AnIn Situ transmission electron microscope deformation study of the slip transfer mechanisms in metals

The slip transfer mechanisms across grain boundaries in 310 stainless steel, high-purity aluminum, and a Ni-S alloy have been studied by using thein situ transmission electron microscope (TEM) deformation technique. Several interactions between mobile lattice dislocations and grain boundaries have been observed, including the transfer and generation of dislocations at grain boundaries and the nucleation and propagation of a grain boundary crack. Quantitative conditions have been established to correctly predict the slip transfer mechanism. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Atomic structure and energy of ∑ = 5 tilt boundaries in gold

The atomic structure of 2 = 5(θ = 36.9 deg) [001] tilt boundaries in gold has been investigated by high-resolution electron microscopy (HREM). Image simulations and experimental conditions for observing grain boundary atomic structure in gold are presented. Two preferred orientations corresponding to symmetric {310} and asymmetric {430}//{100} inclinations have been observed frequently. A single symmetric {210} inclination has also been observed. The atomic structures of these three boundaries are presented. An average ratio of grain boundary to surface energy, γ_gb/γ_s, of 0.62 has been measured at 200 °C. Unique atomic structures are observed for {310}, {430}//{100}, and {210} inclinations, and multiplicities in atomic structures have not been detected. Interfacial volume expansion of these interfaces is presented and compared to computational models. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Phase transformation in an Fe-10.1Al-28.6Mn-0.46C alloy

The microstructure of an (α + γ) duplex Fe-10.1Al-28.6Mn-0.46C alloy has been investigated by means of optical microscopy and transmission electron microscopy (TEM). In the as-quenched condition, extremely fine D0₃ particles could be observed within the ferrite phase. During the early stage of isothermal aging at 550 °C, the D0₃ particles grew rapidly, especially the D0₃ particles in the vicinity of the α/γ grain boundary. After prolonged aging at 550 °C, coarse K’-phase (Fe, Mn)₃AlC precipitates began to appear at the regions contiguous to the D0₃ particles, and —Mn precipitates occurred on the α/γ and α/α grain boundaries. Subsequently, the grain boundary β-Mn precipitates grew into the adjacent austenite grains accompanied by a γ→ α + β-Mn transition. When the alloy was aged at 650 °C for short times, coarse. K-phase precipitates were formed on the α/γ grain boundary. With increasing the aging time, the α/γ grain boundary migrated into the adjacent austenite grain, owing to the heterogeneous precipitation of the Mn-enrichedK phase on the grain boundary. However, the α/γ grain boundary migrated into the adjacent ferrite grain, even though coarse K-phase precipitates were also formed on the α/γ grain boundary in the specimen aged at 750 °C.     

Orientation relationships associated with austenite formation from ferrite in a coarse-grained duplex stainless steel

Studies on the morphology and crystallography of austenite precipitated from ferrite were performed in an Fe-22.5 pct Cr-4.7 pct Ni-3 pct Mo-duplex stainless steel. Samples were solution treated at 1325°C (yielding a grain size of approximately 2 mm), water quenched, and then aged at temperatures ranging from 700 °C to 1100 °C for times ranging from 5000 to 30,000 seconds. The morphology of grain boundary precipitates depends on the grain boundary segment at which the precipitates are formed, and may be adequately described by the Dubé classification system. Orientation relationships (ORs) between austenite and the ferritic matrix were determined with electron backscattered diffraction (EBSD) analysis, employing graphic and algebraic methods. Grain boundary precipitates exhibited Kurdjumov-Sachs (K-S) or Nishyiama-Wassermann (N-W) ORs with at least one of the adjacent grains, and in some cases relationships intermediate between K-S and N-W appeared. In approximately 60 pct of the cases examined, grain boundary precipitates show K-S or N-W ORs with both grains forming a boundary, though with small deviations (up to 5 deg) from the exact relationships. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.     

Orientation relationships associated with austenite formation from ferrite in a coarse-grained duplex stainless steel

Studies on the morphology and crystallography of austenite precipitated from ferrite were performed in an Fe-22.5 pct Cr-4.7 pct Ni-3 pct Mo-duplex stainless steel. Samples were solution treated at 1325°C (yielding a grain size of approximately 2 mm), water quenched, and then aged at temperatures ranging from 700 °C to 1100 °C for times ranging from 5000 to 30,000 seconds. The morphology of grain boundary precipitates depends on the grain boundary segment at which the precipitates are formed, and may be adequately described by the Dubé classification system. Orientation relationships (ORs) between austenite and the ferritic matrix were determined with electron backscattered diffraction (EBSD) analysis, employing graphic and algebraic methods. Grain boundary precipitates exhibited Kurdjumov-Sachs (K-S) or Nishyiama-Wassermann (N-W) ORs with at least one of the adjacent grains, and in some cases relationships intermediate between K-S and N-W appeared. In approximately 60 pct of the cases examined, grain boundary precipitates show K-S or N-W ORs with both grains forming a boundary, though with small deviations (up to 5 deg) from the exact relationships. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.     

