Effect of fiber volume fraction on the fracture behavior of Nb-1 Wt pct Zr/218W composites at elevated temperatures

Nb-1 wt pct Zr/218W long-fiber composite monotapes, nominally containing 0 to 70 vol pct of 218 tungsten fibers, were fabricated by arc spraying the Nb-1 pct Zr matrix onto the tungsten fibers. The monotapes were consolidated by hot pressing and hot isostatic pressing techniques. Tensile tests conducted between 1400 and 1600 K, under engineering strain rates varying between 1.5×10⁻⁵ and 1.5×10⁻³ s⁻¹, demonstrated that composites containing 70 vol pct of fibers had the highest strength-to-density ratio. Microstructural observations of specimens tested at 1400 K revealed that composites containing less than 50 vol pct of fibers showed extensive matrix cavitation, fiber-matrix debonding, and necking of the fibers. Above 50 vol pct, the composite matrix was less prone to cavitation, with an increasing tendency toward shear deformation of the fibers as the fiber volume fraction increased. No fiber damage was observed at 1400 K away from the fractured end, but significant fiber damage was observed at higher temperatures. A phenomenological model is presented to rationalize these observations. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Effect of fiber volume fraction on the fracture behavior of Nb-1 wt pct Zr/218W composites at elevated temperatures

Nb-1 wt pct Zr/218W long-fiber composite monotapes, nominally containing 0 to 70 vol pct of 218 tungsten fibers, were fabricated by arc spraying the Nb-1 pct Zr matrix onto the tungsten fibers. The monotapes were consolidated by hot pressing and hot isostatic pressing techniques. Tensile tests conducted between 1400 and 1600 K, under engineering strain rates varying between 1.5×10⁻⁵ and 1.5×10⁻³ s⁻¹, demonstrated that composites containing 70 vol pct of fibers had the highest strength-to-density ratio. Microstructural observations of specimens tested at 1400 K revealed that composites containing less than 50 vol pct of fibers showed extensive matrix cavitation, fiber-matrix debonding, and necking of the fibers. Above 50 vol pct, the composite matrix was less prone to cavitation, with an increasing tendency toward shear deformation of the fibers as the fiber volume fraction increased. No fiber damage was observed at 1400 K away from the fractured end, but significant fiber damage was observed at higher temperatures. A phenomenological model is presented to rationalize these observations. L.J. GHOSN, formerly Researcher with Case Western Reserve University, Cleveland, OH 44115 This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

High temperature embrittlement of NI and Ni-Cr alloys by trace elements

The effects of Sb, Sn, and Zr additions on the creep properties of Ni and Ni + 20 pct Cr are reported. Antimony and tin additions (~1 wt pct) induce extensive grain boundary cavitation in nickel, while smaller antimony additions had little effect on Ni + 20 pct Cr. Addition of 0.11 pct Zr to Ni + 20 pct Cr greatly inhibited grain boundary cavitation and reduced its Coble creep rate. Auger electron spectroscopy of cavitated specimens provided direct evidence of impurity segregation to cavity surfaces. Residual sulfur segregated most strongly, and was observed on cavity surfaces in all cavitated specimens. Tin segregated somewhat less intensely than sulfur, and antimony segregated only slightly. Segregation of antimony and sulfur to uncavitated portions of Ni + 1 pct Sb grain boundaries was also observed. These results are discussed in terms of segregation effects on energetic and transport properties of the grain boundaries and cavity surfaces. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

Diffusion in the titanium-nickel system: II. calculations of chemical and intrinsic diffusion coefficients

