Effective Interdiffusion Coefficients Research Articles

Interdiffusion Studies in Alloy 617 and 10Cr Steel Joints Using Diffusion Couple Approach and Simulations

In the steam turbine circuit of advanced ultra supercritical power plants dissimilar joints of alloy 617 and 10Cr steel are unavoidable due to economic reasons. In these joints diffusional interaction causing change in microstructure is identified as possible reason for failure during service. To investigate the interdiffusion driven structural changes, alloy 617/10Cr steel diffusion couples were fabricated. To achieve good metallurgical bond between Fe- and Ni-based alloys and to study diffusional transformations under accelerated conditions, diffusion couples were prepared by annealing in the temperature range of 1000-1100 °C for 3-8 h. For all conditions heat treatment interaction zones were wider in alloy 617 (150-200 μm at 1050 °C, 8 h) than in 10Cr steel (15-16 μm at 1050 °C, 8 h) and the phase stability at the interface was studied using electron microprobe and x-ray diffraction. Average effective interdiffusion coefficients were calculated using Dayananda’s approach. While the diffusivities of substitutional solutes were similar in alloy 617 (0.31-0.42 × 10−15 m2/s at 1050 °C), they differed in 10Cr steel in the following sequence: D~Cr\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ilde{D}_{{{\ ext{Cr}}}}$$\\end{document} > D~Fe\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ilde{D}_{{{\ ext{Fe}}}}$$\\end{document}≈D~Ni\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ilde{D}_{{{\ ext{Ni}}}}$$\\end{document} > D~Co.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ilde{D}_{{{\ ext{Co}}}}.$$\\end{document} Further, multicomponent interdiffusion profiles were predicted using homogenization model in DICTRA and an integrated approach combining DICTRA with Thermo-Calc helped in understanding the experimental observations on the interface microstructure.

Hydrogen diffusion in proton-exchanged congruent Lithium Niobate during post-annealing

We investigated the diffusion of hydrogen during post annealing of proton-exchanged congruent LiNbO3 single crystals in the temperature range between 200 and 400 °C. Proton exchange was done in benzoic acid with 3.6 mol% lithium benzoate added. This process results in a 180 nm thin surface layer with a sharp interface where Li is substituted by H for about 45%. Depth profile analysis was carried out by secondary ion mass spectrometry. From the experimental results, diffusivities were extracted which follow the Arrhenius law with an activation enthalpy of diffusion of (1.39 ± 0.17) eV. The diffusivities are about two orders of magnitude lower than hydrogen tracer diffusivities in pure and in proton-exchanged LiNbO3 but are in good agreement to effective interdiffusion coefficients, governing the proton exchange. Also the activation energy for the latter case of (1.24 ± 0.04) eV is identical within error limits. The results indicate that the diffusional redistribution of high amounts of hydrogen in LiNbO3 is an interdiffusion process between H and Li that is governed by the migration of Li.

Microstructural evolution and elemental interdiffusion behaviour near the interface of NiAlTa alloy and single crystal superalloy DD5 at 1000 °C, 1050 °C, and 1100 °C

The NiAlTa system has been identified as a possible candidate for a protective coating material for single crystal superalloy blade tips. The microstructural evolution and elemental interdiffusion behaviour of NiAlTa/DD5 diffusion couples at 1000 °C, 1050 °C, and 1100 °C were investigated. Significant elemental interdiffusion occurred between the NiAlTa alloy and DD5 alloy. γ'-Ni3Al, AlNi2Ta, and TCP phases were generated in the NiAlTa/DD5 diffusion couples. Ni had the highest average effective interdiffusion coefficient and the lowest diffusion activation energy, while the opposite was true for Ta. The precipitation of the TCP phase raised the potential for cracking in the substrate.

Atom Probe Study of the Miscibility Gap in CuNi Thin Films and Microstructure Development.

The unclear miscibility of CuNi alloys was investigated with atom probe tomography (APT). Multilayered thin film samples were prepared by ion beam sputtering (IBS) and focused ion beam (FIB) shaping. Long-term isothermal annealing treatments in a UHV furnace were conducted at temperatures of 573, 623, and 673 K to investigate the mixing process. The effective interdiffusion coefficient of the nanocrystalline microstructure (including defect diffusion) was determined to be Deff = 1.86 × 10−10 m2/s × exp(−164 kJ/mol/RT) by fitting periodic composition profiles through a Fourier series. In nonequilibrium states, microstructural defects like grain boundaries and precipitates were observed. While at the two higher temperatures total mixing is observed, a clear experimental evidence is found for a miscibility gap at 573 K with the boundary concentrations of 26 and 66 at%. These two compositions are used in a subregular solution model to reconstruct the phase miscibility gap. So, the critical temperature TC of the miscibility gap is found to be 608 K at a concentration of 45 at% Ni.

