Iron Self-diffusion Research Articles

The effects of tungsten addition on the microstructural stability of 9Cr–Mo Steels

The effects of tungsten addition on the microstructure and high-temperature tensile strength of 9Cr–Mo steels have been investigated by using three different steels: M10 (9Cr–1Mo), W18 (9Cr–0.5Mo–1.8W), and W27 (9Cr–0.1Mo–2.7W) steels. The tungsten-added 9Cr steels have revealed better high-temperature tensile strength. Microchemical analysis for (Cr,Fe) 2 (C,N) revealed that the tungsten addition increased the Cr/Fe ratio, which resulted in the lattice expansion of (Cr,Fe) 2 (C,N), and then the enhanced pinning effect on the glide of dislocation. In addition, in M10 steel, the M 23C 6 carbides quickly grew and agglomerated, while the tungsten-added 9Cr steels revealed a fine and uniform distribution of M 23C 6 carbides. Dislocation recovery during tempering treatments was delayed in tungsten-added 9Cr steels, which was correlated with the stabilized precipitates and the decreased self-diffusivity of iron. It is, thus, believed that the excellent high-temperature tensile strength of tungsten-added 9Cr steels is attributed to the stabilized M 2X carbo-nitrides and M 23C 6 carbides and the decreased self-diffusivity of iron.

Advances in Physical Metallurgy and Processing of Steels. Design of Ferritic Creep-resistant Steels.

Creep resistant steels must be reliable over very long periods of time in severe environments. Their microstructures have to be very stable, both in the wrought and in the welded states. This paper reviews the quantitative methods for the design of steels for elevated temperature applications. A methodology is described for the calculation of complex precipitation reactions over periods extending many tens of years. However, microstructure alone is not enough in the design of alloys. The estimation of the creep rupture stress using a neural network technique is described in the second part of this review. The calculation of the influence of solute-elements on the self-diffusivity of iron, which features in many creep equations, is an emerging area in alloy design. The methodology for such calculations is reviewed in the final section of the paper.

Hot hardness and indentation creep studies of a Fe–28Al–3Cr–0.2C alloy

The hot hardness behaviour of a Fe–28Al–3Cr–0.2C (at.%) alloy was evaluated from room temperature to 1273 K. Indentation creep measurements were also carried out in the temperature range of 843–963 K. The hardness–temperature plot of this alloy showed five distinct regions. The mechanism of deformation operating in each of these regions has been suggested. Indentation creep measurements showed that the stress exponent ( n) obtained from the hardness–time plots was weakly temperature dependent. The activation energy for high temperature creep was found to be in good agreement with that for self-diffusion of pure iron and in excellent agreement with previous studies. These values of n and activation energy were found to be consistent with dislocation climb as the rate controlling creep mechanism.

Static recrystallization of shock-wave (explosively) deformed Fe–40 at% Al–Zr–B intermetallic

A B2 Fe–40 at% Al–Zr–B iron aluminide was subjected to shock-wave (explosive) loading at ∼6 GPa shock pressure. The applied shock pressure was sufficient to generate high dislocation density in the material which, in turn, gave rise to primary static recrystallization process upon subsequent isothermal annealing at 700, 750 and 800°C for various lengths of time. At various annealing temperatures recrystallization occurs rather inhomogeneously. Nucleation occurs mostly at the triple points and grain boundaries, showing tendency for clustering. Recrystallization kinetics follows a general behavior according to the Johnson, Mehl, Avrami and Kolmogorov theory (JMAK) with the Avrami exponent, n∼2.3–2.9. The estimated apparent activation energy for recrystallization, ∼259 kJ/mol, is very close to the activation energy for self-diffusion in pure iron and iron in FeAl. The results of microhardness studies of partially recrystallized specimens indicate that the average Vickers microhardness values for recrystallized and unrecrystallized areas differ significantly, the former having higher microhardness. This phenomenon is discussed in terms of higher density of retained sinks for quenched-in vacancies in the unrecrystallized areas and lower efficiency of vacancy hardening.