Diffusion coefficients in the Ti-Ni system have been calculated by the aid of equations given by Sauer and Freise, and Wagner. Values for the TiNi (50 at. pct Ni) phase were found to be:D ^u (cm²/s) = 0.0020 exp - 142,000/R for the temperature range between 650 and 940°C. The heat of activation, expressed in J/mol, has an accuracy of ±6000. For the β-Ti(Ni) phase containing 6 at. pct Ni the temperature dependence of the diffusion coefficient is expressed by:D ^u (cm²/s) = 0.0688 exp - 141,000/RT. The uncertainty in the energy of activation is ±12000 J/mol. No clear variation of the diffusion coefficient with concentration could be detected. It was found that Ni is by far the fastest moving component in β-Ti(Ni), Ti₂Ni and TiNi (at least in the composition range between 50 and 53 at. pct Ni). Values ofD _Ni/D _Ti have been calculated with an equation derived by van Loo. The significance of the calculated values is critically examined. By means of a practical example it is shown that the calculated ratio of the intrinsic diffusion coefficients can be extremely sensitive to slight variations in the position of the marker interface.Diffusion coefficients in the Ti-Ni system have been calculated by the aid of equations given by Sauer and Freise, and Wagner. Values for the TiNi (50 at. pct Ni) phase were found to be:D ^u (cm²/s) = 0.0020 exp - 142,000/R for the temperature range between 650 and 940°C. The heat of activation, expressed in J/mol, has an accuracy of ±6000. For the β-Ti(Ni) phase containing 6 at. pct Ni the temperature dependence of the diffusion coefficient is expressed by:D ^u (cm²/s) = 0.0688 exp - 141,000/RT. The uncertainty in the energy of activation is ±12000 J/mol. No clear variation of the diffusion coefficient with concentration could be detected. It was found that Ni is by far the fastest moving component in β-Ti(Ni), Ti₂Ni and TiNi (at least in the composition range between 50 and 53 at. pct Ni). Values ofD _Ni/D _Ti have been calculated with an equation derived by van Loo. The significance of the calculated values is critically examined. By means of a practical example it is shown that the calculated ratio of the intrinsic diffusion coefficients can be extremely sensitive to slight variations in the position of the marker interface. This paper is based on a Thesis submitted by G. F. BASTIN in fulfillment of requirements for the degree of Doctor in Technological Sciences.     

Determination of the Fe−Ni phase diagram below 400°C

The phase diagram for the Fe−Ni system below 400°C has been determined experimentally in the composition range from 0 to 52 wt pct Ni using analytical electron microscopy techniques. High spatial resolution X-ray microanalysis and electron diffraction were conducted on the Fe−Ni regions of meteorites. Both stable and metastable phase boundaries were defined. Our phase diagram is consistent with the available theoretical diagram in that firm experimental evidence was found for a miscibility gap and an associated, asymmetrical spinodal decomposition region. The spinodal decomposition resulted in a two-phase, isotropic microstructure, as expected. The miscibility gap is a metastable construction arising from the presence of a tricritical point due to magnetic interactions. Our experimental diagram differs from the theoretical diagram in three ways. First, observations of meteorite structures show that Fe−Ni solid solution containing 4.0 wt pct Ni is in local equilibrium with ordered FeNi containing 51.4 wt pct Ni and not Ni₃Fe as in the theoretical diagram. Second, our miscibility gap below 400°C, located between 11.7 and 51.9 wt pct Ni at 200°C, is wider than the calculated miscribility gap, especially at the high Ni end. Third, we also find evidence for an ordered structure around ∼25 wt pct Ni. This structure may be either Fe₃Ni or a two-phase structure incorporating ordered FeNi.     

Cavitation Erosion Behavior of Nitrogen Ion Implanted 13Cr4Ni Steel

Nitrogen ions (N⁺) with five different energies (100–600 keV) were implanted on the 13Cr4Ni steel (base) samples under high vacuum at temperature <100 °C. The base and implanted samples were also annealed at 600 °C for 6 h at high vacuum (~10^?9 bar). Energy dependent change in structure and mechanical properties of implanted samples were observed after annealing process. Structural study suggested formation of nitrides and implantation induced surface segregation of nitrogen. The nano-indentation hardness and elastic modulus were increased from 5 to 13 and 183 to 314 GPa respectively with increasing N⁺ energy. The N⁺ implantation process had significantly enhanced the cavitation erosion resistance of the base steel. The minimum cumulative weight loss and maximum erosion resistance were obtained for the sample implanted at 600 keV energy. The roughness values of surfaces at various erosion periods were correlated with erosion process to understand erosion mechanism. The lowest roughness values (R_a = 164.42 nm, R_q = 214.75 nm) after 12 h of cavitation erosion test were obtained for the sample implanted at 600 keV energy.     

镍基等离子熔覆堆焊层的空蚀特征与性能

利用等离子熔覆设备堆焊制备了三种不同成分的镍基合金层(Ni46、Ni67、Ni60/35WC),制样后在旋转圆盘空蚀试验机上对制备的合金层进行了空蚀磨损实验.采用SEM、XRD、显微硬度、失重分析法对空蚀前后的合金层进行了对比分析.结果表明:所有堆焊层的失重均大于对比的304不锈钢;SEM形貌观察堆焊层组织中存在缺陷或孔隙,空蚀后组织中的缺陷呈裂纹状发展,因此空蚀伴随着强烈的疲劳损失过程;XRD分析表明空蚀过程诱发了Ni60/35WC表面的相变;另外,空蚀还引起了材料Ni67和Ni60/35WC加工硬化,而Ni46出现了加工软化.     