Investigation of interdiffusion behavior in the Ti–Zr–Cu ternary system

Abstract The interdiffusion behavior of diffusion couples Ti-25Zr/Cu, Ti-50Zr/Cu and Ti-75Zr/Cu (at.%) at 973–1073 K was investigated by using electron probe microanalysis. The results show that the diffusion paths in all diffusion couples deviate away from the faster-diffusing Cu–Ti side. The diffusion paths pass the tie-line, across or parallel to, corresponding to the different diffusion layer structures and phase morphologies. The average effective interdiffusion coefficients and activation energies of Cu in both solid solutions and intermetallic compounds were determined. The diffusion coefficients of Cu in β(Ti, Zr) solid solution increase slightly and then decrease with the increase in Zr content, while the diffusion coefficients of Cu in Cu(Ti,Zr)2 intermetallic compound decrease with the increase in Zr content. The diffusion coefficients of Cu in Cu51Zr14 are the largest among all the intermetallic compounds in diffusion couples.

Formation of Cu-=SUB=-6-=/SUB=-Sn-=SUB=-5-=/SUB=- intermetallic in Cu/Sn thin films

The study of the formation of the Cu6Sn5 intermetallic compound in Sn(55 nm)/Cu(30 nm) thin bilayer films was carried out directly in the column of a transmission electron microscope (electron diffraction mode) by heating the film sample from room temperature to 300oC and recording the electron diffraction patterns. The thin films formed as a result of a solid state reaction were monophase and consisted of the eta-Cu6Sn5 hexagonal phase. The temperature range for the formation of the eta-Cu6Sn5 phase was determined. The estimate of the effective interdiffusion coefficient of the reaction suggests that the main mechanism for the formation of the Cu6Sn5 intermetallic is diffusion along the grain boundaries and dislocations. Keywords: thin films, Cu6Sn5 intermetallic compound, transmission electron microscopy, electron diffraction.

Intermetallic Phases Identification and Diffusion Simulation in Twin-Roll Cast Al-Fe Clad Sheet.

Aluminium steel clad materials have high potential for industrial applications. Their mechanical properties are governed by an intermetallic layer, which forms upon heat treatment at the Al-Fe interface. Transmission electron microscopy was employed to identify the phases present at the interface by selective area electron diffraction and energy dispersive spectroscopy. Three phases were identified: orthorhombic Al5Fe2, monoclinic Al13Fe4 and cubic Al19Fe4MnSi2. An effective interdiffusion coefficient dependent on concentration was determined according to the Boltzmann–Matano method. The highest value of the interdiffusion coefficient was reached at the composition of the intermetallic phases. Afterwards, the process of diffusion considering the evaluated interdiffusion coefficient was simulated using the finite element method. Results of the simulations revealed that growth of the intermetallic phases proceeds preferentially in the direction of aluminium.

Measurement of Interdiffusion and Tracer Diffusion Coefficients in FCC Co-Cr-Fe-Ni Multi-Principal Element Alloy

Average effective interdiffusion coefficients of individual components and tracer diffusion coefficient of Ni were experimentally determined in FCC CoCrFeNi high entropy alloys. Average effective interdiffusion coefficients of individual elements in CoCrFeNi alloys were determined using the Dayananda-Sohn approach. Cr had the highest while Ni had the lowest magnitude of the average effective interdiffusion coefficient in the CoCrFeNi alloy. The tracer diffusion coefficient of Ni was determined without the application of radiotracers using the novel analytical method proposed by Belova et al. based on linear response theory coupled with the Boltzmann-Matano approach and Gaussian distribution function. Both average effective interdiffusion coefficients and tracer diffusion coefficients in CoCrFeNi were compared with the relevant high entropy alloys with varying entropy of mixing. The diffusion of individual elements in Al-containing high entropy alloys (e.g., Al-Co-Cr-Fe-Ni and Al-Co-Cr-Fe-Ni-Mn), with higher entropy of mixing, was higher than that in CoCrFeNi alloy, with relatively lower entropy of mixing. Therefore, diffusion cannot a priori consider to be sluggish in alloys with higher configurational entropy. Moreover, potential energy fluctuations determined in quinary and senary Al-containing high entropy alloys were higher than that in quaternary CoCrFeNi alloy. This also suggests that the diffusivity of a component may not be always lower in the alloys which possess higher fluctuations in potential energy or configurational entropy of mixing.

