Phase transformations in near equiatomic Ta−Ru alloys

Phase transitions in near equiatomic alloys of Ta-Ru have been investigated with X-ray powder patterns at room temperature and resistance, metallography, and susceptibility measurements as a function of temperature. The results suggest a “two-step” cubic-to-tetragonal-to-orthorhombic martensitic transformation with decreasing temperature in alloys with compositions in the immediate vicinity of 50–50 at. pct while those slightly farther away in composition exhibit a single transition from cubic-to-tetragonal on cooling to room temperature.     

Hydrogen transport in nickel base stainless alloys

Hydrogen transport parameters have been measured in two nickel base stainless alloys, HASTELLOY Alloys C-276 and G. Hydrogen diffusivity and permeability were determined by means of the electrolytic permeability technique over the temperature range of 17 to 90 °C. Although the two alloys are similar in composition and structure, they exhibited dramatically different hydrogen behavior. For Alloy C-276, the diffusivity in both the cold worked and annealed conditions decreased by a factor of two following low temperature (500 °C) aging. That behavior was related to ordering in the alloy. Unexpectedly, hydrogen trapping was not observed in Alloy C-276. An analysis of hydrogen transport in Alloy G indicated reversible and irreversible trapping of hydrogen by niobium substitutional atoms and second phase carbides, respectively. The hydrogen transport results were related to the hydrogen embrittlement tendencies of the two nickel base alloys. DAVID A. MEZZANOTTE, formerly Graduate Assistant with the Department of Metallurgical Engineering and Materials Science, University of Notre Dame. NICHOLAS F. FIORE, formerly Professor and Chairman of the Department of Metallurgical Engineering and Materials Science, University of Notre Dame.     

Prediction of the crystal structure types of equiatomic ternary silicides and germanides

New unsynthesized equiatomic ABX (A and B are various metals; X = Si or Ge) compounds are predicted, and their types of crystal structure are forecasted under standard conditions. Only the data on the properties of the elements—components of compounds are used for their prediction. The calculations are performed using a special-purpose software package of computer analysis of information intended for searching for regularities in databases on the properties of inorganic compounds, and this package is based on the methods of precedent pattern recognition. Computer analysis of the data on the well-known compounds shows that the functions that are most important for the classification of systems in the sign of formation or absence of equiatomic compounds are M(A) × M(B) and I(A) × I(X), where M is the Mendeleev-Pettifor number of element A or B and I is the thermal conductivity of element A or X. The parameters that most strongly separate compounds for crystal chemical classification are functions T(A) + T(B) (where T is the melting temperature of element A or B), I(A), M(A) × M(B), and I(A) × I(X).     

Microstructure and phase identification in type 304 stainless steel-zirconium alloys

Stainless steel-zirconium alloys have been developed at Argonne National Laboratory to contain radioactive metal isotopes isolated from spent nuclear fuel. This article discusses the various phases that are formed in as-cast alloys of type 304 stainless steel and zirconium that contain up to 92 wt pct Zr. Microstructural characterization was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and crystal structure information was obtained by X-ray diffraction. Type 304SS-Zr alloys with 5 and 10 wt pct Zr have a three-phase microstructure—austenite, ferrite, and the Laves intermetallic, Zr(Fe,Cr,Ni)_2+x. whereas alloys with 15, 20, and 30 wt pct Zr contain only two phases—ferrite and Zr(Fe,Cr,Ni)_2+x. Alloys with 45 to 67 wt pct Zr contain a mixture of Zr(Fe,Cr,Ni)_2+x and Zr₂(Ni,Fe), whereas alloys with 83 and 92 wt pct Zr contain three phases—α-Zr, Zr₂(Ni,Fe), and Zr(Fe,Cr,Ni)_2+x. Fe₃Zr-type and Zr₃Fe-type phases were not observed in the type 304SS-Zr alloys. The changes in alloy microstructure with zirconium content have been correlated to the Fe-Zr binary phase diagram.     

