Microstructure and corrosion behavior of as-cast and heat-treated Al-4.5 Wt pct Cu-2.0 wt pct Mn alloys

The microstructure and corrosion behavior of as-cast and heat-treated Al-4.5 pct Cu-2.0 pct Mn alloy specimens solidified at various cooling rates were investigated. The equilibrium phases Al₆Mn and θ-Al₂Cu, which are observed in the conventionally solidified alloy in the as-cast condition, were not detected in rapidly solidified (melt-spun) material. Instead, the ternary compound Al₂₀Cu₂Mn₃ was present in addition to the α phase, which was present in all cases. The morphological and kinetic nature of corrosion was investigated metallographically and through potentiostatic techniques in 3.5 wt pct NaCl aqueous solution. Corrosion of the as-cast material was described by two anodic reactions: corrosion of the intermetallic phases and pitting of the α-Al solid solution. The corrosion rate increased with cooling rate from that for the furnace-cooled alloy to that for the copper mold-cast alloy and, subsequently, decreased in the rapidly solidified alloy. In the heat-treated material, corrosion could be described by two anodic reactions: corrosion of Al₂₀Cu₂Mn₃ precipitate particles and pitting of the α-Al matrix. S.M. Skolianos, formerly Graduate Student, Department of Metallurgy, University of Connecticut     

Microstructure and its development in Cu-Al-Ni alloys

The microstructure of as-cast Cu-AI-Ni alloys, based on copper containing 9 to 10 wt pct Al and up to 5 wt pct Ni, has been examined. The development of the microstructure on continuous cooling has also been investigated. For alloys with 9.2 to 9.3 wt pct Al, and less than 1 wt pct Ni, the as-cast microstructure consists of proeutectoid α solid solution, α + γ₂ eutectoid, and martensitic β. If the nickel content is more than 2.5 wt pct, the α + γ₂ eutectoid is replaced by α + β ₂ ^′ eutectoid, and no martensitic β is observed in the as-cast alloys. The morphologies of the β ₂ ^′ and γ₂ eutectoid phases are similar; both have the Kurdjumov-Sachs (K-S) orientation relationship with the a phase. Two eutectoid reactions, involving β to α + γ₂ and β to α + β′₂, have been observed in an alloy containing 9.7 wt pct Al and 2.7 wt pct Ni. When both eutectoid reactions occur, the Nishiyama-Wassermann (N-W) orientation relationship exists between γ₂ or β ₂ ^′ and the α phase. During continuous cooling, proeutectoid α solid solution is the first phase to precipitate from the high-temperature β phase. The β to α + β ₂ ^′ eutectoid reaction starts at higher temperatures than the β to α + γ₂ reaction. Tempering of the as-cast alloys results in the elimination of the martensitic β. Y.S. SUN formerly Research Associate with the Manchester Materials Science Centre.     

Effect of welding variables and solidification substructure on weld metal porosity

Porosity is defined as cavity-type discontinuities formed by gas entrapment during solidification. Causes of porosity in fusion welds are the dissolved gases in weld metal and welding process variables that control the solidification rate. To study the mechanisms of porosity formation in weld metal, single-pass gas tungsten-arc weld metal was produced using the bead-on-plate technique on three nickel-copper alloys (80 wt pct Ni-20 wt pct Cu, 65 wt pct Ni-35 wt pct Cu, 35 wt pct Ni-65 wt pct Cu). Four different welding speeds were used under various amounts of nitrogen content in argon-shielding atmosphere. A qualitative model was proposed to characterize the effect of welding variables and solidification substructure on bulk and interdendritic porosity formation. Increasing amounts of nitrogen gas (from 0.2 pct to 6.0 pct in volume) introduced in argon-shielding atmosphere increased the amount of porosity in weld metal. The amount of bulk and total porosity increased as the solubility of nitrogen in the weld metal alloy decreased. The solidification rate of the weld pool is the most important factor controlling the mechanism of porosity formation. The observed amount of bulk pores in this study increased with the increase of welding speed; that is, if the time is insufficient for dissolved and evolved gases to escape during solidification, porosity will result. However, a decrease in the amount of interdendritic pores was observed with increasing welding speed in the 80Ni-20Cu and 35Ni-65Cu alloys. This decrease can be related to the effect of solidification rate on the balance between the disjoining pressure, resistance of the liquid film to be disrupted, repulsion of the bubble from the solidification front, and the hydrodynamic force resisting the movement of the bubble. This balance determines the ability of the cellular solidification front to “equilibrium” capture the pores. Furthermore, the observed decrease of interdendritic porosity with increasing welding speed (80Ni-20Cu and 35Ni-65Cu alloys) can also be related to the time for nucleation and growth of pores in the molten weld metal and their entrapment in the interdendritic channels of a dendritic solidification front. This phenomenon is considered a “nonequilibrium capture” of pores. On the other hand, the 65Ni-35Cu alloy that exhibited a structural transition in solidification substructure with the variation of welding speed showed a slight increase in the amount of interdendritic pores. This increase was correlated to the change of pore-capture mechanism from an equilibrium to a nonequilibrium mode as the solidification substructure changed from cellular to cellular dendritic. To substantiate that the controlling mechanism of interdendritic porosity formation is the nonequilibrium capture, a good correlation between the measured mean pore radius and the interdendritic arm spacing was found.     