Formation and nanomechanical properties of intermetallic compounds in electrodeposited Cu–Sn–Co multilayers

The properties of the intermetallic compounds (IMCs) formed at the interface between the solder and the Under Bump Metallization (UBM) are crucial for the reliability of solder joints. Cobalt (Co) is a potential candidate for UBM since it can act as a diffusion barrier and hence can improve the performance of tin-based solder joints. Growth kinetics, mechanism of formation, and properties of IMC in the copper-tin-cobalt (Cu–Sn–Co) system that is encountered with Co UBM are presented in this paper. Cu–Sn–Co systems were prepared by electrodeposition of Cu, Sn and Co multilayers on Cu substrates followed by reflow at 250 °C for varying durations (1–30 min). Microscopic observation revealed the formation of defect-free (Co,Cu)Sn3, crack-containing (Cu,Co)6Sn5 and thin irregular Cu3Sn IMCs after 1 min reflow. As reflow duration increased, (Co,Cu)Sn3 and Cu3Sn grew at the expense of (Cu, Co)6Sn5. The consumption of the entire Sn layer after 1 min and the calculated effective interdiffusion coefficient of (Co,Cu)Sn3 suggested that the growth of (Co,Cu)Sn3 after 1 min is controlled by solid-state diffusion of Sn. The average Young’s Modulus values of (Co,Cu)Sn3, (Cu,Co)6Sn5 and Cu3Sn are 99.5 ± 3.2 GPa, 110.8 ± 7.3 GPa and 109.4 ± 0.3 GPa, respectively, while the nanohardness values are 4.15 ± 0.34 GPa, 6.74 ± 0.62 GPa and 4.96 ± 1.09 GPa, respectively. Increasing the reflow duration in the Cu–Sn–Co system is expected to improve the performance of the solder joint, through the replacement of the crack-containing (Cu,Co)6Sn5 by the defect-free (Co,Cu)Sn3.

Coating-associated microstructure evolution and elemental interdiffusion behavior at a Mo-rich nickel-based superalloy

Substituting Mo for W in superalloy (SA) design can effectively decrease the alloy density and improve thrust-to-weight ratio of aero engines. However, the outer diffusion of Mo was proved to deteriorate the oxidation resistance and mechanical properties of the coated alloys. In this paper, single phase NiAl alloys (Ni/Al = 50/50 and 60/40, marked as Ni50Al and Ni40Al) and two-phase NiAl alloys (Ni/Al = 70/30, marked as Ni30Al), which always worked as coatings for superalloy, were prepared and employed to form diffusion couples with a Mo-rich superalloy. The average effective interdiffusion coefficients were measured to describe the interdiffusion behavior, and the Ni-30Al/SA diffusion couple showed the lowest average effective interdiffusion coefficients for all elements. Besides, distinct congregation of refractory elements (Mo, Cr, Re, Ta) was observed at the superalloy side in Ni-50Al/SA and Ni-40Al/SA diffusion couples. The different interface phase structure was thought to cause the results, and the initial coating compositions – interface microstructure – elemental interdiffusion behavior relationship was established. This should provide insight into the coat-superalloy interaction and usefully guide designing suitable coats for Mo-rich single crystal superalloy.

Формирование интерметаллида Cu-=SUB=-6-=/SUB=-Sn-=SUB=-5-=/SUB=- в тонких пленках Cu/Sn

The study of the formation of the Cu6Sn5 intermetallic compound in Sn(55nm)/Cu(30nm) thin bilayer films was carried out directly in the column of a transmission electron microscope (electron diffraction mode) by heating the film sample from room temperature to 300 °C and recording the electron diffraction patterns. The thin films formed as a result of a solid state reaction were monophase and consisted of the η-Cu6Sn5 hexagonal phase. The temperature range for the formation of the η-Cu6Sn5 phase was determined. The estimate of the effective interdiffusion coefficient of the reaction suggests that the main mechanism for the formation of the Cu6Sn5 intermetallic is diffusion along the grain boundaries and dislocations.

