Interdiffusion and Reaction between Pure Magnesium and Aluminum Alloy 6061

Using solid-to-solid couples investigation, this study characterized the reaction products evolved and quantified the diffusion kinetics when pure Mg bonded to AA6061 is subjected to thermal treatment at 300°C for 720 hours, 350°C for 360 hours, and 400°C for 240 hours. Characterization techniques include optical microscopy, scanning electron microscopy with X-ray energy dispersive spectroscopy, and transmission electron microscopy. Parabolic growth constants were determined for γ-Mg17Al12, β-Mg2Al3, and the elusive ε-phase. Similarly, the average effective interdiffusion coefficients of major constituents were calculated for Mg (ss), γ-Mg17Al12, β-Mg2Al3, and AA6061. The activation energies and pre-exponential factors for both parabolic growth constant and average effective interdiffusion coefficients were computed using the Arrhenius relationship. The activation energy for growth of γ-Mg17Al12 was significantly higher than that for β-Mg2Al3 while the activation energy for interdiffusion of γ-Mg17Al12 was only slightly higher than that for β-Mg2Al3. Comparisons are made between the results of this study and those of diffusion studies between pure Mg and pure Al [1] to examine the influence of alloying additions in AA6061.

Modelling of the Growth of Fe2B Layers in AISI 1018 Steel

In this work, a simulation of the growth kinetics of layers on AISI 1018 steel was done by means of a kinetic model. This model considers a solid diffusion of boron into a semi-infinite medium where the boron solubility in the Fe phase depends on the process temperature. An expression of the parabolic growth constant was then obtained through an application of the mass balance equation at the (/substrate) interface. The present model was validated by the experimental data available in the reference work (I. Campos-Silva et al: Kovove Mater. Vol.47 (2009), p.1-9). A good concordance was observed between the experimental parabolic growth constants and the predicted ones by the model for an upper limit of boron in the phase equal to 8.91 wt.% ( as a fitting parameter of the model). In addition, the generated weight gain was estimated at the surface of the borided AISI 1018 steel as a function of the upper limit of boron in the phase and the temperature.

Diffusion of Hard Coatings on Ductile Cast Iron

ABSTRACTThis work estimate the growth kinetics of Fe2B coatings created on surface nodular cast iron ASTM A-536 class 80-56-06. The Fe2B coatings were formed by power packaging boriding process, considering three temperatures and exposure times different treatment. The hard coatings were evaluated through X-ray diffraction (XRD) and scanning electron microscopy (SEM). The model of diffusion employs the mass balance equation at the (Fe2B/substrate) interface to evaluate the boron diffusion coefficient in the Fe2B coating DFe2B, an expression of the parabolic growth constant, the instantaneous velocity of the Fe2B/substrate interface, and the weight gain in the boriding sample were establish as a function of the parameter ε(T) and η(T), dependents of boriding process in function of the temperature related and the time of boriding t0 (T), respectively in the Fe2B coating. Model validation was extended considering the treatment of 1273 and 1123 K for 10 h respectively, obtaining a good correlation with experimental data.

A Model for the Growth of Fe2B Layers on a Steel Substrate: Effect of the Surface Boron Concentration

The growth kinetics of Fe2B layers formed at the surface of AISI 1018 was simulated. The paste-boriding (with a paste thickness of 4 mm) was applied to produce the Fe2B phase at the material surface; considering four temperatures (1123, 1173, 1223 and 1273 K) for 2, 4, 5, 6 and 8 h. The suggested model was based on the mass balance equation at the (Fe2B /substrate) interface. As a fitting parameter of the model, the surface boron concentration (12.16 wt. %B) was obtained in order to predict with a good agreement the experimental parabolic growth constants at the (Fe2B /substrate) interface derived from the literature. An expression of the parabolic growth constant at the (Fe2B /substrate) interface was obtained as a function of the two parameters: and . In addition, a relationship of the Fe2B layer thickness was also deduced that showed a good concordance with the experimental results from the literature.

Study on the Growth of Nb3Sn Superconductor in Cu(Sn)/Nb Diffusion Couple

Nb3Sn growth following the bronze technique, (i.e. by interdiffusion between Cu(Sn) alloy (bronze) and Nb) is one of the important methodologies to produce this superconductor. In this study, we have addressed the confusion over the growth rate of the Nb3Sn phase. Furthermore, a possible explanation for the corrugated layer in the multifilamentary structure is discussed. Kirkendall marker experiments were conducted to study the relative mobilities of the species, which also explained the reason for finding pores in the product phase layer. Based on the parabolic growth constant at different temperatures, the activation energy for the growth is determined. We have further explained the dramatic increase in the growth rate of the product phase by changing just one atomic percentage of Sn in the Cu-Sn bronze alloy.

Microstructural Evolution of Durable Thermal Barrier Coatings with Hf and/or Y Modified CMSX-4TM Superalloy Substrates

Thermal cyclic lifetime and microstructural degradation of thermal barrier coatings (TBCs) with (Ni,Pt)Al bond coat and Hf- and/or Y-modified CSMX-4 superalloy substrates were examined. Thermal cyclic lifetime of TBCs was measured using a furnace thermal cycle test that consisted of 10-minute heat-up, 50-minute dwell at 1135°C, and 10-minute forced-air-quench. TBC lifetime was observed to improve from 600 cycles to over 3200 cycles with appropriate Hf- and/or Y-alloying of CMSX-4 superalloys. This significant improvement in TBC lifetime is the highest reported lifetime in literature with similar testing parameters. Cross-sectional microstructure of TBC specimens were examined by scanning electron microscopy (SEM) after the spallation failure. While undulation of TGO/bond coat interface (e.g., rumpling and racheting) was observed to be main damage mechanisms for TBCs on baseline CMSX-4, the same interface remained relatively flat for durable TBCs on Hf- and/or Y-modified CSMX-4. The parabolic growth constant of the TGO scale was slightly lower for TBCs with Hfand/ or Y-modified CSMX-4.