Diffusion Of Substitutional Solutes Research Articles

Oxidation and carburization behaviour of two type 316H stainless steel casts in simulated AGR gas environment at 550 and 600 °C

• Duplex oxide and carburized layers form on steels exposed to AGR reactor coolant • Cracking in the oxide layer increases the oxidation and carburization rates • Carbon locates in the austenite lattice and predominately M 23 C 6 carbides • Diffusion of substitutional solutes is the rate-limiting mechanism of carburization • Small precipitates lead to a reduced carburization rate at 550ºC The time-dependent oxidation and carburization behaviour of two type 316H austenitic stainless steels with varying Mn content and differing average austenite grain size of 142 µm in ‘Low Mn’ (0.98 wt.% Mn) and 81 µm in ‘High Mn’ (1.52 wt.% Mn), were assessed at 550 °C and 600 °C in the presence of simulated reactor primary gas coolant containing 500 vppm H 2 O, 100 vppm H 2 , 300 vppm CH 4 and 1 vol.% CO, balanced with CO 2 . Rupture and spallation of the initial protective chromia layer occurs in the ‘High Mn’ steel after 2000 h at 550 °C, and leads to the formation of a magnetite/spinel oxide layer that reaches 75% surface coverage only after 8000 h. In contrast, ‘Low Mn’ steel reaches ~ 85% coverage after only 1000 h at the same temperature. The development of an inner carburized layer occurs gradually in both steels once the duplex oxide layer is forming. The differences in steel behaviour are reduced at 600 °C, where both oxidation and carburization are significantly accelerated. Only 0.02 wt.% carbon remains in solid solution in the austenite lattice in the carburized layer, the excess carbon atoms precipitating out in the form of Cr-rich M 23 C 6 particles, present both inter- and intragranularly. The experimental values of the activation energy for carburization suggest that diffusion of substitutional solutes as the rate-limiting mechanism of the process. Previously observed carburization depths of ≥ 200 µm in ex-service austenitic stainless steels, may be achieved by pre-conditioning 316H steel in simulated reactor gas coolant for ≥ 6000 h at 600 °C.

Effect of dynamic strain aging and precipitation on the hot deformation behavior of 253MA heat-resistant alloy

In this research, we performed an experimental investigation into the influence of dynamic strain aging (DSA) and precipitation behavior on the plastic deformation and microstructure evolution in alloy 253MA considering its service temperature. The serrated flow behavior, stress anomaly behavior and negative strain rate sensitivity are considered as the manifestation of DSA within a certain temperature range (TDSA), which can be classified as low (550–700 °C), intermediate (600–750 °C) and high temperatures (700–790 °C) at 0.001 s−1, 0.01 s−1 and 0.1 s−1, respectively. The activation energy of DSA was determined to be 242 kJ/mol, indicating that the diffusion of substitutional solutes, such as Cr and Ni, is mainly responsible for the DSA phenomenon. The characteristic of dislocation substructures is prone to change from linear structure to cellularization structure with a rising deformation temperature in the DSA regime. The stress anomaly behavior can be attributed to the rapid dissolution of σ phase. The initiation of dynamic recrystallization should contribute to the disappearance of DSA by suppressing the dynamical solute atom–dislocation interaction.

Stress-dependent solute energetics in W–Re alloys from first-principles calculations

We present a systematic study of Re solute transport energetics in W using density functional theory calculations. The study focuses on substitutional solute diffusion in the presence of dislocation strain fields as a first step toward capturing the essential physics of solid solution hardening/softening in W–Re alloys. We calculate the heat of solution, the vacancy formation energy and the solute migration energy as functions of both hydrostatic and shear strains. Our results show that the vacancy formation energy scales with hydrostatic deformation, whereas it decreases with increasing shear strain. The migration energy decreases with hydrostatic deformation, whereas it displays path-length-dependent behavior under shear deformation. In addition, we compute the binding energies of an Re solute atom to the cores of 1/2〈111〉 screw and edge dislocations, and find the binding energy to be highest in the tensile lobe of the edge core. Finally, we obtain the dilatational stress due to a solute atom as a function of distance. Our calculations are then used to parameterize the jump rate of Re atoms in W as a function of the underlying stress state.