Orientation relationships associated with austenite formation from ferrite in a coarse-grained duplex stainless steel

Studies on the morphology and crystallography of austenite precipitated from ferrite were performed in an Fe-22.5 pct Cr-4.7 pct Ni-3 pct Mo-duplex stainless steel. Samples were solution treated at 1325°C (yielding a grain size of approximately 2 mm), water quenched, and then aged at temperatures ranging from 700 °C to 1100 °C for times ranging from 5000 to 30,000 seconds. The morphology of grain boundary precipitates depends on the grain boundary segment at which the precipitates are formed, and may be adequately described by the Dubé classification system. Orientation relationships (ORs) between austenite and the ferritic matrix were determined with electron backscattered diffraction (EBSD) analysis, employing graphic and algebraic methods. Grain boundary precipitates exhibited Kurdjumov-Sachs (K-S) or Nishyiama-Wassermann (N-W) ORs with at least one of the adjacent grains, and in some cases relationships intermediate between K-S and N-W appeared. In approximately 60 pct of the cases examined, grain boundary precipitates show K-S or N-W ORs with both grains forming a boundary, though with small deviations (up to 5 deg) from the exact relationships. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.     

Mechanical alloying of IN-738

Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y₂O₃. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.     

Coherent <Emphasis Type="Italic">γ′</Emphasis>-precipitates at singular and rough grain boundaries in a model Ni-base superalloy

In a model Ni-base superalloy of Ni-24Co-4Al-4Ti-5Cr-5Mo (by wt pct), the singular and rough grain boundaries that exist at a temperature range between 1050 °C and 1200 °C can be identified by observing the shapes of coherent γ′-precipitates by transmission electron microscopy (TEM). Some grain boundaries become locally curved with the impinging spherical γ′-precipitates. These grain boundaries must be rough. Some grain boundaries maintain their flat shapes even at the contact areas with the γ′-precipitates and triple junctions. Such flat grain boundary shapes indicate that these grain boundaries are singular. Some grain boundaries have hill-and-valley shapes and some of their segments also show flat shapes with impinging precipitates. These boundary segments must also be singular. The results show that the singular and rough grain boundaries in this alloy can be clearly identified by the shape distortions produced by the impinging γ′-precipitates.     

The relationship between microstructural and plastic instability in AI-4.0 Wt Pct Cu alloy

The investigation of the effect of plastic deformation on the stability of theθ′ precipitates in an aluminum-4.0 wt pct copper alloy was performed. The alloy was produced by directional solidification, with Ti added as a grain refiner. Hot compression tests were performed at 200 °C in the strain rate range of 10_-3 to 10_-5 s₁ and equivalent strain up to 0.7 on specimens that had been initially heat treated, also at 200 °C, in order to obtain a uniform distribution of theθ0′ precipitates within the matrix. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the plastically deformed specimens revealed a very heterogeneous distribution of strain. Also, the regions with localized strain contained randomly distributedθ precipitates of nearly equiaxed shape without any preferred orientation relationships to the matrix. Thus, the plastic deformation initiated the transformationθ′ →θ. The flow stress was reduced in the regions in which this transformation had occurred, which further accentuated the localization tendency of the strain. The combined process,θ′ →θ transformation/strain localization, thus developed in an avalanching way.     