Wettability, Diffusion Behaviors, and Modeling of Ni Nanoparticles and Nanowires in Brazing Inconel 718

Ni nanoparticles (NPs) and nanowires (NWs) are compared as brazing filler metals for joining Inconel 718. NWs significantly enhance the strength of brazed joints compared to NP joints (270 MPa). Herein, joints formed with a pure NW pellet and a NW–NP mixed pellet yield average lap shear bonding strengths of 319.2 and 370 MPa, respectively. The experiment displays that the contact angle in brazing with NPs is higher than that with NWs. A modified Hart's equation is also proposed for predicting the effective interdiffusion coefficients of nanomaterials that account for surface diffusion in addition to lattice and grain boundary diffusion. The diffusion coefficients calculated by the modified Hart's equation are consistent with the values determined by Sauer–Friese analysis for NPs, but for NWs, the diffusion coefficients determined by Sauer–Friese are mainly controlled by grain boundary diffusion, significantly higher than those predicted by the modified Hart's equation due to size‐dependent transient liquid phase diffusion. This explains the enhanced brazing strength with NW fillers.

Interdiffusion, Solubility Limit, and Role of Entropy in FCC Al-Co-Cr-Fe-Ni Alloys

High entropy and sluggish diffusion “core” effects were investigated in an FCC Al-Co-Cr-Fe-Ni alloy by examining the nonequiatomic compositions generated by the concentration profiles within the solid-to-solid diffusion couple, Al48Ni52vs Co25Cr25Fe25Ni25, annealed at 900 °C, 1000 °C, 1100 °C, and 1200 °C. The average effective interdiffusion coefficients of individual components and the maximum solubility limit of Al in nonequiatomic Al-Co-Cr-Fe-Ni alloys were determined as a function of temperature. The magnitudes of the average effective interdiffusion coefficients in Al-Co-Cr-Fe-Ni alloys were compared to the interdiffusion coefficients in relevant ternary and quaternary alloys. The solubility limit of Al in nonequiatomic AlpCoqCrrFesNit alloys was compared to that of Al in equiatomic AlxCoCrFeNi determined from the equilibrium pseudo-binary phase diagram. A reduction in the magnitude of interdiffusion coefficients was not observed for individual components in Al-Co-Cr-Fe-Ni alloys. The maximum solubility of Al in nonequiatomic AlpCoqCrrFesNit alloys was observed to be higher than that in equiatomic AlxCoCrFeNi alloys at a temperature of 1100 °C or above. Correspondingly, the free energy of mixing for nonequiatomic AlpCoqCrrFesNit alloys was determined to be lower than that of equiatomic AlxCoCrFeNi alloys at a temperature of 1100 °C or above. At a temperature of 1100 °C or above, the role of enthalpy of mixing was estimated to be significant in achieving higher thermodynamic stability of the nonequiatomic AlpCoqCrrFesNit alloy than in the equiatomic AlxCoCrFeNi alloy for the compositions corresponding to the highest solubility limit for Al. The compositions of nonequiatomic AlpCoqCrrFesNit alloys were observed to follow the existing empirical rules for the formation of single phase in high entropy alloys (HEAs).

In Situ Electron Diffraction Investigation of Solid State Synthesis of Co-In2O3 Ferromagnetic Nanocomposite Thin Films

In situ electron diffraction was used to study structural transformations during the formation of Co-In2O3 ferromagnetic nanocomposite thin films in a thermite reaction of In/Co3O4 bilayer thin films. Heating was performed from room temperature to 600°C at a rate of 4°C/min, while simultaneously electron diffraction patterns were recorded at a speed of 4 frames/min. This made it possible to determine the initiation, 185°C, and finishing, 550°C, temperatures of the solid-state synthesis, as well as the change in the phase composition during the thermite reaction. The synthesized Co-In2O3 film nanocomposite contained ferromagnetic cobalt nanoclusters surrounded by an In2O3 layer, with an average size of 20 nm, and had a magnetization of 400 emu/cm3 and a coercivity of 50 Oe at room temperature. The estimate of the effective interdiffusion coefficient of the reaction suggests that the main mechanism for the formation of the Co-In2O3 nanocomposite is diffusion along the grain boundaries and dislocations.