Pressure and temperature dependence of nickel diffusion in Fe 3Al: effect of ordering

The pressure and temperature dependence of 63Ni heterodiffusion was investigated in the temperature range 888–1282 K, which includes the critical temperature of the A2–B2 order–disorder transition. All the experiments were carried out using the radiotracer method combined with a serial sectioning technique. The high-pressure measurements have shown that the diffusion process is controlled by mono-vacancies both in the ordered and disordered structures. The Arrhenius plot presents a downward curvature as the result of B2-type atomic ordering and the lattice diffusion coefficients can be fitted by a continuous curve as a function of temperature and long-range order parameter. Because of the site preference of Ni for α-sites, nickel diffusivity is slower and the activation energy of diffusion is higher than the corresponding values for iron self-diffusion.

Microestructura y propiedades mecánicas de dos aceros para herramientas con ultra alto contenido de boro

In the present work, two selected tool steels have been modified by a boron addition of 0.5 and 1 mass %. Both steels were processed by powder metallurgy methods, including argon atomization and hot isostatic pressing. The Consolidated materials presented a microstructure consisting of a fine and homogeneous distribution borocarbides M23(C,B)6 in a ferrite-martensite matrix. No changes are observed in the microstructure after deformation by compression-strain-rate-change tests at temperatures ranging from 700 to 1,100 °C. For the Fe-lB-lC steel, a stress exponent of 4.5 was obtained, that suggests that slip creep is the controlling deformation mechanism. On the other hand, a stress exponent between 2 and 3 was obtained for the Fe-0.5B-1.5C steel that suggests that grain boundary sliding is the controlling deformation mechanism. In both cases, the activation energy for creep was related to the activation energy for iron self-diffusion.

Effect of Atomic Order on Iron and Cobalt Grain Boundary Diffusion in the FeCo Equiatomic Compound

ABSTRACTSimultaneous grain boundary self-diffusion of iron and cobalt in FeCo was investigated in the B-kinetic regime by the radiotracer method combined with a serial sectioning technique over the temperature range 873-1133 K, which includes the A2-B2 bulk order-disorder transition temperature. This transition leads to a decrease in both iron and cobalt mass transport mainly owing to a change in the pre-exponential factor, but not in the diffusion activation energy. Moreover, the breaking point in the Arrhenius plots is observed below the bulk order-disorder transition temperature. In view of recent surface segregation studies on Fe-Co, and according to the Fe-Co phase diagram, the results are discussed assuming an iron segregation, which decreases the A2-B2 transition temperature in grain boundaries.

High strain rate superplasticity in the fine-grained duplex stainless steel Fe-22Cr-5Ni-3Mo-0.3N

The fine-grained duplex stainless steel Fe-22Cr-5Ni-3Mo-0.3N consisting of α- and γ-Fe(Cr,Ni,Mo) solid solutions exhibits structural superplasticity at deformation temperatures of 900 to 1050°C. The equiaxed microstructure with an average grain size of d α,γ = 3 μm was produced by thermomechanical processing. This steel shows also superior superplastic properties at high strain rates up to e 5.10 -2 s -1 . Maximum strain rate exponents of m = 0.5 and elongations to failure of more than 800% were achieved. The superplastic deformability (m > 0.3) of this steel in a wide strain rate range enables near net shape deep drawing or blow forming of parts with complex shape applying low flow stresses. A deformation model is presented to describe the superplastic behaviour at high strain rates. Grain and interphase boundary sliding is accommodated by sequential steps of dislocation glide and climb. The maximum m-value of about 0.5 and an activation energy of 260 kJ/mol, which is comparable to that of self diffusion of iron in γ-Fe (270 kJ/mol), and high dislocation densities indicate that dislocation climb in the slightly solid solution hardened γ-Fe phase (solid solution class II type of material) is the rate controlling step for superplastic flow.

