Oswald N. C. Uwakweh

Mössbauer study of the distribution of carbon interstitiels in iron alloys and the isochronal kinetics of the aging of martensite: The clustering-ordering synergy

Conversion electron Mössbauer spectroscopy allows the resolution of all components in spectra of 100 pct retained austenite for varying carbon contents (>6.5 at. pct C). An off-stoichiometry ordered structure, Fe8Ci1−y, is proposed which confirms interstitial repulsion in y iron. Transmission spectroscopy is used to follow the nonisothermal kinetics of the concomitant clustering and ordering processes during aging and of the first stage of tempering in 8.5 and 8 at. pct iron martensite. Activation energies of 75 ∓ 2.5, 74 ∓ 1.5, and 121.5 ∓ 1.5 kJ/mole are observed. The synergy between the clustering and ordering within the frame of the carbon multiplet model is emphasized. The ε- or η-carbide obtained by tempering is analyzed to be Fe2.4C.

Electron microscopy study of the aging and first stage of tempering of high-carbon Fe-C martensite

Transmission electron microscopy (TEM) observations of 1.95 wt pct C martensite samples, aged at 330 and 352 K for 1 and 2 hours, respectively, confirm the fact that the two steps of aging, as evidenced by Mössbauer spectroscopy kinetics study, are due to clustering and subsequent long-range ordering. Streaks are observed along 〈203〉* directions during clustering, and in addition, two types of split superstructure spots appear during ordering at the advanced stage of aging. These are tentatively explained by a 1C-2Fe-1C-5Fe-1C-5Fe sequence of carbon and iron atoms arranged along [001]α′, which gives rise to antiphase domains with a 12a superperiod within the alternating carbon-rich and carbon-depleted regions, wherea is the lattice parameter of body-centered cubic (bcc) iron. The associated formula is Fe6C. In the first stage of tempering, the orthorhombic structure of the precipitated carbide is confirmed, while evidence of ordering as in Co2N is lacking. The Fe9C4 stoichiometry, which is close to the experimental Fe2.4C, is instead proposed forε- orη-carbide.

Cems study of the carbon distribution in austenite

CEMS resolution allows a careful study of Fe−C pure austenite spectra with high carbon content. Three Fe environments are detected which are ascribed to Fe atoms with one carbon first nearest neighbour and zero carbon second nearest neighbour and two environments with no carbon first nearest neighbour but zero carbon second nearest neighbour and one to four carbon second nearest neighbours respectively. This confirms the repulsive interaction between carbon interstitials and the tendency towards Fe8C ordering is suggested.

Kinetics of phase evolution of Zn-Fe intermetallics

The intermetallic phases, Γ (Fe33Zn10), Γ1, (Fe5Zn21), δ (FeZn7, and ζ (FeZn13), are mechanically alloyed through ball milling of pure elemental Fe and Zn powders under a controlled atmosphere of argon gas. The state of the as-ball-milled materials was crystalline, except for the Γ phase, which was amorphous. Phase-evolution kinetics through differential scanning calorimeter (DSC) measurements of the as-ball-milled powders show three characteristic transition temperatures for the Γ1, and ζ phases, two for the Γ phase, and only one for the δ phase. The activation energies for the evolution of the milled powders to their equilibrium crystalline phases are 170 ± 1, 251 ±2, 176± 1, and 737 ±4 kJ/mol for the Γ, Γ1, δ, and ζ phases, respectively. These values show that the mechanisms for the metastable-to-stable phase transition in these intermetallics are different, whereas diffusion over short distances may be part of the entire process in all cases.

Corrosion Fatigue of High-Strength Aircraft Structural Alloys

Results of corrosion fatigue characterization of AA7075-T6 and AF1410 steel under different simulated marine environments and loading conditions are presented. In comparison with baseline tests conducted in laboratory air, corrosion fatigue experiments performed at 1-Hz frequency in the presence of 1% NaCl environment indicated a substantial reduction in fatigue lifetime in the case of AA7075-T6, whereas AF1410 corrosion fatigue life was found to be statistically unaffected at 1-Hz frequency in the presence of 1% and 3.5%NaCl. However, a reduction of frequency to 0.5 Hz significantly reduced the lifetime of AF1410 steel. On the other hand, Cd-plated AF1410 tested to study fatigue characteristics in a hydrogen-rich metal surface environment yielded minimal change in the lifetime. Atomic force microscope analysis was performed to discern features in fracture surface morphology leading to changes in lifetime of AF1410 and AA7075-T6 alloys.

