Diffusion in the nickel-rich part of the Ni−Al system at 1000° to 1300°C; Ni3Al layer growth,diffusion coefficients,and interface concentrations

The formation of the Ni₃Al layer in NiAl (55 at. pct Ni)-pure Ni diffusion couples at temperatures above 1000°C has been found to be controlled almost completely by volume diffusion. At 1000°C and below, the relatively small grain size of the Ni₃Al compound in the layers caused such a large contribution from grain boundary diffusion, that the layer growth rates at 1000°C exceeded those at 1100°C and even those at 1200°C. In Ni₃Al (75at. pct Ni)-pure Ni diffusion couples the Ni₃Al compound rapidly converted into the solid solution of aluminum in nickel. Volume-diffusion coefficients calculated by the Boltzmann-Matano method yielded heats of activation of 55, 64, and 65 kcal·mol^?1 for NiAl, Ni₃Al and the solid solution of aluminum in nickel, respectively. In addition, eleven different types of diffusion couples were prepared from various Ni?Al alloys and annealed at 1000°C. Marker interface displacements and observations of porosity in these couples yielded a more detailed picture of the Kirkendall-effect than earlier work had done. The ratio of the intrinsic diffusion coefficients at the marker interface,D _NI/D _Al, is greater than one in the nickel-rich NiAl phase. For the Ni₃Al phase no statement can be made on the basis of this work. When the marker interface is located in the nickel solid solution,D _Ni/D _Al is smaller than one. The phase boundary concentrations in these couples did not show the expected deviation from the equilibrium concentrations in two-phase alloys; this finding is discussed with regard to the free-energycomposition diagram.     

Interdiffusion in Ni-Rich,Ni-Cr-Al alloys at 1100 and 1200 °C: Part I. Diffusion paths and microstructures

Interdiffusion in Ni-rich, Ni-Cr-Al diffusion couples was studied after annealing at 1100 and 1200 °C. Recession of γ′ (Ni₃Al structure), β (NiAl structure), or α (bcc) phases was also measured. Aluminum and chromium concentration profiles were measured in the γ (fcc) phase for most of the diffusion couples. The amount and location of Kirkendall porosity suggests that Al diffuses more rapidly than Cr which diffuses more rapidly than Ni in the γ phase of Ni-Cr-Al alloys. The location of maxima and minima in the concentration profiles of several of the diffusion couples indicates that both cross-term diffusion coefficients for Cr and Al are positive and that D_CrAl has a greater effect on the diffusion of Cr than does D_A1Cr on the diffusion of Al. The γ/γ + β phase boundary has also been determined for 1200 °C through the use of numerous γ/γ+ β diffusion couples.     

Influence of rare-earth metals on the high-temperature strength of Ni<Subscript>3</Subscript>Al-based alloys

The influence of the content of reaction- and surface-active alloying elements (rare-earth metals (REMs)) and the method of their introduction into cast high-temperature γ′-Ni₃Al-based intermetallic alloys, which are thermally stable natural eutectic composites, on their structure-phase state and the mechanical properties is studied. The life of low-alloy heterophase γ′ + γ cast high-temperature light Ni₃Al-based alloys is shown can be increased at temperatures exceeding 0.8T _m (T _m is the melting temperature of Ni3Al) due to additional stabilization of the single-crystal structure of these alloys with submicron and nanometer-sized particles of the phases formed by refractory and active REMs. It is also shown that stage-by-stage fractional introduction of all components into alloys during vacuum induction melting with allowance for their reaction activities (most refractory metals are introduced in the form of low-melting-point master alloys at the first stage of vacuum induction melting, and lanthanum is introduced with a master alloy in the optimal contents of 0.1–2 wt % into the charge of VKNA-1V and VKNA-25 alloys at the final stage) leads to the formation of a modified structure stabilized by nanoprecipitates of nickel and aluminum lanthanides and the phases formed by refractory metals. This method increases the life of VKNV-1V-type alloys (0.5 wt % Re) at 1000–1200°C by a factor of ∼1.7 and that of VKNA-25-type alloys (1.2 wt % Re and Co) by a factor of ∼3.     