Solidification of chill-cast Al-Zn-Mg alloys to be used as sacrificial anodes

In the present research, Al-Zn-Mg alloys were vacuum induction melted and gravity cast into steel molds. Ingots were microstructurally and electrochemically characterized to evaluate their performance as Al-sacrificial anodes for cathodic protection of structures exposed to marine environments. The microstructure observed in as-cast ingots consisted mainly of α-Al dendrites with 0.68 to 2.25 vol pct of τ phase in α-Al matrix and eutectic in interdendritic regions. After heat treatment, the presence of the τ phase increased up to 5 vol pct. Electrochemical efficiencies obtained in Al alloys showed maximum values of 73 and 87 pct in as-cast ingots and heat-treated ingots, respectively. In order to contribute to the development of Al-Zn-Mg anodes, the Al-5.3 at. pct Zn-6.2 at. pct Mg (Al-12 wt pct Zn-5.4 wt pct Mg) alloy was monitored to identify the temperature changes as it cools through phase transformation intervals. Growth temperatures of the phases present in this alloy were employed to predict the structure growing at fixed growth velocity. Predictions of variation of solute concentration with growth velocity in α-Al dendrites were included, too. The results of these analyses help to select alloy composition and to control microstructure in order to develop a new generation of Al-sacrificial anodes free of In and Hg.     

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (h _i), secondary dendrite arm spacing (λ₂), corrosion potential (E _Corr), corrosion rate (i _Corr), polarization resistance (R ₁), capacitances values (Z _CPE), ultimate tensile strength (UTS, σ _u ), yield strength (YS, σ _y ), and elongation. It is shown that σ _U decreases with increasing λ₂ while the CR increases with increasing λ₂, for both alloys experimentally examined. A combined plot of CR and σ _U as a function of λ₂ is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.     

A comparison of the microstructures of As-cast and laser surface melted Ti-8Al-4Y

A Ti-8 at. pct Al-4 at. pct Y alloy has been cast in small quantities and subjected to laser surface melting treatments in an effort to produce a uniform distribution of fine-scale dispersoids by internal oxidation. The microstructures of both as-cast and laser-modified material have been characterized using transmission electron microscopy, energy dispersive X-ray spectroscopy, and convergent beam electron diffraction, and the identity and distribution of second phase dispersoids established. An ability to refine significantly the dendritic structure of the as-cast alloy by laser surface treatment is demonstrated. The matrix phase of the modified alloy is martensitic α′, in contrast to the α phase of the as-cast material. The interdendritic phase observed in both as-cast and laser-modified material and the fine-scale dispersed particles (50 to 70 nm) in the as-cast alloy are shown to be rich in Y, with a structure consistent with that of the oxide Y₂O₃. Dispersoids within the laser modified zone are typically 8 to 20 nm in diameter and show a tendency to form preferentially at interlath boundaries of the martensite. The origins of these structures and their potential effects on the mechanical properties of the alloy are discussed briefly.     