Neutron diffraction and phase evolution of the mechanically alloyed intermetallic compound ζ-FeZn13

High-energy ball milling with subsequent annealing is used to synthesize the intermetallic compound ζ-FeZn13. The mechanically alloyed phase in the as-milled state is determined to be nonequilibrium, or metastable. Transmission electron microscopy (TEM) studies show a highly defective microstructure with undefined grain areas, and the alloy can be described as a mechanical mixture of elemental Fe and Zn, based on neutron diffraction measurements. Characteristic stages associated with its transformation to the equilibrium state are identified based on differential scanning calorimetry (DSC) measurements. The activation energies corresponding to these stages are 128, 202, and 737 kJ/mole, respectively, with increasing transformation temperatures. The first stage is related to limited atomic diffusion or rearrangements, such as recovery, during thermal treatment, while the second stage depicts continued recrystallization and long-range atomic diffusion leading to a stable phase formation. The third and final stage marks structural decomposition of the equilibrium structure due to phase transition. Neutron diffraction of the equilibrium alloy confirmed that the structure is C2/m, with lattice parameters of a=13.40995 Å, b=7.60586 Å, c=5.07629 Å, and β=127 deg 18 minutes. The atomic positions of Fe and Zn compared well to reported values.

Application of metastable transformation of mechanically alloyed Fe-Zn-Si in equilibrium phase studies

A series of six Fe-Zn-Si alloys was investigated with Fe content varying through the range 5.60 to 25.37 wt.% and fixed Si content of 0.12 wt.%. These alloys were formed by mechanical alloying of pure elemental powders—specifically by ball milling. After preparation, uniformity of microstructure was verified by electron microscopy and microprobe analysis. Annealing progress was followed by DSC and XRD. Differential scanning calorimetry measurements through the range 200 to 600 ‡C were used to locate the temperatures of relaxation phenomenon and the structural evolution. X-ray diffraction patterns were used to identify the phases present at various stages of the evolution. An invariant reaction was found near 423 ‡C and was identified as the peritectic melting of Zn. Identification of the formation of FeSi in a broad composition range below 9.1 wt.% Fe and at temperatures through 400 ‡C suggests the existence of phase field change. The formation of this FeSi phase is suggested as a possible cause for the Sandelin effect.

Morphology and aging of the martensite induced by cathodic hydrogen charging of high-carbon austenitic steels

The transformation by cathodic hydrogen charging of a 1.95 wt pct C austenite is studied by X-ray diffraction and Mössbauer spectroscopy, and the morphology of the induced martensite, analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), is compared to that of the same alloy quenched at 78 K. Optimal charging current density is found to be at 2200 A/m2, and charging time is varied. Martensite crystals have three different shapes: long thin plates, “spear-like,” and “palm frond.” The habit plane is essentially {225}γ, and the serratures of the palm frond are obtained by (111)γ accommodation slip. The state of aging of the hydrogen-induced martensite at room temperature is similar to that of a martensite obtained by quench at 78 K and subsequently aged at much higher temperature, e.g., 97 °C for 1 hour, and displays the “tweed-like” or “salt and pepper” morphology of Génin’s Fe6C “carbon extended multiplets” which transform in situ into ε- or η-carbide precipitates at the same tem-perature, i.e. 117 °C, for 1 hour. The role of hydrogen in catalyzing the martensitic transfor-mation and affecting the aging kinetics is explored.

Mössbauer spectroscopy study of the synthesis of SnFe2O4 by high energy ball milling (HEBM) of SnO and α-Fe2O3

The formation of single phase nanoparticles of spinel structured ferrite, SnFe2O4, by mechanochemical syntheses using HEBM of stoichiometric amounts of solid SnO and α-Fe2O3 with acetone as surfactant was achieved progressively as function of ball milling time. Single phase SnFe2O4 formation commenced from five hours of continuous ball milling, and reached completion after 22 hours, thereby yielding a material with a lattice parameter of 8.543 A, and particle size of 10.91 nm. The coercivity was 4.44 mT, magnetic saturation value of 17.75 Am2/kg, and remanent magnetizations of 1.50 Am2/kg, correspondingly. The nanosized particles exhibited superparamagnetic behavior phenomenon based on Mossbauer spectroscopy measurements. The kinetic analyses based on the modified Kissinger method yielded four characteristic stages during the thermal evolution of the 22 hours milled state with activation energies of 0.23 kJ/mol, 2.52 kJ/mol, 0.024 kJ/mol, and 1.57 kJ/mol respectively.

Weld thermal simulation and its effect upon the microstructure of as-cast FeAl-based materials

Abstract Gleeble simulation studies of the effects of welding on the microstructural change of four FeAl-based materials with Mo, Zr, C, and B additions based on light optical metallography and x-ray diffraction techniques have been undertaken. These materials, designated Fe-Al-Mo (heat 564), Fe-Al-Mo-B (heat 565), Fe-Al-Mo-Zr-C (heat 567), and Fe-Al-Mo-Zr-C-B (heat 568), exhibit behaviors suggesting that the additions of B, Zr, and C are beneficial (individually or in combination) to the resistance of cracking following-weld thermal cycles. The microstructure is significantly altered due to the addition of Zr and C. The relation and correlation between the effects of minor alloying elements on FeAl materials and weldability is established. Published by Elsevier Science Inc.