Wrought aluminum-nickel alloys for high strength-high conductivity applications

Aluminum-Nickel alloys ranging from 0.06 pct to 6.1 pct (by wt) Ni have been developed for high strength-high conductivity applications. These alloys were produced by solidification in a permanent mold followed by homogenization, hot extrusion or hot rolling and cold drawing to wire form. This sequence of fabrication a) led to the production of fine fibrous dispersoids of NiAl₃ as part of the Al-NiAl₃ eutectic during the initial casting operation, b) permitted the retention of fine fibrous dispersiods of NiAl₃ produced during casting without any significant coarsening during processing and c) led to uniform dispersion and general alignment of these fibrous dispersoids along a given direction in the product without any measurable fiber-matrix separation, extensive fiber-fragmentation or crack production in the matrix. These alloys can be processed to wire form as easily as aluminum and when processed by the above sequence, possess very attractive combination of high strength-high electrical conductivity. Tensile strengths range from 173 N/mm² (at 0.6 pct Ni) to 241 N/mm² (at 6.1 pct Ni) in combination with corresponding conductivity values between 62 pct IACS and 55.5 pct IACS. The wires also possess attractive yield strength; for instance, the 0.2 pct off-set strength of Al-6.1 pct Ni wire is 213 N/mm². Using simple composite rules, the estimated strength and the conductivity of NiAl₃ fibers were found to be 1380 N/mm² and 18 pct IACS respectively, in these wires.     

Cr32Ni7Mo3N特级双相不锈钢的空蚀行为

利用磁致伸缩空蚀试验机对Cr32Ni7Mo3N特级双相不锈钢在蒸馏水和人工海水中进行了空蚀实验,并采用扫描电镜跟踪观察了经不同时间段空蚀后试样的形貌.通过测量失重绘制了材料的累积失重量和失重率曲线.经电化学工作站测量了材料在静态与空蚀条件下的极化曲线和腐蚀电位变化.对比分析了Cr32Ni7Mo3N与SAF2205双相不锈钢在人工海水的抗空蚀能力.结果表明：Cr32Ni7Mo3N特级双相不锈钢空蚀破坏首先在铁素体薄弱区以及铁素体和奥氏体相界发生,并向铁素体内扩展,铁素体发生解离断裂脱落;奥氏体随着空蚀的进行,滑移线增多,显微硬度值增加,且人工海水中奥氏体显微硬度值比在蒸馏水中的高;铁素体大面积破坏后,奥氏体才失稳产生延性断裂脱落,奥氏体的存在延缓了破坏在整个材料表面上的扩展.空蚀与腐蚀交互影响导致材料在人工海水中加速破坏.Cr32Ni7Mo3N特级双相不锈钢在人工海水中的抗空蚀能力优于SAF2205双相不锈钢.     

Wrought aluminum-nickel alloys for high strength-high conductivity applications

Aluminum-Nickel alloys ranging from 0.06 pct to 6.1 pct (by wt) Ni have been developed for high strength-high conductivity applications. These alloys were produced by solidification in a permanent mold followed by homogenization, hot extrusion or hot rolling and cold drawing to wire form. This seguence of fabrication a) led to the production of fine fibrous dispersoids of NiAl₃ as part of the Al-NiAl₃ eutectic during the initial casting operation, b) permitted the retention of fine fibrous dispersiods of NiAl₃ produced during casting without any significant coarsening during processing and c) led to uniform dispersion and general alignment of these fibrous dispersoids along a given direction in the product without any measurable fiber-matrix separation, extensive fiber-fragmentation or crack production in the matrix. These alloys can be processed to wire form as easily as aluminum and when processed by the above sequence, possess very attractive combination of high strength-high electrical conductivity. Tensile strengths range from 173 N/mm² (at 0.6 pct Ni) to 241 N/mm² (at 6.1 pct Ni) in combination with corresponding conductivity values between 62 pct IACS and 55.5 pct IACS. The wires also possess attractive yield strength; for instance, the 0.2 pct off-set strength of Al-6.1 pet Ni wire is 213 N/mm². Using simple composite rules, the estimated strength and the conductivity of NiAl₃ fibers were found to be 1380 N/mm² and 18 pct IACS respectively, in these wires.     