A structural study of oxidation in a zirconia-toughened alumina fiber-reinforced NiAl composite

A composite of NiAl reinforced with continuous zirconia-toughened alumina (PRD-166) fibers was fabricated by pressure casting. The chemical stability of the composite at 1100 °C in vacuum and air was investigated by optical and transmission electron microscopy and energy-dispersive spectroscopy (EDS). Exposure of the fiber to the molten metal caused ZrO₂ particles in the fiber to move to the surface, thus permitting dissolution of ZrO₂ into the molten metal. The dissolved Zr reacted with A1₂O₃ of the fiber and formed ZrO₂ particles in some regions at the fiber/matrix interface. Vacuum annealing did not result in any noticeable change in the microstructure. Air annealing led to the precipitation of ZrO₂ within the matrix near the fiber/matrix interface. A thin layer of A1₂O₃ was observed to envelop the ZrO₂ particles and cover the fiber. During air annealing, Al oxidized preferentially, thereby continually reducing the Al content of the β-NiAl. This caused a progressive change in the microstructure of the matrix from β-NiAl to premartensitic microstructure, to martensitic structure, followed by nucleation and growth of Ni₃Al, to the development of a two-phase microstructure consisting of Ni₃Al cuboids dispersed in a disordered α-Ni(Al) and, subsequently, the formation of single-phase α-Ni(Al). The orientation relationship between Ni₃Al and NiAl was . Internal oxidation of α-Ni(Al) led to precipitation of A1₂O₃ particles which subsequently reacted with Ni, in the presence of O, to form NiO · A1₂O₃ spinel. The Ni was oxidized to formβ-NiO. Titanium-containing, platelike precipitates with a {111} habit plane were occasionally observed in NiO. Some larger NiTiO₃ particles were also formed within NiO. Diffusion of O through the interphase and grain boundaries of the fiber is believed to be responsible for the rapid oxidation of the composite.     

NiAl powder alloys: I. Production of NiAl powders

The influence of five methods of production of Ni₅₀Al₅₀ powder alloys on the processes occurring during reactive alloy formation of nickel monoaluminide during heating is considered. It is shown that, when powder mixtures obtained by agitation in ball mills and cladded composite powders with a low level of internal stresses are used, it is possible to produce a material with a nearly equilibrium phase composition in the course of reactive sintering due to an exothermic effect with the participation of a liquid phase (aluminum melt) in the reaction. The sintered material is porous and has an island structure. Mechanical alloying in a high-energy ball mill (attritor) results in the formation of layered Ni/Al granules with a developed interface and a high level of internal stresses and defects, which makes it possible to decrease the temperatures of initiation of reactive interaction by ∼300°C. This interaction develops in the solid phase according to a slow diffusive mechanism leading to the formation of intermediate nickel aluminides and hindering the achievement of equilibrium phase composition. The microingot granules (∼80 wt % particles 100–400 μm in size) produced by melt spraying by gases (N, Ar) has the composition of the melt, but grain boundaries are depleted of aluminum in comparison with the volume. The NiAl powders (∼90 wt % particles <40 μm in size) produced by combined hydride-calcium reduction are characterized by a highly homogeneous nickel and aluminum distribution, and their composition is close to equilibrium. These two types of powders are selected as the initial material for investigating the compacting and production of NiAl-based alloys.     

Transient liquid-phase bonding in the NiAl/Cu/Ni system-A microstructural investigation

A transmission electron microscopy based investigation of microstructural development in NiAl-Ni transient liquid bonding, using commercial purity copper interlayers, is presented in this article. The article considers the mechanisms of isothermal solidification in NiAl/Cu/Ni joints and the influence of copper diffusion from the joint centerline on the microstructures of the adjacent NiAl and Ni substrates. Changes in the microstructure of the bond centerline due to entry of aluminum (from the NiAl substrate) and Ni (from both the NiAl and the Ni substrates) are discussed. Transfer of aluminum from the NiAl substrate to the Ni substrate is also examined. The precipitation of bothLl₂ typeγ′ andB ₂ typeβ phases at the joint centerline is investigated. Precipitation ofγ′ within both the NiAl and Ni substrates is considered. The formation ofAl typeγ phases in the NiAl substrate is also examined.     