The morphology,crystallography, and chemistry of phases in as-cast nickel-aluminum bronze

The morphology, crystallography, and composition of the phases present in as-cast nickel-aluminum bronze of nominal composition copper-10 wt pct aluminum-5 wt pct nickel-5 wt pct iron have been investigated using optical, electron optical, and microprobe analysis techniques. The as-cast microstructure consists of copper-rich α, martensitic β, and κ-phases based on Fe₃Al and NiAl. The κ_z precipitates have a dendritic morphology and are cored; the composition ranges from iron-rich solid solution to Fe₃Al. The κ_II and κ_iv precipitates have, respectively, a dendritic and an equiaxed/dendritic morphology, and are based on Fe₃Al, while κ_III is a eutectoidal decomposition product of lamellar or globular morphology based on NiAl. The κ_I, κ_II, and κ_III precipitates have the Kurdjumov-Sachs orientation relationship with α matrix. Small κ_IV precipitates exhibit the Nishiyama-Wasserman orientation relationship with the α matrix, while large κ_iv precipitates have an orientation relationship which lies between Kurdjumov-Sachs and Nishiyama-Wasserman.     

The effect of oxygen on the structure and mechanical behavior of Aged Ti-8 Wt pct Al

It is shown that for a Ti-8 wt pct Al alloy aged at a temperature high in the two-phase region (695°C) to precipitate the ordered α₂ phase, an increase in oxygen content from 600 ppm to 1200 ppm decreases the fracture strain from 20 to 1 pct elongation at room temperature and slightly increases the yield strength. The fracture mode is changed from dimpled rupture to predominantly cleavage. Further increase in oxygen content to 3000 ppm does not produce significant additional changes in ductility or yield strength. It is demonstrated that oxygen additions alter the position of the α/α + α₂ coherent solvus, resulting in formation of coherent α₂ in specimens containing ⪞ 1000 ppm oxygen aged at 968 K (695°C). For a given aging time the volume fraction of α₂ increases with increasing oxygen up to 1300 wt ppm and then levels off. The changes in mechanical behavior are attributed to the presence of α₂. The experimental evidence suggests that oxygen partitions preferentially into α₂. Formerly a Graduate Student in the Department of Metallurgy and Materials Science, University of Pennsylvania     

Cast microstructure and fatigue behavior of a high strength aluminum alloy (KO-1)

Microstructure and fatigue behavior were studied in sand-cast and end-chilled KO-1 high strength aluminum alloy. Secondary dendrite arm spacing and volume percent microporosity increase with distance from the chill, whereas volume percent inter dendritic nonequilibrium secondary phase decreases. Solution kinetics of the secondary phase depend on the dimensionless parameter [Dt/L ²], where:D is the diffusivity of copper in this alloy at the solution temperature,L is half the dendrite arm spacing, andt the solutionizing time. The fatigue life of the alloy at room temperature was measured in reversed bending on unnotched specimens at stress levels of 17,300 and 19,000 psi and was found to decrease with increasing distance from the chill. Unnotched solutionized specimens exhibited a longer fatigue life than as-cast specimens, even though crack growth studies showed that cracks grew more rapidly in the solutionized material. This would be attributed to a delay in crack initiation resulting from the decrease in the amount of microporosity and the rounding of micropores during heat treatment. Micropores and inclusions acted as sources of stress concentration for fatigue crack initiation. Cracks then usually grew transgranularly.     

Fracture behavior of thixoformed 357-T5 Al alloys

The effects of microstructural features on the fracture behaviors, including impact, high-cycle fatigue, fatigue crack propagation, and stress corrosion cracking, of thixoformed 357-T5 (Al-7 pct Si-0.6 pct Mg) alloy were examined. The resistance to impact and high-cycle fatigue of thixoformed 357-T5 tended to improve greatly with increasing volume fraction of primary α. An almost threefold increase in impact energy value was, for example, o served with increasing volume fraction of primary α from 59 to 70 pct. The improvement in both impact and fatigue properties of thixoformed 357-T5 with increasing volume fraction of primary α in the present study appears to be related to the magnitude of stress concentration at the interface between primary α and eutectic phase, by which the fracture process is largely influenced. The higher volume fraction of primary α was also beneficial for improving the resistance to stress corrosion cracking (SCC) in 3.5 pct NaCl solution. The in-situ slow strain rate test results of thix oformed 357-T5 in air and 3.5 pct NaCl solution at various applied potential values demonstrated that the percent change in tesile elongation with exposure decreased linearly with increasing volume fraction of primary α within the range studied in the present study. Based on the fractographic and micrographic observations, the mechanism associated with the beneficial effect of high volume fraction of primary α in thixoformed 375-T5 alloy was discussed.     