Effect of Fe on ductility and cavitation in the superplastic Zn-22 Pct Al eutectoid

In this investigation, the ductility and cavitation behavior of five grades of the superplastic Zn-22 pct Al were studied under identical conditions of grain size, temperature, and initial strain rate. These five grades were prepared from high-purity Al and Zn using the same procedure but different Fe impurity levels; grades A, B, C, D, and E contain 1, 40, 120, 400, and 1460 ppm of Fe, respectively. A comparison between the present results and those reported earlier for the creep behavior of the five grades of Zn 22 pct Al demonstrates that while the presence of Fe has no noticeable effect on the steady-state creep rates during deformation in the superplastic region (where maximum ductility occurs), it has a pronounced effect on the ductility and fracture behavior of the alloy. The experimental data show that there is a significant drop in the average elongation to fracture when Fe concentration exceeds 125 ppm, and that the ductility of grade E is approximately 50 pct of grade A for initial strain rates less than 10⁻² s⁻¹. In addition, the data reveal two important findings regarding cavitation behavior. First, neither the initial grain size nor the occurrence of grain growth has an effect on cavitation that is as significant as that arising from the presence of excessive impurities. Second, the presence of other impurities in addition to Fe in Zn-22 pct Al has the effect of enhancing cavitation. The effect of Fe, alone or in the presence of other impurities, on ductility and cavitation in Zn 22 pct Al is examined in terms of phenomena associated with impurity segregation at boundaries. Formerly Research Associate, Materials Section, Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA Formerly Graduate Research Assistant, Materials Science & Engineering, Department of Chemical and Biochemical Engineering, University of California, Irvine, CA This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium, as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Continuous Extraction of Nickel from Superalloy Scraps Using Zinc Circulation

A novel technique for the continuous extraction of nickel (Ni) from Ni-based superalloy scraps using molten zinc (Zn) has been proposed, and its feasibility was experimentally demonstrated. The newly developed approach allows for extraction of Ni metal directly from superalloy scraps with simultaneous separation of the Zn from the resulting Zn-Ni alloy. The optimal conditions for the extraction of Ni and separation of valuable elements such as rhenium (Re), tantalum (Ta), and tungsten (W) were determined by varying major process parameters including the reaction time and configuration of the reaction chamber. The proposed method has been successfully utilized for the production of the superalloy containing 62.8 mass pct of Ni and 15.5 mass pct of refractory metals (Re, W, and Ta). Under certain conditions, 41 pct of the Ni contained in the superalloy could be extracted at 1173 K (900 °C) over 48 hours, producing an alloy containing 84.0 mass pct of Ni and 0.2 mass pct of the refractory metals.     

Superplastic flow and cavitation in Zn-22 pct Al doped with Cu

The sigmoidal relationship between stress and steady-state strain rate that has been reported for micrograin superplastic alloys is characterized by the presence of three regions: region I at low stresses, region II (the superplastic region) at intermediate stresses, and region III at high stresses. Recent results on the superplastic Zn-22 pct Al eutectoid have shown that the characteristics of region I are influenced by the impurity level of the alloy, and that neither region I nor significant cavitation is observed when such a level is reduced to about 6 ppm. These observations are in agreement with the suggestion that the origin of region I is related to strong impurity segregation at boundaries. The present investigation was conducted to study the effect of Cu, as a selected impurity, on superplastic deformation and cavitation in Zn-22 pct Al. The results show that Zn-22 pct Al-0.13 pct Cu exhibits two primary characteristics: region I is absent and cavitation is not extensive. These characteristics, which are essentially similar to those reported previously for high-purity Zn-22 pct Al but are different from those documented for a grade of the alloy containing a comparable atomic concentration of Fe, suggest that Cu has little or no tendency to segregate at boundaries. Indirect evidence in support of this suggestion is inferred from studying the effect of impurities on former α boundaries that form in the microstructure of Zn-22 pct Al as a result of solution treatment above the eutectoid temperature. Although further studies are needed to provide direct evidence for the absence of Cu segregation at boundaries, the present results clearly indicate that superplastic flow and cavitation at low stresses are controlled not only by the impurity level, but also by its type.     

The effect of temperature and microstructure on the creep damage found in low alloy ferritic steels

Constant strain rate tests at 10^-5 s^-1 have been carried out in the temperature range 723 to 973 K on two 1 1/2 pct Cr · 1/2 pct V ferritic steels, the first steel with a 20 pct bainite, 80 pct ferrite microstructure and the second with a fully ferritic structure. Measurements of the quantitative strain, ε_gb, due to grain boundary sliding (gbs), were made and in both steels the γ values (where γ = ε_gb/ε_T) increased with increasing temperature. In both structures, sliding was found to occur on all boundaries. A qualitative study of cavitation damage and final fracture mechanisms was also made. It is suggested that in the mixed structure, cavities are nucleated by gbs at carbides whereas in the fully ferritic structure, cavity nucleation is by the interaction of intragranular slip with a grain boundary. Optical observations showed that the large scale cavitation behavior was superficially very similar in both steels, but scanning electron microscope observations showed remarkable differences in the fine scale cavitation damage. The implications of these results are discussed in terms of the relationship between matrix deformation, grain boundary deformation and creep fracture. Formerly of the Department of Metallurgy, University of Manchester.     

Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10⁻⁵ to 10⁻³ s⁻¹. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10⁻⁴ s⁻¹. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q _c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.     

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.     

Cavitation erosion of age-hardenable aluminum alloys

The mechanisms of deformation and failure of age-hardenable aluminum alloys due to ultrasonic cavitation have been investigated. The results indicate that the mode and rate of erosion depend on both composition and heat treatment: increasing the hardness and strength by aging or alloying produces an increase in erosion resistance togetner with a change in failure mode. Alloys of low solute content erode in a manner similar to that of pure FCC metals namely by uniform ductile rupture across the surface. In the samples of 2 to 4.5 pct alloy content, however, material is lost preferentially from apparently random, isolated regions, leaving behind striated, flat-bottomed pits. The fracture surface of these pits resembles those of fatigue failures in aluminum alloys. These pits expand laterally to macroscopic proportions on continued caviation exposure without suffering any changes in their topographical features. Alloys of 9 to 10 pct solute content develop similar striated pits but on a microscopic scale. These different erosion mechanisms are discussed in terms of the microstructure of the alloys and their ability to absorb and attenuate the incident cavitation energy. Formerly Associate Professor at S.U.N.Y., Stony Brook     

Microstructure and formability of ZnNi alloy electrodeposited sheet steel

ZnNi alloy electrodeposited sheet steels were made from a chloride bath using a high-speed flow cell. A Ni-rich flash coating was deposited first, upon which the ZnNi coating, with Ni contents ranging from 8 to 16 wt pct, was subsequently electrodeposited. It is demonstrated that the Ni content of the coating affects the forming properties and microstructure of the ZnNi coatings. The hardness of the ZnNi coating increased with Ni content, leading to poor formability and inferior adhesion of the coated steels, as evident from the large amount of coating loss during swift cupping and coating peel-off during low-temperature adhesion tests. On the other hand, the friction force between the coated steel and cupping die decreased with increasing Ni content. At low Ni contents of 8 wt pct, the coating had a porous equiaxed grain structure. As the Ni content increased, the coating surface changed to dense faceted morphologies. A pyramid morphology was observed for 16 wt pct ZnNi coatings. An X-ray diffraction (XRD) analysis showed that all coatings containing up to 16 wt pct Ni contained only γ phase. Transmission electron microscopy (TEM) observations showed the 8 wt pct Ni coating to have a fine-grained structure, which changed to a columnar structure at 16 wt pct Ni. The formation of the columnar structure is explained by the smaller amount of hydrogen discharge as the bath Ni ion concentration increased.     

Tracer diffusion of59Fe and51Cr in Fe-17 Wt Pet Cr-12 Wt Pet Ni austenitic alloy

The volume and grain-boundary diffusion of⁵⁹Fe and⁵¹Cr have been studied in an austenitic iron alloy containing 17 wt pct Cr and 12 wt pct Ni. The diffusivities in this alloy of these two tracers and⁶³Ni are compared with their diffusivities in pure iron and in other austenitic stainless steels. For volume diffusion at any particular temperature in the present alloy, Cr is the most rapid while Ni is the slowest, and all three tracers diffuse slower than that reported for pure iron or for other austenitic stainless steels. For grain-boundary transport, Fe diffuses most rapidly above 850°C and Ni diffuses most rapidly below that temperature. The activation energies for both volume and grain-bounary diffusion obey the relationshipQ _Ni <Q _Cr <Q _Fe. Formerly Presidential Intern in the Metals and Ceramics Division, Oak Ridge National Laboratory     

Transformation characteristics of low-carbon steels containing 5 Pct Ni

Optical metallography and transmission electron microscopy were used to examine the structure of isothermally transformed steels containing 4 to 5 pct Ni and 0.05 to 0.38 pct C. The steels were investigated after short-time isothermal reaction at temperatures between 700° and 400°C. Two distinct types of ferrite morphology were observed in the steels containing less than 0.2 pct C; both Widmanstätten structures and equiaxed ferrite were observed at the higher temperatures, but the latter morphology predominated at the lower temperatures. In steels containing 0.17 and 0.38 pct C, bainitic structures were observed after transformation at 400°C. The structural features of these transformation products are described and possible explanations of their origin are discussed.