Corrosion-creep interaction of stainless alloys in acid chloride solutions

Rupture of passive film is considered as an essential step in the stress corrosion cracking (SCC) process. At constant load, accumulation of creep strain is often associated with the strain to passive film rupture. Therefore, low-temperature creep behavior of a material is important from an SCC point of view. Constant load creep studies carried out on alloy 22 (a Ni-22Cr-13Mo-4W alloy) in acidified chloride environments at 80 °C showed a logarithmic creep behavior. The creep strain decayed logarithmically and reached values less than 4×10⁻⁹/s, which is lower than the detectable limit of laboratory scale SCC tests. 304 SS showed SCC failure in acidified chloride solutions in simulated open circuit conditions. A steady-state creep strain rate could be observed during SCC failures, of the order of 10⁻⁵ to 10⁻⁶/s. The high creep strain rate of 304 SS can be correlated to the observed higher corrosion currents, which were more than 40 times that observed in alloy 22. When the dissolution rate of alloy 22 was increased by impressing about 1 mA/cm² anodic current, a steady-state creep strain rate of 6.5×10⁻⁸/s was observed. The results indicated that anodic dissolution increased the localized plasticity of the material, resulting in creep strain. However, alloy 22 did not show SCC. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

Transmission electron microscopy crystal structure study of the Cr-rich phase in a laser-clad Ni alloy

The point group and space group of an unknown Cr-rich phase in Ni alloy laser clads were identified with convergent beam electron diffraction (CBED). Zone axis symmetry analysis showed that the point group of the phase was 6/mmm. Further examination by zero intensity lines (GM lines) revealed the existence of a 6₃ screw axis and a glide plane. The space group was then identified as P6₃/mmc. By referring to [0001] zone axis high-resolution electron microscopy (HREM) images, a structure model with 12 Cr atoms at 12k positions and 2 Ni atoms at 2b positions, as listed in space group P6₃/mmc, was proposed. The alloying elements were considered to be distributed in the Ni lattice. Simulated [0001] and [11–20] HREM images using the proposed model showed a close match with corresponding transmission electron microscopy (TEM) images.     

Cracking kinetics of two-phase stainless steel alloys in hydrogen gas

The kinetics of hydrogen-induced slow crack growth (SCG) under constant load was studied in two stainless steel alloys containing mixtures of bcc and fcc phases. FERRALIUM 255, a duplex stainless steel, consisting of ∼50 pct austenite in a ferrite matrix, was tested in hydrogen gas at 0 to 100 °C with the loading axis both perpendicular and parallel to the rolling direction. In addition, specimens of AISI 301 were deformed in air in different ways to produce various amounts of bcc phase in an austenite matrix prior to testing in H₂ gas at room temperature. The kinetics of subcritical slow crack growth (SCG) in these alloys was compared with that for austenitic and for ferritic stainless steels. The SCG rates were rationalized in terms of differences in hydrogen permeation in the two phases. The results confirm that a higher rate of supply and accumulation of hydrogen in the region ahead of the crack tip allows a higher cracking velocity.     

Dilution control in gas-tungsten-arc welds involving superaustenitic stainless steels and nickel-based alloys

Fusion welds were prepared between a superaustenitic stainless steel, (the AL-6XN alloy) and two Ni-based filler metals (IN625 and IN622) using the gas-tungsten-arc welding (GTAW) process. Fusionzone compositions over the full range of dilution levels (0 to 100 pct) were produced by varying the independent welding parameters of arc power and volumetric filler-metal feed rate. Microstructural characterization of the welds was conducted via light optical microscopy, with quantitative chemical information obtained through electron-probe microanalysis (EPMA). The dilution level of each weld was determined from the EPMA data as well as through geometric measurements of the weld cross-sectional areas. The dilution level was observed to decrease with increasing filler-metal feed rate and decreasing arc power. These effects are quantitatively interpreted based on a previously proposed processing model. The model is used to demonstrate that, in terms of welding parameters, the dilution level can be correlated exclusively to the ratio of the volumetric filler-metal feed rate (V _fm) to arc power (VI), i.e., the individual values of V _fm and VI are not important in controlling the dilution and resultant weld-metal composition. Good agreement is obtained between experimental and calculated dilution values using the model. It is also demonstrated that the melting enthalpies of the filler metal and substrate have only a minor influence on dilution at dilution levels in the range from 40 to 100 pct. This knowledge facilitates estimates of dilution levels in this range when the substrate and fillermetal thermal properties are not accurately known. The results presented from this study provide guidelines for controlling the weld-metal composition in these fusion-zone combinations.     