Observations on void nucleation and growth in α/β Ti-Mn alloys

Void nucleation and growth was studied in three binary equiaxed α-β Ti-Mn alloys containing 1.8 wt pct Mn (alloy 2), 3.9 wt pct Mn (alloy 3), and 5.8 wt pct Mn (alloy 4) given heat treatments to vary the alpha size at constant volume fraction of alpha. Void nucleation rates expressed as number of voids per unit volume,N _v, increased exponentially with true strain, ε. WhenN _v was normalized with respect to the number of alpha particles or grains per unit volume, N_α ^T,N _v/N_α ^T was found to be largest for the largest alpha size in each alloy series. Void size distributions as a function of strain for one alloy containing 3.9 wt pct Mn (alloy 3 given heat treatment B,3B) were presented and, as expected, the largest number of voids occurred at the smallest void sizes. Void growth rates for alloys 3 and 4 were found to increase with increasing particle size and this was ascribed to decreasing constraints to slip with increasing particle size. For alloy 2C with the largestα grain size void growth rates were smallest and this behavior was attributed to the growth inhibiting effects of multiple twinning. Evidence was adduced to show that nucleating voids grow more rapidly than established voids. T. V. Vijayaraghavan, Formerly Graduate Student, Polytechnic University, Brooklyn, NY     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

Measurements of the bulk velocity during solidification of metallic alloys

The ultrasound Doppler velocimetry (UDV) was applied to measure the bulk flow in a Sn-15 wt pct Pb alloy solidified directionally from a water-cooled copper chill. The flow is driven by a rotating magnetic field (RMF). Our results show that the velocity profiles undergo distinct modifications during solidification indicating the occurrence of more sophisticated flow patterns as known from the isothermal case. Furthermore, the UDV data allow an assessment of the current position of the dendritic solidification front. This research is supported by the Deutsche Forschungsgemeinschaft (DFG) in the form of the SFB 609 “Electromagnetic Flow Control in Metallurgy, Crystal Growth and Electrochemistry.” This support is gratefully acknowledged by the authors.     

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al. Formerly with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University     

Structure and properties of a rapidly solidified Al-Li-Mn-Zr Alloy for high-temperature applications: Part I. inert gas atomization processing

A new Al-Li alloy containing 2.3 wt pct Li, 6.5 wt pct Mn, and 0.65 wt pet Zr, for high-temperature applications, has been processed by a rapid solidification (RS) technique (as powders by inert gas atomization) and then thermomechanically treated by hot isostatic pressing (hipping) and hot extrusion. As-received and thermomechanically treated powders (of various size fractions) were characterized by X-ray diffraction and scanning and transmission electron microscopy (SEM and TEM, respectively). Phase analyses in the as-processed materials revealed the presence of two Mn phases (Al₄Mn and Al₆Mn), one Zr phase (Al₃Zr), two Li phases (the stable AlLi and the metastable Al₃Li), and the αAl solid solution with high excess in Mn solubility (up to close the nominal composition in the as-atomized powders). Extruded pieces were solutionized at 370 °C and 530 °C for various soaking times (2 to 24 hours). A variety of aging treatments was practiced to check for the optimal (for tensile properties) aging procedure, which was found to be the following: solutioning at 370 °C for 2 hours and water quenching + 1 pct mechanical stretching + one step aging at 120 °C for 3 hours. The mechanical properties, at room and elevated temperatures, of the “hipped” and hot extruded powders are compared following the optimal solutioning and aging treatments. The results indicate that Mn is indeed a favorable alloying element for rapidly solidified Al-Li alloys to retain about 85 to 95 pct of the room-temperature tensile properties even at 250 °C, though room-temperature strength is not satisfactory in itself. However, specific moduli are by 20 to 25 pet higher than those of the 2024 series duralumin-type alloys. Ductilities at room temperatures are in the low 1 to 2.5 pct range and show no improvement over other Al-Li alloys.     

