction decays from a normal value between 1 and 0.67 to less than 0.3 as stress relief by recovery dominates the more protracted stages of the reaction. Above 200°C the more stable nitride γ^’ (Fe₄N) forms at an increasing rate as plates on 012 habit planes, accompanied by marked softening.     

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.     

Elevated temperature deformation behavior of an Al-8.4 wt pct Fe-3.6 wt pct Ce alloy

The elevated temperature deformation characteristics of a rapidly solidified Al-8.4 wt pct Fe-3.6 wt pct Ce alloy have been investigated. Constant true strain rate compression tests were performed between 523 and 823 K at strain rates ranging from 10⁻⁶ to 10⁻³ s⁻¹. At temperatures below approximately 723 K, the alloy is significantly stronger than oxide dispersion strengthened (ODS) aluminum. However, at higher temperatures, the strength of the Al-Fe-Ce alloy falls rapidly with increasing temperature while ODS aluminum exhibits an apparent threshold stress. It is shown that particle coarsening cannot fully account for the reduction in strength of the Al-Fe-Ce alloy at elevated temperatures. The true activation energy for deformation of the Al-Fe-Ce alloy at temperatures between 723 and 773 K is significantly greater than that for self-diffusion in the matrix. This is unlike the behavior of ODS alloys, which contain nondeformable particles and exhibit true activation energies close to that for self-diffusion in the matrix. Since abnormally high true activation energies for deformation are also exhibited by materials containing deformable particles, such as γ^′ strengthened superalloys, it is concluded that elevated temperature deformation in ythe Al-Fe-Ce alloy involves deformation of both the matrix and the precipitates. The loss of strength of the Al-Fe-Ce alloy appears to be related to a reduction in strength of at least some of the second phase particles at temperatures above 723 K. Formerly Research Assistant, Department of Materials Science and Engineering, Stanford University.     

Molybdenum accumulation at ferrite: Austenite interfaces during isothermal transformation of an Fe-0.24 pct C-0.93 pct Mo alloy

A scanning transmission electron microscope (STEM) technique was used to measure Mo concentrations at ferrite:austenite (α:γ) interfaces in an Fe-0.24 pct C-0.93 pct Mo alloy partially transformed at 650°C, 630°C, and 610°C. These concentrations were quite small at 650°C, which is just below the bay temperature of the time-temperature-transformation (TTT) curve for the initiation of ferrite formation. There were larger concentrations at 630°C, a temperature at which transformation stasis (incomplete transformation) occurred. Concentrations at 610°C were intermediate between the values observed at 650°C and 630°C. The average accumulation at the latter temperatures increased appreciably as a function of transformation time. After each heat treatment, there was considerable variation in Mo accumulation from one α:γ interface to another and, to a lesser extent, from one region to another along the same interface. These higher Mo concentrations were deduced to have developed largely through volume diffusion of Mo, mainly through ferrite, to interfaces whose ledgewise growth had been interrupted by growth stasis. (Mo₂C precipitation at α:γ boundaries occurred only at the end of growth stasis.) It appears that only a very small amount of Mo segregation is needed, probably at specific interfacial sites, in order to produce growth cessation. Growth kinetics anomalies of this kind continue to provide the best evidence available for the operation of a coupled-solute drag effect. This article is based on a presentation given in the symposium “The Effect of Alloying Elements on the Gamma to Alpha Transformation in Steels,” October 6, 2002, at the TMS Fall Meeting in Columbus, Ohio, under the auspices of the McMaster Centre for Steel Research and the ASM-TMS Phase Transformations Committee.     