Interphase boundary structures associated with diffusional phase transformations in Ti-base alloys

Interphase boundary structures generated during diffusional transformations in Ti-base alloys, especially the proeutectoid α and eutectoid reactions in a β-phase matrix, are reviewed. Partially coherent boundaries are shown to be present whether the orientation relationship between precipitate and matrix phases is rational or irrational. Usually, these structures include both misfit dislocations and growth ledges. However, grain boundary α allotriomorphs (GBA’s) do not appear to develop misfit dislocations at partially coherent boundaries. Evidently, these dislocations can be replaced by ledges which provide a strain vector in the plane of the interphase boundary. The bainite reaction in Ti-X alloys produces a mixture of eutectoid α and eutectoid intermetallic compound. Both eutectoid phases are partially coherent with theβ matrix, and both grow by means of the ledge mechanism, though unlike pearlite the ledge systems of the two phases are structurally independent. Even after deformation and recrystallization, the boundaries between the eutectoid phases and theβ matrix, as well as between these phases, are partially coherent. Titanium and zirconium hydrides have partially coherent interphase boundaries with respect to theirβ matrix. The recent observation of ledgewise growth of γ TiH within situ high-resolution transmission electron microscopy (HRTEM) suggests that, repeated suggestions to the contrary, these hydrides do not grow by means of shear transport of Ti atoms at rates paced by hydrogen diffusion. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Formation and stability of fe-rich precipitates in dilute Zr(Fe) single-crystal alloys

The formation and stability of Fe-rich precipitates in two α-Zr(Fe) single-crystal alloys with nominal compositions I, 50 parts per million by atom (ppma) Fe, and II, 650 ppma Fe, have been investigated. Optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to examine the characteristics of Fe-rich precipitates. The SEM and TEM micrographs showed that in as-grown alloy II, Zr₂Fe precipitates were located at “stringers. ”Precipitates were not observed in as-grown alloy I. Annealing treatments below 700 °C, for alloy I, and 820 °C, for alloy II, resulted in the diffusion of excess Fe (above the α-phase solution limit) to the free surface with the subsequent formation of Zr₃Fe precipitates in both alloys. Dissolution of Zr₃Fe surface precipitates of alloy I (annealing above the solvus) left precipitate-like features on the surfaces. Zr₂Fe precipitates in as-grown alloy II were readily dissolved by β-phase annealing.     

Isothermal transformations in an Fe-9 pct Ni alloy

Isothermal transformation from austenite in an Fe-9.14 pct Ni alloy has been studied by optical metallography and examination by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range 565 °C and 545 °C, massive ferrite (α _q ) forms first at prior austenite grain boundaries, followed by Widmanst?tten ferrite (α _W ) growing from this grain boundary ferrite. Between 495 °C and 535 °C, Widmanst?tten ferrite is thought to grow directly from the austenite grain boundaries. Both these transformations do not go to completion and reasons for this are discussed. These composition invariant transformations occur below T ₀ in the two-phase field (α+γ). Previous work on the same alloy showed that transformation occurred to α _q > and α _W on furnace cooling, while analytical TEM showed an increase of Ni at the massive ferrite grain boundaries, indicating local partitioning of Ni at the transformation interface. An Fe-3.47 pct Ni alloy transformed to equiaxed ferrite at 707 °C ±5 °C inside the single-phase field on air cooling. This is in agreement with data from other sources, although equiaxed ferrite in Fe-C alloys forms in the two-phase region. The application of theories of growth of two types of massive transformation by Hillert and his colleagues are discussed. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Composition and stability of Ω phase in an Al-Cu-Mg-Ag Alloy

Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were employed to determine the stability and composition of the Ω phase in an Al-Cu-Mg-Ag alloy. It is shown that the Ω. phase can nucleate at isothermal aging temperatures up to 470 °C in the α + θ phase region of the Al-Cu phase diagram. Furthermore, Ω precipitates formed by aging solution-treated and quenched samples at 130 °C were found to exist after reversion treatments at temperatures close to the θ-phase solvus. The TEM/EDS analysis of relief-etched Ω, precip-itates shows that the composition of Ω phase is statistically the same as that of θ-Al₂Cu phase in a binary Al-Cu alloy, with Ag and Mg below the limits of detectability. These results indicate that the Ω, phase is probably a metastable {111} variant of the equilibrium θ phase. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, University of Virginia, Scientist     

Stress corrosion cracking of copper bicrystals with 〈110〉-Tilt ∑3, ∑9, and ∑11 coincident site lattice boundaries