The relative effects of chromium,molybdenum, and tungsten on the occurrence of σ phase in Ni-Co-Cr alloys

The relative effects of chromium, molybdenum, and tungsten on the occurrence of σ phase have been studied in Ni-Co-Cr alloys. These alloys were designed to simulate the γ matrix in commercial nickel-base superalloys that are strengthened primarily by precipitation of the γ phase, based on Ni₃Al. Three alloy series were studied. The first series comprised four alloys varying in chromium content from 34.63 to 43.65 at. pct. The other two series contained separate molybdenum and tungsten additions of 1, 2, 3, and 4 at. pct at constant chromium contents of 37.5 at. pct. In each of the 12 alloys, the atomic percentages of nickel and cobalt were equal. The alloys were aged in both the annealed and cold-rolled conditions at 1400°F (760°C), 1550°F (845°C), and 1700°F (925°C) for times up to 3000 h. The contributions of the chromium-group elements to σ formation were evaluated both by measuring the volume percentage of σ phase and by determining the final composition of the y matrix after σ precipitation. By these two techniques, critical values of the average electron vacancy number, •N _v , for σ formation at 1550°F (845°C) were found to be 2.518 and 2.512, respectively; σ precipitation was most rapid at 1550°F (845°C). Both techniques in-dicated that under conditions approaching equilibrium, molybdenum and tungsten are equiv-alent in inducing σ formation and about 1.5 to 2 times as potent as chromium. The approxi-mate electron vacancy coefficients(N _v ) for molybdenum and tungsten, as derived from volume-fraction measurements of σ phase, are as follows: 7.35 at 1400°F (760°C) and 1550°F (845°C), and 8.7 at 1700°F (925°C). The values derived from final compositions of the γ matrix after σ precipitation are 7.9 at 1550°F (845°C) and 8.6 at 1700°F (925°C). The bulk diffusion of aluminum into alloys that were otherwise not σ-prone at 1700°F (925°C) caused extensive σ precipitation during aging. This was due to copious precipitation of γ-Ni₃Al and β-NiAl, resulting in enrichment of the matrix in elements of the chromium group. This paper is based on a dissertation submitted by GARY N. KIRBY in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Metallurgical Engineering, The University of Michigan, 1971. The study was conducted in the Ann Arbor Research Labora-tory of the Climax Molybdenum Company of Michigan, a subsidiary of American Metal Climax, Inc.     

Structure of As-cast aluminum matrix composite containing Ni3AI-type intermetallic ribbons

An aluminum matrix composite containing rapidly solidified Ni₇₅Al₂₃B₁Zr₁ (at. pct) ribbons has been fabricated by casting at 700 °C, 715 °C, 730 °C, and 875 °C. Microstructural investigation has shown that the matrix contains particles with a composition between Al₃Ni and eutectic. The interfacial zones composed of several layers with different aluminum and nickel contents are observed around the ribbons. The sequence of layers from the ribbon outward in the specimens fabricated at 700 °C, 715 °C, and 730 °C is as follows: AINi → Al₃Ni₂ → the outer layer between Al₃Ni and eutectic. Composite specimens fabricated at 875 °C contain two types of interfacial zones: a single-layer AINi and a triple-layer zone. The first two layers in the triplelayer zone are exactly the same as their counterparts in the specimens fabricated at lower temperatures. The outer layer has a composition close to the Al₃Ni compound. The thickness of the AINi layer increases continuously with the increasing casting temperature. Within the experimental error, the thickness of the Al₃Ni₂ layer seems to be independent of casting temperature. The thickness of the outer layer in the specimens fabricated at 700 °C to 730 °C (Al₃Ni plus eutectic) increases with the casting temperature. However, the outer layer in the 875 °C specimen (Al₃Ni) is much thinner than the others.     

The synthesis of nickel aluminides by multilayer self-propagating combustion

The synthesis of Ni-Al intermetallic thin films by self-propagating combustion reactions was investigated for the 1:1 and 3:1 Ni/Al stoichiometries. The dependence of the combustion wave velocity on the individual layer thickness was determined. The marked decrease in velocity with layer thickness was consistent with results of modeling studies on multilayer systems. Activation energies for the synthesis of NiAl were determined to be in the range 127.9 to 149.8 kJ · mol₋₁, and those for the synthesis of Ni₃Al were found to be in the range 133.8 to 146.3 kJ · mol₋₁. In the case of NiAl, the experimental value is attributed to a diffusion process of Al in NiAl. Differential thermal analysis (DTA) showed the sequence of steps in the formation of NiAl and Ni₃Al. The dependence of the thermal peaks on the heating rate for both cases was found to be consistent with theory. The activation energies obtained from the DTA analysis were compared to previous results obtained with relatively thin layers.     