Experimental and Finite Element Simulation Study of Thermal Relaxation of Residual Stresses in Laser Shock Peened IN718 SPF Superalloy

An integrated experimental and modeling/simulation approach was developed to investigate and secure a quantified knowledge of the impact of high temperature exposures on the stability of residual stresses in a laser shock peened (LSP) high temperature aero-engine alloy, IN718 SPF (super-plastically formed). Single dimple LSP and overlap LSP treatments were carried out utilizing a Nd:Glass laser (λ = 1.052 μm), and subsequent heat treatments on the LSP-treated coupons were conducted at different temperatures between 550 and 700 °C. A 3-D nonlinear finite element (FE) computational model and the rate-dependent Johnson-Cook material model were calibrated using the experimental results of residual stress from the single dimple LSP and thermal relaxation treatments, and were further extended to the overlap LSP treatment case. Both experimental and FE simulations show that: a) a high level of compressive residual stress (~700 MPa at surface) and residual stress depth (~0.4–0.6 mm) were achieved following LSP, and b) the overlap LSP treatment gave higher residual stress and greater depth. The magnitudes of the initial residual stress (and plastic strain), heating temperature and exposure time were identified as the key parameters controlling the thermal relaxation behavior. The stress relaxation mainly occurs initially before 20 min exposure and the extent of relaxation increases with an increase in temperature and a higher magnitude of the initial as-peened residual stress. In addition, in regions deeper than ~300 μm or after initial thermal exposure where the residual stress was lower than ~300 MPa, stress relaxation was found to be negligible. Kinetic analysis of the experimental thermal relaxation data based on Zener-Wert-Avrami model gave an activation enthalpy of 2.87 to 3.77 eV, which is near that reported in the literatures for volume and/or substitutional solute diffusion in Nickel. These results suggest that thermal relaxation of the LSP-induced residual stress occurs by a creep-like mechanism involving recovery, rearrangement and annihilation of dislocations by climb.

Serrated flow behavior in AL6XN austenitic stainless steel

Serrated flow behavior of the AL6XN austenitic stainless steel has been investigated at different temperatures and strain rates. The results show the serrated flow, peak/plateau in flow stress and negative strain rate sensitivity appearing in tensile deformation of the AL6XN steel at 773–973 K and 3.3 × 10 −5–3.3 × 10 −3 s −1 (excluding 873 K, 3.3 × 10 −5 s −1), suggesting the occurrence of dynamic strain aging (DSA). The activation energy for type-A and -(A + B) serrations was calculated to be 304 kJ/mol and diffusion of substitutional solutes, such as chromium and molybdenum is considered as the mechanism of serrated flow. TEM observations further revealed a typical planar slip mode in the regime of DSA of the deformed AL6XN steel.

Diffusion in Metallic Elements and Intermetallics

Starting from some fundamentals of solid-state diffusion, we remind the reader to the major techniques for lattice diffusion measurements. Self-diffusion is the most basic diffusion phenomenon in any solid. The paper covers main features of self-diffusion in pure fcc and bcc metals and some important facts about diffusion of substitutional solutes in metals. Binary intermetallics are compounds of two metals or of a metal and a semimetal. Their structures are different from those of the constituents. Some intermetallics are interesting functional materials others have attracted attention as high-temperature structural materials. The paper reviews some results mainly from our laboratory on diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si and Co-Nb, which have been published in detail elsewhere. Some results for the ternary system Ni-Fe-Al are also mentioned.