The stress corrosion cracking (SCC) behavior of copper bicrystals with 〈HO〉-tilt ∑3(111), ∑9(221), and ∑11(311) coincident site lattice (CSL) boundaries was investigated. Stress corrosion cracking tests were carried out in 1 N NaNO₂ aqueous solution at 303 ±2 K using a slow strain rate technique (SSRT). Transgranular SCC occurred along the primary slip traces on the top surface of the bicrystal having a ∑3(111) coincidence boundary. No cracks initiated on the grain boundary except for very small and shallow corrosion pits. In contrast, for the bicrystals with ∑9(221) or ∑11(311) coincidence boundaries, corrosion pits and cracks initiated on the grain boundary and propagated into the crystal interior along {110} traces, which are almost perpendicular to the tensile axis. The SCC behavior is closely related to the activated slip systems and the degrees of crystal rotation owing to deformation. Susceptibility to intergranular SCC is affected by the angle between the Burgers vector of the primary slip system and the grain boundary plane. The susceptibility of the ∑23(111) boundary to SCC is remarkably low in comparison with the other two types of grain boundaries. Y. Nakazawa, formerly Graduate Student, Department of Mechanical Engineering, Doshisha University This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Microstructure and mechanical properties of sputter-deposited Cu1−x Ta x alloys

The microstructures and mechanical properties of a family of sputter-deposited Cu_1−x Ta _x (0<x<0.18) alloys have been investigated. The as-deposited microstructures for all film compositions consisted of a polycrystalline, face-centered-cubic (fcc) Cu matrix, with varying levels of Ta in solid solution, plus a very high density of discrete, 1 to 3 nm, fcc Ta particles. Decreased deposition temperature (−120 °C vs 100 °C) increased the level of Ta in solid solution. After annealing (900 °C for 1 hour) the as-deposited 6 at. pct Ta films, the Cu matrix grains remained submicron and the Ta particles remained fcc with no apparent particle coarsening. Additionally, the fcc Ta particles were found before and after annealing to be oriented identically with the Cu matrix and aligned on {111} and {100} habit planes. Annealing 17 at. pct Ta films at 900 °C for 1 hour resulted in the formation of body-centered-cubic (bcc) Ta particles (>50-nm diameter) in addition to the much smaller fcc Ta particles. Annealing the low and high Ta composition films at 900 °C for as long as 100 hours produced no observed change in either the Cu matrix grain size or the size and distribution of the fcc and bcc Ta particles. Microhardness and nanoindentation mechanical property evaluations of bulk hot-pressed materials indicated that the high strengths of the composites were unchanged, even after annealing for 100 hours at 900 °C.     

The effect of annealing on the microstructure and mechanical properties of Cu-X microcomposites

The effect of annealing on the microstructure, texture, and room-temperature mechanical properties ofin situ processed copper-based microcomposites has been investigated. These copper microcomposites, containing 15 vol pct Nb, Cr, or Ta, were produced by rolling of cast material. Annealing was carried out in vacuum for 10 hours at 250 °C, 400 °C, and 650 °C. Evidence of microstructural coarsening was found even at the lowest annealing temperature. The through-thickness microstructure of the composites was examined by transmission electron microscopy both before and after the annealing treatments. Texture of the as-processed micro-composites was assessed using X-ray diffraction methods. The strength of the composites following annealing was found to scale with the melting point of the second component. This article is based on a presentation made in the symposium “High Performance Copper-Base Materials” as part of the 1991 TMS Annual Meeting, February 17–21, 1991, New Orleans, LA, under the auspices of the TMS Structural Materials Committee.     

The effect of sheet processing on the microstructure, tensile, and creep behavior of INCONEL alloy 718

The grain size, grain boundary character distribution (GBCD), creep, and tensile behavior of INCONEL alloy 718 (IN 718) were characterized to identify processing-microstructure-property relationships. The alloy was sequentially cold rolled (CR) to 0, 10, 20, 30, 40, 60, and 80 pct followed by annealing at temperatures between 954 °C and 1050 °C and the traditional aging schedule used for this alloy. In addition, this alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and the corresponding microstructure and mechanical behavior were evaluated. The creep behavior was evaluated in the applied stress (σ _a ) range of 300 to 758 MPa and the temperature range of 638 °C to 670 °C. Constant-load tensile creep experiments were used to measure the values of the steady-state creep rate and the consecutive load reduction method was used to determine the values of backstress (σ₀). The values for the effective stress exponent and activation energy suggested that the transition between the rate-controlling creep mechanisms was dependent on effective stresses (σ _e =σ _a σ₀) and the transition occurred at σ _e ≅ 135 MPa. The 10 to 40 pct CR samples exhibited the greatest 650 °C strength, while IN 718SPF exhibited the greatest room-temperature (RT) tensile strength (>1550 MPa) and ductility (ε _f >16 pct). After the 954 °C annealing treatment, the 20 pct CR and 30 pct CR microstructures exhibited the most attractive combination of elevated-temperature tensile and creep strength, while the most severely cold-rolled materials exhibited the poorest elevated-temperature properties. After the 1050 °C annealing treatment, the IN 718SPF material exhibited the greatest backstress and best creep resistance. Electron backscattered diffraction was performed to identify the GBCD as a function of CR and annealing. The data indicated that annealing above 1010 °C increased the grain size and resulted in a greater fraction of twin boundaries, which in turn increased the fraction of coincident site lattice boundaries. This result is discussed in light of the potential to grain boundary engineer this alloy. INCONEL is a registered trademark of Special Metals Corp., Huntington, WV. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Subnanometer-scale chemistry and structure of α-iron/molybdenum nitride heterophase interfaces