Mechanical properties of two ultrahigh carbon-boron tool steels

The microstructure and the mechanical behavior of two modified boron-containing tool steels were investigated at low and high temperatures. Both steels were processed by powder metallurgy methods, involving gas atomization and hot isostatic pressing at 180 MPa for 2 h at 900, 1000 and 1100 °C. The microstructure of the consolidated tool steels consist of a ferrite matrix and carboboride M 23(C,B) 6 particles. The 1wt.%C-1wt.%B tool steel contains, in addition, small particles of vanadium carbide and vanadium boride. An ultimate tensile strength of 300 MPa at 700 °C was obtained in the 1wt.%C-1wt.%B tool steel. At testing temperatures in the austenitic phase, a stress exponent of about 5 was obtained and the activation energy for creep was related to the activation energy for iron self-diffusion in austenite. This suggests that slip creep is the controlling deformation mechanism in the 1wt.%C-1wt.%B tool steel tested in this temperature range. At testing temperatures in the ferritic phase the creep rate was slower than that predicted by the slip creep equation. This was attributed to the presence of fine second phase particles. On the other hand, the 1.5wt.%C-0.5wt.%B tool steel showed an ultimate tensile strength somewhat lower than 300 MPa at 700 °C. At testing temperatures in the austenitic phase, a stress exponent of about 2 was obtained and the activation energy for creep was related to the activation energy for iron self-diffusion in austenite. This suggests that grain boundary sliding is the controlling deformation mechanism in the 1.5wt.%C-0.5wt.%B tool steel tested in this temperature range. A contribution from slip creep exists at testing temperatures in the ferritic phase.

Dislocation creep controlled superplasticity in the high carbon steel 140NiCr16-6

superplasticity in the alloyed high carbon-steel 140NiCr16-6 with phosphorus additions and a fine grained microduplex structure-containing cementite in volume fractions of 22 % (Fe,Cr,Ni) 3 C, particle size of about 1 μm and with a medium ferrite grain size of about 2 μm - has been investigated in the temperature regime of 550 to 675°C and in the strain rate range of 10 -5 to 5.10 -2 s -1 . Maximum strain rate exponents of m = 0.45 at 675°C with strain rates of the order of 10 4 s -1 have been determined. Maximum superplastic elongations of about 700 % were detected. At higher strain rates of 10 3 s -1 superplastic elongations of about 570 % were achieved. At relatively low test temperatures of 550°C elongations up to 230 % were recorded. The activation analysis in the temperature regime of 550 to 650°C show an activation energy for superplastic flow of 250 ± 20 kJ/mol. This is in agreement with the activation energy for lattice self diffusion of iron in α-iron. Above 650°C the activation energy decreases to 70 kJ/mol. This is due to a stress induced decrease in the eutectoid o-'/-transformation temperature from 685°C to somewhat lower temperatures during superplastic deformation. The superplastic deformability (m > 0.3) of this steel in a wide strain rate range at relatively low temperatures above 550°C allows near net shape forming of complex parts applying low flow stresses.

Effect of phosphorus on Fe grain boundary self-diffusion in austenitic Fe-20Ni-10Cr-xP alloys

Austenitic Ni-Cr steels are extensively used as refractory and corrosion-resistant materials. Frequently, they are used at temperatures approaching T[sub m]/2 (T[sub m] is melting point). In this temperature range, all transport processes controlled by diffusion almost exclusively involve high-diffusivity paths, while the lattice diffusion is frozen. In a single-phase material without interphase interfaces, grain boundaries (GB's) and dislocation pipes act as high-diffusivity paths. It is well known that impurities, the solubility limit of which are low, concentrate at GB's. Because of the very small thickness of a GB, [delta], they form relatively highly concentrated thin zones along GB's even if their mean concentration is very low. Moreover, the changes of chemical composition of these zones may be even more complicated owing to e.g., cooperative and site-competitive interactions between atoms in the GB, and/or to heterogeneous precipitation on GB's induced by segregation. Therefore, diffusion along GB's is strongly affected by the chemistry of the GB, especially by segregated so-called surface active elements. At present, there is no universal theory describing all details of diffusion along segregated GB's, which is, partially, caused by lack of experimental data. In the present paper, the effect of small concentrations of phosphorus, that is one ofmore » most deleterious impurities in iron alloys, on GB self-diffusion of iron in Fe-Ni-Cr-xP system is studied.« less

The Creep Behaviour of 310-Type Stainless Steel

Abstract The primary and steady-state creep behaviour of 310-type stainless steel has been investigated over the temperature range 500–700°C (0.46–0.58 Tm ). The steady-state stress exponent, n, increased slightly with increasing grain size, d, from n= 5.9 at d = 40 μm to n = 6.4 at d = 100 μm at a test temperature T = 600°C. The activation energy for steady-state creep, Q c, determined at σ = 300 MPa was 250±6.4 kJ mol−1 for d = 40 μm. This value of Q c is approximately equal to that for the volume self-diffusion of iron (280 kJ mol−1). The activation energy determined for primary creep is almost the same as that for secondary creep. The grain diameter parameter, m, in the equation ec = Aσn d m exp (− Q c/RT) was determined to be –1.67 for σ = 300 MPa and T = 600°C. This value is slightly different from a previously published value of m= −2, which was obtained at a lower applied stress. Using the strain-time data obtained in these tests, constitutive equations have been developed to describe both the pri...