Interdiffusion behavior in 304L SS/Ti–5Ta–2Nb system

This paper presents the results of an experimental and computational study on interdiffusion and related structure-property changes in 304 L stainless steel/Ti–5Ta–2Nb system. Diffusion annealing treatments were carried out in the temperature range of 873–1173 K, above and below the β transus temperature of Ti. The formation of σ, FeTi and Fe2Ti phases at the interface as well as stabilization of α-Fe and β-Ti in 304 L SS and TiTaNb respectively has been unambiguously established in this study. Growth kinetics of the reaction zones has been determined and effective interdiffusion coefficients in various phases were calculated using Wagner's approach. Temperature dependant variations in the mechanical property of the base materials and diffusion couples have also been predicted using JMatPro® and compared with literature. Finally, the evolution of microstructure at the interface and diffusion path in the diffusion couples were compared with SS/Ti based diffusion bonded joints and explosive clads to understand the interdiffusion behavior under equilibrium and non-equilibrium conditions.

Investigation of diffusion behavior and mechanical properties of Mg-Zn system

The growth kinetics, diffusion kinetics and mechanical properties of intermetallics in Mg-Zn binary systems have been investigated in this work. Four intermetallic compounds (IMCs) Mg2Zn11, MgZn2, Mg21Zn25, and Mg4Zn7 have been observed in the Mg-Zn diffusion zone at the temperature range of 523–593 K. The square of thickness for IMCs diffusion layer increases linearly with time, which is consistent with the parabolic growth law, and also indicates that the growth of IMCs is controlled by diffusion. Growth constants and growth activation energies of Mg-Zn IMCs have been evaluated. Average effective interdiffusion coefficient of each phase has been calculated by the Wagner's method based on the concentration distribution obtained by EPMA. The diffusion activation energy has been estimated and compared with the available results. Finally, displacement-unloading curve measured by nanoindentation was used to estimate the hardness and Young's modulus to characterize the mechanical and creep properties of Mg2Zn11, MgZn2, Mg21Zn25 and Mg4Zn7. The present results should be beneficial to the design and performance of Mg-based alloys.

Interdiffusion behaviors and mechanical properties of Cu-Zr system

In this work, solid-to-solid diffusion couples were assembled and annealed to investigate diffusion behavior and mechanical properties of the Cu-Zr system in temperatures range from 1043 K to 1113 K. Six intermetallic compounds (IMCs) Cu9Zr2, Cu51Zr14, Cu8Zr3, Cu10Zr7, CuZr, and CuZr2 were observed in the diffusion zone. Composition-dependent interdiffusion coefficients of IMCs have been calculated based on the measured composition Cu profiles of the diffusion zones by using Sauer–Freise method. And the average effective interdiffusion coefficients for each phase were also calculated by using Wagner method. The activation energies of diffusion are evaluated according to the average effective interdiffusion coefficient. Finally, the load-displacement curves measured by nano-indentation are obtained to characterize mechanical properties of Cu9Zr2 and Cu51Zr14, which have similar hardness and elastic moduli.

Diffusion behavior and reactions between Al and Ca in Mg alloys by diffusion couples

The diffusion behavior and reactions between Al and Ca in Mg alloys by diffusion couple method were investigated. Results demonstrate that Al2Ca is the only phase existing in the diffusion reaction layers. The volume fraction of Al2Ca in diffusion reaction layers increases linearly with temperature. The standard enthalpy of formation for intermetallic compounds was rationalized on the basis of the Miedema model. Al-Ca intermetallic compounds were preferable to form in the Mg-Al-Ca ternary system under the same conditions. Over the range of 350–400°C, the structure of Al2Ca is more stable than that of Al4Ca, Al14Ca13and Al3Ca8. The growth constants of the layer I, layer II and entire diffusion reaction layers were determined. The activation energies for the growth of the layer I, layer II and entire diffusion reaction layers were (80.74±3.01)kJ/mol, (93.45±2.12)kJ/mol and (83.52±1.50)kJ/mol, respectively. In layer I and II, Al has higher integrated interdiffusion coefficients D˜iInt, layer than Ca. The average effective interdiffusion coefficients D˜Aleff values are higher than D˜Caeffin the layer I and II.