Composite materials with an intermetallic matrix based on nickel and titanium monoaluminides hardened by oxide particles or fibers

Hardening phase/intermetallic matrix pairs are chosen for composite materials (CMs) intended for long-term high-temperature operation. These materials must have high and stable mechanical properties during a long time at high temperatures and loads. The compatibility of the physicochemical and mechanical properties of CM components is estimated to choose hardening phase/intermetallic matrix pairs in which the matrix is represented by an alloy based on NiAl or TiAl monoaluminide and the hardening phase is a refractory thermodynamically stable oxide of a Group III transition metal M ₂O₃. The following two schemes are used to perform hardening of a CM with a matrix consisting of a TiAl or NiAl alloy by the most thermodynamically stable interstitial phases, i.e., refractory oxides, at temperatures higher than the operating temperature (T _op) of the IMM. The first scheme consists in creating Al₂O₃/TiAl CMs hardened by continuous single-crystal sapphire fibers using the impregnation of a bundle of single-crystal fibers with a matrix melt followed by directional solidification. The TiAl-based matrix in these CMs serves as a binder connecting oxide phase fibers and preventing them from fracture due to high adhesion forces between oxide fibers and the matrix and a high fiber/matrix interface strength. In the second scheme, Y₂O₃/NiAl CMs are produced by powder metallurgy methods, which include severe deformation by extrusion accompanied by the formation of deformation texture and subsequent recrystallization annealing. In these CMs, disperse refractory oxide particles stabilize grain boundaries in a recrystallized matrix material and lead to the formation of directional structures with coarse elongated grains and a low fraction of transverse boundaries. Al₂O₃/TiAl CMs containing 20–25 vol % hardening single-crystal sapphire Al₂O₃ fibers can operate at temperatures of 1000–1050°C (∼0.7T _m of matrix), which is 250–300°C higher than the maximum values of T _op of a TiAl-based matrix and 400-450°C higher than the maximum values of T _op of a Ti-based matrix. An Y₂O₃/NiAl composite with a directionally recrystallized structure of a NiAl-based matrix hardened by 2.5 vol % Y{ia2}O₃ particles can be recommended for operation at temperatures of 1400–1500°C ((0.8–0.9)T _m of matrix), which are higher by 100–400°C than not only T _op but also T _m of Ni superalloys.     

Precipitation of an intermetallic phase with Pt2Mo-type structure in alloy 625

The microstructure of Alloy 625, which has undergone prolonged (∼70,000 hours) service at temperatures close to but less than 600 °C, has been characterized by transmission electron microscopy. The precipitation of an intermetallic phase Ni₂(Cr, Mo) with Pt₂Mo-type structure has been observed in addition to that of the γ″ phase. Six variants of Ni₂(Cr, Mo) precipitates have been found to occur in the austenite grains. These particles exhibit a snowflake-like morphology and are uniformly distributed in the matrix. They have been found to dissolve when the alloy is subjected to short heat treatments at 700 °C. The occurrence of the Ni₂(Cr, Mo) phase has been discussed by taking the alloy chemistry into consideration. Apart from the intermetallic phases, the precipitation of a M₆C-type carbide phase within the matrix and the formation of near continuous films, comprising discrete M₆C/M₂₃C₆ carbide particles, at the austenite grain boundaries have been noticed in the alloy after prolonged service.     

Kinetics of phase layer growth during aluminide coating of nickel

The diffusion coating of nickel with aluminum was studied by a two-step aluminizing pack process involving initially an influx of aluminum at the surface (step 1) and later a partial honiogenization of the aluminum-rich region under conditions of zero surface flux (step 2). The process was studied in the temperature range from 870 to 1000°C. Step 1 was characterized mainly as the rapid, parabolic growth (after an initial transient period) of the Ni₂Al₃ phase (γ) as a surface layer with concurrent growth of a thinner NiAl (δ) layer. Step 2 was characterized mainly as the rapid loss of the aluminum gradient in the γ layer followed by parabolic growth of the δ layer primarily by the solution of they phase. Mathematical models were developed, in which numerical methods and computer techniques as well as closed-form solutions were utilized. The models yielded growth rate predictions in agreement with the experimental data and were used to define the critical parameters controlling growth kinetics for the aluminide layers formed during this process.     