Phase Equilibria and Interstitial Effects in the Ti-V-Mo Alloy System

The stability of theβ phase in the Ti-V, Ti-Mo, and Ti-V-Mo alloy systems was investi-gated, and theβ/α + β phase boundaries in these systems were determined in the range 300 to 600° C. The results indicate that Mo is more potent than V in stabilizing theβ phase with respect to α phase formation and in retarding the β → α reaction kinetics. It is shown that increasing the oxygen concentration in the alloys tends to enhance α phase formation in Mo-lean alloys (Mo contents < 15 wt pct), whereas it leads to the formation of an oxide phase in Mo-rich alloys (Mo contents ≥15 wt pct). Formerly Research Assistant, Department of Materials Science, University of Southern California     

A metallographic study of porosity and fracture behavior in relation to the tensile properties in 319.2 end chill castings

A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al₅FeSi, α- Al₁₅(Fe,Mn)₃Si₂, and Al₂Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.     

The intergranular microstructure of cast Mg-Zn and Mg-Zn-rare earth alloys

The solidification path and microstructure of cast Mg-9Zn and Mg-8Zn-1.5MM (misch metal) alloys have been investigated by a combination of thermal analysis and analytical electron microscopy. The addition of 1.5 wt pct MM had a strong influence on the as-cast microstructure with the introduction of new “ternary” interdendritic phases and structural modification of known binary phases. The temperature ranges for formation of these phases from the melt were identified, their crystal structures determined, and their compositions analyzed. Products from eutectoidal decomposition of the interdendritic phase in the binary Mg-9Zn alloy were also identified.     

The microstructure of rapidly solidified Ti-Fe melt-spun ribbons

Ti-Fe binary alloys were rapidly solidified by the melt-spinning technique, and four compositions were examined: Ti-5 wt pct Fe, which is the critical composition for theβ to ω athermal transformation; Ti-10 wt pct Fe, which represents a hypoeutectoid composition; the eutectoid composition Ti-15 wt pct Fe; and Ti-20 wt pct Fe, as an example of a hypereutectoid alloy. The Ti-5 wt pct Fe rapidly solidified ribbons are composed of two different structures. The first consists of α′-martensite plates inβ matrix and the second, athermal ω particles inβ matrix. The Ti-10, 15, and 20 wt pct Fe alloys are also composed of two structures. These areβ grains and isothermal-like ω particles inβ matrix. A solidification model is suggested which explains the existence of two different microstructures at the same composition and the for-mation of two kinds of ω particles.     

Diffusivity and solubility of hydrogen as a function of composition in Fe-Ni alloys

An electrochemical method has been used to determine the permeability,P, diffusion coefficient,D, and solubility,c, of hydrogen in alloys of the Fe-Ni system. The heats of activation for diffusion and the heats of solution have been derived.D falls from ≃10⁻⁴ sq cm per sec for pure iron to ≃10⁻¹⁰ sq cm per sec for 40 wt pct of Ni in the alloy. Thereafter it rises slightly to that for pure nickel,c rises by about 10³ between pure iron and 40 wt pct Ni, then remains constant up to pure nickel. The resultantP doubles at 5 wt pct Ni and then falls by 10³ times up to 40 wt pct Ni, afterwards rising slightly to that for pure nickel. Between 0 and 40 wt pct Ni the dominant factor in controlling the value ofP is the fall of the mole fraction of the α phase in the alloy. This hypothesis gives a reasonable quantitative calculation of theP-composition relation. Between 40 and 100 wt pct, the crystallographic phase is allγ and the major effect is the bonding of hydrogen in the alloy, the small changes noted being reasonably calculable. The negligible change of solubility in this region reflects the negligible change ind character of the alloy from 40 to 100 wt pct Ni. The hydrogen permeability of Fe-Ni (5 wt pct) is greater than that of palladium atT > 200°C. The corrosion rate and hydrogen permeability (hence, susceptibility to hydrogen embrittlement) pass through a minimum at about 50 wt pct Ni. A remarkable parallelism exists between corrosion rate and hydrogen permeation in Fe-Ni alloys. An interpretation is suggested. Formerly with the University of Pennsylvania Formerly with the University of Pennsylvania Work carried out by P. K. SUBRAMANYAN in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University of Pennsylvania, 1970.