The Soret effect in diffusion in crystals

The Soret effect, as it occurs in the diffusion of solutes in crystals, is analyzed using the principle of microscopic reversibility whereby an approach is developed for interpreting and computing Q ∗, the heat of transport. To compute the transport of energy during the diffusion jump process, and then Q ∗, the processes of thermal activation to the transition state and the decay from the transition state are considered to be inverses; thus the net process is reduced to the analysis of the purely mechanical decay process. Molecular statics and dynamics are then suggested as the means to simulate the decay process and the case of carbon diffusion in body-centered cubic α-iron is so analyzed as an example. Our results show that, for the case Q ∗ ≈ − Q m, where Q m is the activation energy for carbon diffusion, this is in agreement with experimental evidence reported in the literature. Thus both the sign and magnitude of Q ∗ are correctly predicted. Various cases, such as the diffusion of substitutional solutes and vacancies, are also considered within our approach along with implications for future study.

Bulk and interface boundary diffusion in group IV hexagonal close-packed metals and alloys

Bulk and grain boundary (GB) self-diffusion and substitutional solute diffusion in group IV hexagonal close-packed (hcp) metals (α-Ti, α-Zr, and α-Hf) are reviewed. The recent results obtained on high-purity materials are shown to approach closely the “intrinsic” diffusion characteristics. The enhancement effect of fast-diffusing impurities (such as Fe, Ni, or Co) is discussed for both self- and substitutional bulk solute diffusion in terms of the interstitial solubility of the impurity atoms. In GB self-diffusion, the impurity effect is found to be less dramatic. The results obtained on high-purity hcp materials can be interpreted in terms of intrinsically ‘normal’ vacancy-mediated GB diffusion, with the ratio of GB to volume diffusion activation enthalpies of Qgb/Q ≈ 0.6. The GB self-diffusion in group IV hcp metals reveals distinct systematics. Bulk self-diffusion and fast interstitial solute diffusion (Fe and Ni) in the hcp phase α2-Ti3Al are reviewed. Interphase boundary diffusion of Ti in the unidirectional lamellar α2/γ structure of the two-phase Ti48Al52 alloy is analyzed with respect to the phase boundary structure and GB self-diffusion in α2-Ti3Al.

Intrinsic self-diffusion and substitutional Al diffusion in α-Ti

Self-diffusion and Al impurity diffusion were studied in the α (h.c.p.) phase of Ti. We used four Ti materials with different impurity contents, including ultrapure Ti with extremely small concentrations of interstitial impurities (Fe, Co and Ni). The self-diffusion measurements are performed with the radiotracer 44Ti and the ion beam sputtering technique. For Al diffusion measurements in-depth profiling by secondary ion mass spectrometry is applied. The measurements are made both perpendicular (⊥) and parallel (‖) to the c axis using single crystals and coarse-grained polycrystals. The measurements on ultrapure α-Ti yield the Arrhenius parameters D 0⊥ = 1.35 × 10 −3m 2/s and Q ⊥ = 303 ± 2kJ/mol for self-diffusion and D 0⊥ = 6.6 × 10 −3m 2/s and Q ⊥ = 329 ± 2kJ/mol for Al diffusion. The anisotropy factor D |/ D ⊥ ≈ 0.5 for self-diffusion and ≈ 0.65 for Al diffusion. These results are treated as intrinsic diffusion properties of α-Ti. It is demonstrated that they are well consistent with the normal diffusion behaviour in other h.c.p. metals. We conclude that both self-diffusion and substitutional solute diffusion in α-Ti are intrinsically normal and dominated by the vacancy mechanism. Diffusion in less pure α-Ti occurs faster and with a smaller activation energy. This effect is explained by the enhancement of atomic mobility in the matrix material owing to the interstitially dissolved fast-diffusing impurities.

Diffusion of Substitutional Solutes in α-Ti Measured by RBS and HIRBS

Diffusion of Substitutional Solutes in α-Ti Measured by RBS and HIRBS