Effect of alloying elements on interdiffusion phenomena in explosive clads of 304LSS/Ti–5Ta–2Nb alloy

Dissimilar joints of 304L austenitic stainless steel (SS) and Ti–5Ta–2Nb alloy were fabricated using explosive cladding process with an aim to avoid the formation of brittle intermetallic phases at the interface. Subsequently, diffusion annealing heat treatments were carried out at temperatures in the range of 550–800 °C for various durations. In the present study, concentration and temperature dependence of the distinct diffusion zones, formed at the clad interface, due to interdiffusion of the alloying elements, have been established using Electron Probe Micro Analysis and imaging of the interface. Molar volume showed a close match with the ideal Vegard’s law at low temperatures while a non-linear negative deviation has been observed at high temperatures (800 °C) due to the formation of secondary phases such as FeTi, Fe2Ti and βTi(Fe) phases. Interdiffusion parameters evaluated by Wagner’s method showed sluggish diffusion kinetics of Fe and Ti at concentration corresponding to a two phase mixture of FeTi and Fe2Ti. Further, an attempt has been made to understand multicomponent diffusion using Dayananda’s approach and estimation of effective interdiffusion coefficients revealed ~22 µm shift of the interface from Matano plane at 800 °C.

Ankerite grains with dolomite cores: A diffusion chronometer for low- to medium-grade regionally metamorphosed clastic sediments

Ankerite grains with dolomite cores occur in marls, pelites, and psammites from a Buchan terrain in Maine and a Barrovian terrain in Vermont (U.S.A.). Dolomite cores are typically ≤20 μm in diameter, have sharp but irregular contacts with ankerite, and have the same crystallographic orientation as ankerite rims. Ankerite grains with dolomite cores are common in the chlorite zone, less abundant in the biotite and garnet zones, and rare (Vermont) or absent (Maine) at higher grades. The texture and crystallographic orientation of dolomite and ankerite and the sharpness of the dolomite-ankerite contact are consistent with partial replacement of detrital dolomite by ankerite by solution-reprecipitation. Metamorphic biotite is in Fe-Mg exchange equilibrium with ankerite rims but not with dolomite cores, implying that ankerite did not form long after biotite (biotite has no phlogopite cores). Possible sources of iron for the formation of ankerite are reduction of ferric iron hydroxide or the smectite-to-illite reaction during diagenesis. The sharpness of the dolomite-ankerite contact is a diffusion chronometer that constrains timescales of metamorphic process. Relatively low spatial resolution analyses of Fe/Mg across the contact with a NanoSIMS instrument and a FEG TEM give upper bounds on the thickness of the transition from ankerite to dolomite of ~2 and ~0.5 μm, respectively. Higher resolution analysis of BSE grayscale contrast with a FEG SEM gives a thickness ~100 nm. Fit of the grayscale profile to a model of one-dimensional diffusion across an infinite plane gives Dt = 10^(−15) m^2 (± a factor of 5), where D is the effective Fe-Mg interdiffusion coefficient and t is the duration of diffusion. Using the published experimental determination of D, upper bounds on the residence time of ankerite grains with dolomite cores at peak T = 400–500 °C, on the duration of linear cooling from peak T to 100 °C, and on the duration of linear heating from 100 °C to peak T followed by linear cooling to 100 °C are all 100 °C. The question is what explains the occurrence of ultrasteep composition gradients between dolomite and ankerite. Regional metamorphism on a timescale of a year or less is unrealistic. No barrier to diffusion at the dolomite-ankerite contact was observed in TEM images. Post-metamorphic formation of ankerite at very low temperature is ruled out by Fe-Mg exchange equilibrium between biotite and ankerite but not dolomite. It is unlikely that the steep composition gradients were preserved by intracrystalline pressure gradients. Alternatively, the steep composition gradients would be consistent with timescales of metamorphic process ~10^6 years or longer if D values during metamorphism were approximately six orders of magnitude or more smaller than those measured in the laboratory. The error of measurement is much less, approximately ± a factor of 2. A correction to D for the difference in P between measurements (0.1 MPa) and metamorphism (350–800 MPa) is likely an order of magnitude or less. Oxygen activity (a_(O2)), however, was 17–20 orders of magnitude larger during the laboratory measurements than during metamorphism. A correction to measured D for the difference in a_(O2_ between experiment and metamorphism appears to be the likeliest way to reconcile the steep composition gradients with realistic timescales of metamorphism. Before ankerite grains with dolomite cores are fully realized as a useful diffusion chronometer for low- and medium-grade metamorphic rocks, the rates of Fe-Mg interdiffusion in ankerite and dolomite need to be calibrated as a function of a_(O2).