The mechanism of hydrogen induced cleavage in Fe-3 pct Si alloy

Hydrogen promotes the cleavage fracture of Fe-3 pct Si alloy. Hydrogen atmospheres moving alone with a dislocation will reduce the strain energy of the dislocation, resulting in decreasing the exte nal stress necessary to operate the Frank-Read source. Thus, hydrogen can promote dislocation mul tiplication, which has been verified using a dislocation decoration technique. Fractography and mechanics analysis indicate that a set of the [001] sessile dislocations on the [001] plane is just a cleavage microcrack. Hydrogen carried by the gliding dislocations will enter into the microcrack and the hydrogen pressure will augment the external stress and reduce the critical numbern necessary to form a stable cleavage crack and the numberm of the piled-up dislocations. This indicates that hydrogen promotes the formation of the stable cleavage crack before the other slip systems operate     

Tensile flow and fracture behavior of DO3 Fe-25 at. pct Al and Fe-31 at. pct al alloys

The tensile properties, fracture modes, and deformation mechanisms of two DO₃ alloys, Fe-25 and Fe-31 at. pct Al, have been investigated as a function of temperature up to 600°C. The first alloy was produced by powder metallurgy and hot-extrusion, the second by casting and hot-extrusion. At room temperature extensive plastic deformation occurs in these intermetallics, exhibiting an elongation to fracture of 8 pct and 5.6 pct, respectively. In the Fe-25Al alloy the deformation process consisted of motion and extensive cross-slip of ordinary dislocations and associated formation of antiphase-boundary (APB) bands, while in the Fe-31 Al alloy, plasticity occurred by the motion of superlattice dislocations which eventually dissociated to form APB bands. At room temperature both alloys exhibited transgranular cleavage fracture modes. The variation of tensile properties and fracture modes with temperature is presented. H. A. LIPSITT, formerly with the Materials Laboratory of the Air Force Wright Aeronautical Laboratories, Wright-Patterson Air Force Base, OH 45433-6533     

Structural evolution in nanocrystalline Fe-10 Wt pct Cr alloy powder produced by mechanical alloying—An atomic-force microscopy study

A mechanical alloying (MA) method was used to synthesize Fe-10 wt pct Cr alloy powder. The formation of an Fe-Cr solid solution during milling was studied using atomic-force microscopy (AFM), with the help of an atomic-force microscope in acoustic AC (AAC) mode. The AFM amplitude images indicated that the interlamellar spacing in the structure decreased with an increase in the milling time, and finally gave way to a nonlamellar structure. For structures obtained by milling up to 40 hours, AFM phase-contrast images showed regions of inhomogeneity. Surface-topography images of the granular milled powder showed that the powder surfaces were not smooth, but consisted of a typical hills-and-valley structure. The mean height of the hills decreased with an increase in the milling time. Powders milled up to 20 hours showed a structure that contained grains and subgrains. However, as the interlamellar spacing in granules was reduced, the clear definition of the grains disappeared. Only subgrains were observed in powders milled for time intervals ≥40 hours. With the milling time ≥40 hours, the subgrains not only got more and more refined, they also got elongated in the direction of granular flow. The subgrains in the powder milled for 100 hours were found to have an aspect ratio of 2.5 to 3.0; their smaller dimensions varied from 5 to 30 nm.     

Partition of Mn during the growth of proeutectoid ferrite allotriomorphs in an Fe-1.6 at. pct C-2.8 at. pct Mn alloy

A STEM analysis is made of the Mn distribution around grain boundary allotriomorphs of proeutectoid ferrite in an Fe-1.6 at. Pct C-2.8 at. Pct Mn alloy. Whereas the Mn enriched region is readily observed to extend along the austenite grain boundary, no substantial build-up or depletion of Mn near the ferrite : austenite interface is detected, consistent with the electron probe microanalysis previously reported. In the temperature range where the partition-local equilibrium (P-LE) mode has been proposed to prevail, measured parabolic growth rate constantsfall 1 to 2 orders of magnitude above that predicted from this model, but also below that calculated from the paraequilibrium (PE) model by roughly the same amount. A modification of the theory of grain boundary diffusion-aided growth of precipitates,i.e., the collector/rejector plate mechanism, on the other hand, accounts fairly well for the observed growth kinetics of ferrite allotriomorphs. However, only a slightly better accounting than the P-LE model is provided by this mechanism for the temperature dependence of Mn partition. Data on Ni partition, obtained in an Fe-0.5 at. Pct C-3.1 at. Pct Ni alloy, are also analyzed with the rejector plate model.