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.     

Ordering reactions in Ni-Al-Mo-Ta and Ni-Al-Mo-W superalloys

The ordered structures formed in two experimental nickel base superalloys have been determined using selected area electron diffraction. Upon quenching from 1300 °C, the alloys contained ordered γ′ precipitates (L1₂ structure) and the matrix exhibited diffuse intensity at {1 1/2 0} positions, indicating the presence of short range order. The high refractory metal content of the alloys caused the D1a, DO₂₂, and Pt₂Mo prototype structures to form in the matrix following aging at 600, 700, and 800 °C. The detailed structural effects of the Ta and W quaternary additions are similar to those observed in Ni₃(Mo, Al), Ni₃(Mo,Ta), and Ni₃(Mo, W) ternary alloys. The decomposition products observed in the quaternary alloys studied can be explained by considering the partitioning of solutes between the γ′ and the matrix.     

Interdiffusion and intrinsic diffusion in the Ni AI (δ) phase of the Al-Ni system

Interdiffusion coefficients at 950 to 1150°C and the ratio of intrinsic diffusion coefficients at 1100°C were measured as functions of composition in the NiAl (δ) phase of the Al-Ni system, using a vapor-solid technique. Diffusivity values were also obtained for the Ni₃Al (∈)and Ni (Al) solid solution (ζ) phases from 950 to 1150°C. The interdiffusion coefficient in NiAl (δ) varies several orders of magnitude over the δ phase field with a deep minimum in the diffusivity-composition curve at 48 to 49 at. pct Al. The ratio of intrinsic diffusion coefficients, D_ni/D_aI, in the δ phase also varies with composition from a value of 3 to 3.5 below 50 at. pct Al to 0.1 or less above 50 at. pct Al. Formerly Research Assistant, Department of Mate-rials Science, SUNY at Stony Brook, New York.     

Isothermal section of the ternary Sn-Cu-Ni system and interfacial reactions in the Sn-Cu/Ni couples at 800 °C

The isothermal section of the Sn-Cu-Ni system at 800 °C has been experimentally determined. There is no ternary compound. A solid solution with a very wide compositional range, the γ phase is formed between the Ni₃Sn(H) phase and Cu₄Sn(H) phase; however, both of these two binary phases are not stable at 800 °C. The binary Ni₃Sn₂ phase also has extensive ternary solubility. The homogeneity ranges of both the γ and Ni₃Sn₂ phases are very large in parallel to the Cu-Ni side, but relatively narrow along the Sn direction. This phenomenon indicates that Cu and Ni are exchangeable in both phases. Three kinds of reaction couples, Sn-55 at. pct Cu/Ni, Sn-65 at. pct Cu/Ni, and Sn-75 at. pct Cu/Ni, were prepared and reacted at 800 °C for 5 to 20 minutes. The reaction paths are liquid/Ni₃Sn₂/γ/Ni₃Sn(L)/Ni for the Sn-55 at. pct Cu/Ni and Sn-65 at. pct Cu/Ni couples, and the reaction path is liquid/γ/Ni₃Sn(L)/Ni for the Sn-75 at. pct Ni couples.     

Mechanisms of Hf dopant incorporation during the early stage of chemical vapor deposition aluminide coating growth under continuous doping conditions