Quantum chemical examination of the penetration of ions of hydrogen and its isotopes through metals and alloys

UDC 539.2 Simulation of the mechanisms of penetration of ions of hydrogen and its isotopes through metals is a complicated theoretical tasks because it requires to take into account simultaneously short-term interatomic interactions and the effects of transfer and delocalization of the electron charge and lattice relaxation. The greatest difficulties arise in the case of d-metals [1, 2] and their alloys for which detailed investigations have not been carried out. In this work, we conduct quantum chemical calculations of the activation energy and their coefficients of intracrystalline diffusion of hydrogen for a- and y-iron, chrome, silver, and a Fe-Cr alloy. Calculations were carried out using a method employed previously by the authors in examining the effect of hydrogen on the mechanical properties and self-diffusion in iron [3, 4]. The method is based on using quasifermion approximation for evaluating the Hartree-Fock energy. In this case, the expression for the total energy of a polyatomic system assumes the following form ' + •

Grain boundary self-diffusion of 51Cr and 59Fe in austenitic NiFeCr alloys

The grain bpundary (GB) self-diffusion of chromium and iron in polycrystalline austenitic NiFeCr alloys has been studied from 858 to 1208 K using the sectioning method. It appears that the dependence of the GB diffusivity on the average nickel concentration is not so strong as the dependence on the chromium concentration. It is shown that, unlike the volume diffusion coefficients, the GB diffusivities of chromium were lower or equal to those of iron. With respect to their dependence on the chromium content of the alloy, the iron and chromium GB diffusivities reach a maximum and a minimum respectively at 10 wt.% Cr. The Arrhenius parameters for iron and chromium GB self-diffusion tend to increase with increasing chromium content, reaching constant values at about 20 wt.% Cr. The activation energy for chromium GB diffusion for all the chromium isoconcentration sections was always higher than that for iron GB diffusion.

Theoretical examination of the effect of hydrogen on self-diffusion in iron

Theoretical examination of the effect of hydrogen on self-diffusion in iron

Effect of deoxidation practice and heat treatment on the hydrogen attack of carbon steels

The hydrogen attack (HA) kinetics of an electroslag refined (ESR) and a rare earth metal (REM)-treated steel in the Q. and T. condition were investigated by a highly sensitive dilatometer. Measured activation energies for bubble growth of 108 to 203 kJ/mol and pressure exponents of 0.9 to 1.6 are rationalized in terms of surface or grain boundary self-diffusion of iron as the rate controlling mechanisms depending on the external hydrogen pressure and temperature. Comparisons of the HA susceptibility of these steels with published work show that although the HA resistance of the ESR steel is not influenced by the heat treatment, the REM steel shows a significant decrease in the rate of sample expansion. SEM observations indicate that the improvement in the HA resistance of the REM steel is related to the presence of a very low density of methane bubbles.

Impurity Diffusion of Vanadium and Self-Diffusion in Iron

Impurity diffusion of /sup 48/V was measured in - and el-iron. In both phases the vanadium diffusion is faster by a factor of 2 to 3 when compared to iron self-diffusion. In the el-phase the diffusion parameters D/sub 0/=(0.62 +- 0.7) cm/sup 2/ s/sup -1/ and Q=(273.5 +- 6.2) kJ mol/sup -1/ correspond to a normal vacancy diffusion mechanism. The fit of the experimental data of vanadium in the paramagnetic -phase to an Arrhenius relation, D/sub 0/=(124 +- 19) cm/sup 2/ s/sup -1/ and Q=(274 +- 6.6) kJ mol/sup -1/, yields a very large frequency factor, like the results of the additionally measured self-diffusion in the paramagnetic -phase, D/sub 0/=(121 +- 4) cm/sup 2/ s/sup -1/ and Q=(281.6 +- 1.7) kJ mol/sup -1/, which can be understood by an influence of the magnetic ordering on lattice diffusion still above the Curie-temperature. The occurrence of ferromagnetic clusters tends to decrease the diffusivity already in the paramagnetic -phase. Thus the calculated results of D/sub 0/ and Q have to be regarded as effective values only.