Microanalytical study of the heterogeneous phases in commercial Al-Zn-Mg-Cu alloys

X-ray microanalysis and Convergent Beam Diffraction (CBD) studies were conducted on the second phase constituent and dispersoid particles in 7075 and 7475 aluminum alloys. Partial substitution of alloying elements was found to occur in all the second phase particles causing small deviations from the stoichiometric compositions reported for the binary and ternary compounds. The coarse constituent phases were identified to be Al₇Cu₂Fe, (Al,Cu)₆(Fe,Cu), Mg₂Si, α-Al₁₂Fe₃Si, amorphous silicon oxide, and a modified Al₆Fe compound in decreasing order of abundance. The dispersoid particles were Al₁₈Mg₃Cr₂ compound, and they formed in both triangular and spherical morphologies. Their compositions were found to vary slightly with the aging treatment. The crystal structure of the dispersoid phase consisted of a disordered form of a cubic structure (Fd3m) reported for the Al₁₈Mg₃Cr₂ compound. The uniqueness of CBD analysis in the crystal structure determination is emphasized.     

Effect of a catalyst on heterogeneous nucleation in pure and Fe-Ni alloys

In order to elucidate the nature of the heterogeneous nucleation, a differential scanning calorimetry (DSC) thermal analysis of pure Fe and Fe-Ni alloys (Ni content: 1.0 to 29.3 mass pct) containing TiN, Al₂O₃, and Ti₂O₃ was conducted. Then, special attention was paid to the difference in the phase of the primary crystal nucleated by the triggering effect of a catalyst (nucleating agent). The solidification and transformation mode appearing during cooling in these alloys is classified into three cases: F mode, FA mode, and A mode. The change of modes and the critical undercooling (ΔT) depend on the kind of catalyst used as well as the chemical composition (Ni content). In addition, in spite of the kind of primary crystal, the value of ΔT is always small in the order of TiN, Al₂O₃, and Ti₂O₃. As a matter of fact, only TiN has a practical effect as a catalyst on the triggered nucleation of the primary crystal of the δ phase. None of them has a practical effect on the nucleation of the primary crystal of the γ phase. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.     

Influence of low-gravity solidification on heterogeneous nucleation in stable iron-carbon alloys

The processes of nucleation and growth of alloys during solidification are linked to the level of gravitational force. In a low-gravity environment, buoyancy-induced convection becomes negligible, resulting in lower convection as compared to normal or high gravity. In this paper, heterogeneous nucleation and grain multiplication during solidification of gray cast iron, and the effect of gravitational level on them, have been studied by means of directional solidification on ground and under low-gravity (low-g) and high-gravity (high-g) conditions obtained by aircraft parabolic flights. It has been assumed that the final number of eutectic grains results from the contribution of heterogeneous nucleation,N _h , heterogeneous nucleation induced by inoculation,N _i , and heterogeneous nucleation induced by convection,N _c . In turn,N _c has two components, a grain multiplication component,N _c ^m , and a kinetics of chemical reactions component,N _c ^k . In all cases, it was found that a higher number of grains are obtained when solidifying in highg as compared with lowg. This was attributed to higher convection in highg. It was demonstrated that grain multiplication due to convection can contribute 20 to 23 pct from the total number of grains resulting from heterogeneous nucleation of uninoculated samples. For the case of inoculated samples, it was shown that the contribution to the convection-induced nucleation of the kinetics of chemical reactions can be as high as 30 pct but can be zero at very low or very high grain numbers. A possible mechanism and an explanation have been given to those findings. The silicon distribution, graphite morphology, and the influence of soak time on experimental results have also been discussed.     

Intercrystalline structure distribution in alloy 304 stainless steel

The intercrystalline structure distribution function (ISDF) describes the probability density for the occurrence of grain boundaries in the polycrystalline medium with five specified geometrical parameters: three describing intercrystalline lattice misorientation and two describing the orientation of the grain boundary plane. This paper extends the ISDF analysis to Bunge’s formalism which represents the distribution in terms of a series expansion of symmetric generalized spherical harmonics. An exemplary calculation of the ISDF is illustrated for alloy 304 stainless steel tubing. The results confirm the observation that gS3 and gS9 boundaries, arising from twinning, are prevalent in the structure. The distribution of twinning boundaries and other special boundaries is represented by Euler plots in the five geometrical parameters defining boundary structure. One remarkable feature of this material is a nearly isotropic distribution of boundary misorientation in the two parameters defining the boundary plane orientation. These results are compared with other published experimental data and theoretical calculations for the distribution of special boundaries.     

Complex grain structure of metals and alloys

An algorithm is derived for the identification of parameters characterizing the complex grain structure of metals and alloys. The analysis of photographs of sample sections confirms the applicability of the algorithm.