A laboratory-scale chemical vapor deposition (CVD) reactor was used to perform “continuous” Hf doping experiments while the surface of a single-crystal Ni alloy was being aluminized to form an aluminide (β-NiAl) coating matrix for 45 minutes at 1150 °C. The continuous doping procedure, in which HfCl₄ and AlCl₃ were simultaneously introduced with H₂, required a high HfCl₄/AlCl₃ ratio (>∼0.6) to cause the precipitation of Hf-rich particles (∼0.1 μm) at grain boundaries of the coating layer, with the overall Hf concentration of ∼0.05 to 0.25 wt pct measured in the coating layer by glow-discharge mass spectroscopy (GDMS). Below this ratio, Hf did not incorporate as a dopant into the growing coating layer from the gas phase, as the coating matrix appeared to be “saturated” with other refractory elements partitioned from the alloy substrate. In comparison, the Hf concentration in the aluminide coating layer formed on pure Ni was in the range of ∼0.1 wt pct, which was close to the solubility of Hf estimated for bulk NiAl. Interestingly, the segregation of Hf and the formation of a thin γ′-Ni₃Al layer (∼0.5 μm) at the coating surface were consistently observed for both the alloy and pure-Ni substrates. The formation of the thin γ′-Ni₃Al layer was attributed to an increase in the elastic strain of the β-NiAl phase, associated with the segregation of Hf as well as other refractory alloying elements at the coating surface. This phenomenon also implied that the coating layer was actually growing at the interface between the γ′-Ni₃Al layer and the β-NiAl coating matrix, not at the gas/coating interface, during the early stage of the coating growth.     

Characterization of microstructure in laser-surface-alloyed layers of aluminum on nickel

In order to obtain basic understanding of microstructure evolution in laser-surface-alloyed layers, aluminum was surface alloyed on a pure nickel substrate using a CO₂ laser. By varying the laser scanning speed, the composition of the surface layers can be systematically varied. The Ni content in the layer increases with increase in scanning speed. Detailed cross-sectional transmission electron microscopic study reveals complexities in solidification behavior with increased nickel content. It is shown that ordered B2 phase forms over a wide range of composition with subsequent precipitation of Ni₂Al, an ordered ω phase in the B2 matrix, during solid-state cooling. For nickel-rich alloys associated with higher laser scan speed, the fcc γ phase is invariably the first phase to grow from the liquid with solute trapping. The phase reorders in the solid state to yield γ′ Ni₃Al. The phase competes with β AlNi, which forms massively from the liquid. The β AlNi transforms martensitically to a 3R structure during cooling in solid state. The results can be rationalized in terms of a metastable phase diagram proposed earlier. However, the results are at variance with earlier studies of laser processing of nickel-rich alloys.     

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: I. Agitation in a ball mill

The effect of the sintering temperature (1100–1400°C) of NiAl alloy samples with oxide Y₂O₃ produced by hydrostatic pressing on their structure and phase composition and the distribution of oxide particles in a NiAl-based intermetallic matrix alloyed with ～0.5 at % Fe is considered. It is found that dispersed oxide particles in the compact material prepared from a mixture of oxide Y₂O₃ powder and a NiAl alloy (produced by calcium hydride reduction of a mixture of nickel and aluminum oxides) powder in a standard ball mill are nonuniformly distributed in the volume. The morphology of oxides changes during sintering: sintered samples contain rounded particles, which differ strongly from the clearly faceted angular particles of oxide Y₂O₃ added to a mixture (they represent conglomerates of single crystals). In the sintered samples, large aggregates of oxides are revealed along grain boundaries. Mass transfer is possible at the NiAl/Y₂O₃ interface in the system: it leads to partial substitution of aluminum and/or iron atoms for yttrium atoms in the Y₂O₃ lattice and to the formation of submicroscopic particles of (Fe,Al)₅Y₃O₁₂-type oxides.     

The microstructure of an Fe-Mn-Si-Cr-Ni stainless steel shape memory alloy

The microstructure and phase stability of the Fe-15Mn-7Si-9Cr-5Ni stainless steel shape memory alloy in the temperature range of 600 °C to 1200 °C was investigated using optical and transmission electron microscopy, X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and chemical analysis techniques. The microstructural studies show that an austenite single-phase field exists in the temperature range of 1000 °C to 1100 °C, above 1100 °C, there exists a three-phase field consisting of austenite, δ-ferrite, and the (Fe,Mn)₃Si intermetallic phase; within the temperature range of 700 °C to 1000 °C, a two-phase field consisting of austenite and the Fe₅Ni₃Si₂ type intermetallic phase exists; and below 700 °C, there exists a single austenite phase field. Apart from these equilibrium phases, the austenite grains show the presence of athermal ɛ martensite. The athermal α′ martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through the γ-ɛ-α′ transformation sequence.