Self-Diffusion of Iron in Heterogeneous Cu-Fe Alloys

Self-Diffusion of Iron in Heterogeneous Cu-Fe Alloys

Confirmation of the one-interstitial model for α-iron from positron annihilation experiments in thermal equilibrium on pure and carbon doped samples

The value of the vacancy migration enthalpy in α-iron has been the subject of a long standing controversy. In the one-interstitial model, the value of 0.55 eV has been put forward, whereas in the two-interstitial model a value of about 1.3 eV is given. In a review paper l, it was concluded that the majority of experimental data is in favour of the one-interstitial model. It was also pointed out that, if one accepts this model, an important problem remained to be solved: the vacancy migration enthalpy that can be obtained by subtracting the vacancy formation enthalpy from the self diffusion enthalpy (provided that self-diffusion is driven by a vacancy mechanism), was more than twice the vacancy migration enthalpy of the one-interstitial model. We report here on research we performed concerning this problem. The values of the enthalpies of vacancy formation and self diffusion were carefully re-investigated, taking into account: 1) the influence of ferromagnetic ordering, and 2) the influence of the strong interaction between residual carbon atoms and vacancies. To determine the ferromagnetic self diffusion enthalpy QS D,f, we used reliable literature data on self diffusion in iron. The main conclusion of our analysis is that only an upperlimit to QS D,f, can be given 2: QS D,f ⩽2.75 eV. We also performed positron annihilation measurements in thermal equilibrium on pure and carbon doped (50 and 750 appm) samples to determine the ferromagnetic vacancy formation enthalpy H V f,f. In very pure iron no trapping of positrons is observed below the Curie point, so that H V f,f cannot be determined. However in carbon-doped samples the carbon-vacancy pair can be used as a probe to determine H V f,f. We obtained: H V f,f = (2.0 ± 0.2) eV. This value is the first fully experimental determination of this parameter. By combining the data given above, we conclude that an upperlimit to the ferromagnetic vacancy migration enthalpy H V m,f is: H V m,f ⩽ 0.95 eV. On the other hand, it can be shown 3 that H V m,f ⩾ 0.16 eV. We therefore conclude that there is no longer an inconsistency between the one-interstitial model and the vacancy migration enthalpy derived from thermal equilibrium measurements.

Influence of cooling rate from normalizing temperature and tempering on strength of ferrite–pearlite steels

The influence of cooling rate from the normalizing temperature and tempering in the temperature range 450–600°C on the lower yield strength (LYS) and ultimate tensile strength (UTS) of two ferrite–pearlite structural steels has been examined. Falls in both LYS and UTS were obtained as a result of slow cooling or tempering; these falls were not related to grain-size changes. The processes controlling the falls in LYS and UTS were found to have activation energies of ∼170 kJ mol−1 and ∼290 kJ mol–1, respectively, and the kinetics of the processes obeyed t2/3 and t1/3 relationships, respectively (t being the time). The activation energy and kinetics for the process controlling the fall in UTS correspond to those associated with the spheroidization of pearlite, the rate-controlling factor being the self-diffusion of iron in the matrix, while the activation energy for the fall in LYS corresponds to that for diffusion of iron at the grain boundaries. An interesting relationship was also obtained between this fall in LYS and the thickening of the boundary carbides that occurred during tempering below the Ae 1 transformation temperature. The activation energy for the growth of grain-boundary carbides was ∼170 kJ mol−1 and the kinetics of growth also obeyed a t2/3 relationship. On the basis of these results, it is suggested that C that has become trapped at the boundaries on transformation can precipitate out on existing carbides at the boundaries during tempering. The removal of C from solution at the boundaries would then reduce the strength of the steel by lowering the stress required to transfer yielding across grain boundaries. Further work is suggested to examine this hypothesis in more detail.