The local chemistry and structure of α-iron/molybdenum nitride heterophase interfaces is studied on a subnanometer scale by atom-probe field-ion microscopy (APFIM), three-dimensional atom-probe microscopy (3DAPM) and both conventional transmission electron microscopy (CTEM) and highresolution electron microscopy (HREM). Molybdenum nitride precipitates are generated by annealing Fe-2 at. pct Mo-X, where X=0.4 at. pct Sb or 0.5 at. pct Sn, at 550 °C or 600 °C, in an ammonia/hydrogen mixture. Internal nitridation at 550 °C produces thin, coherent platelet-shaped molybdenum nitride precipitates. Nitridation at 600 °C generates a much coarser structure with semicoherent thick plate-shaped and spheroidal precipitates in addition to the thin-platelet structure. The APFIM and 3DAPM analyses of the heterophase interfaces show substantial segregation of the solute species Sn and Sb only at the coarse precipitates, with Gibbsian interfacial excesses of up to 7±3 nm⁻², whereas the broad faces of the thin platelets have no detectable segregation. The TEM and HREM analyses show that the coarse precipitates are semicoherent, whereas the thin platelets are either coherent or have much fewer misfit dislocations than geometrically necessary. This demonstrates that Sn and Sb segregation is related to the presence of misfit dislocations at the interfaces of the coarse precipitates. This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Phase transformations during aging of a nitrogen-strengthened austenitic stainless steel

An analytical electron microscopy study was undertaken in order to characterize intergranular and matrix precipitation accompanying intermediate temperature aging in NITRONIC 50, a nitrogen-strengthened austenitic stainless steel. Extensive precipitation on most grain boundaries had occurred after aging for 24 hours at 675 °C. The primary intergranular phase at that time was Cr-rich M₂₃C₆, and energy dispersive spectra taken on grain boundary segments between these carbides indicated Cr-depletion and Fe- and Ni-enhancement relative to the matrix. After aging for 336 and 1008 hours at 675 °C, M₆C (eta-carbide) precipitates were also present on grain boundaries. These precipitates were distinguished from M₂₃C₆ on the basis of their lattice parameters and chemistries, with M₆C containing less Cr and Fe, and more Ni, Mo, and Si than M₂₃C₆. The differences in chemistry were clarified by a statistical treatment of the spectra. The statistical analysis also showed that precipitates with a range of chemistries between M₂₃C₆ and M₆C coexisted with these phases on the grain boundaries. Associated with this shift in precipitate stoichiometry was an increase in the average concentration of Cr and a decrease in the average concentration of Ni at the grain boundaries. Intergranular sigma phase was also observed after times 24 hours at 675 °C, with sigma precipitating on grain boundaries containing carbides. Intragranular precipitates observed to be stable up to 1008 hours at 675 °C included Z-phase, a complex nitride which had formed during solution annealing; M₇C₃ carbides, which nucleated at Z-phase/austenite interfaces; M₂₃C₆ carbides, which precipitated on incoherent twin boundaries; and Cr-rich MN precipitates, which nucleated on dislocations.     

Phenomenology of precipitation in copper-20 pct nickel-20 pct manganese

The precipitation phenomena in the alloy copper-20 pct nickel-20 pct manganese have been investigated. Utilizing transmission electron microscopy as the principal tool; the effects of aging temperature and time as well as prior cold work were studied. For all aging temperatures the reaction products are the solute depleted fcc solid solution and an ordered structure with fct symmetry. Three aging temperatures characterized by different precipitate morphologies were studied. At 350°C discontinuous precipitation is the predominant mode of decomposition. Precipitate colonies nucleate at grain and twin boundaries and eventually grow through the entire structure. Microtwinning of the colony matrix accompanies the precipitation reaction. At 450°C both grain boundary nucleated discontinuous precipitates and fine periodic homogeneous arrays are observed in the absence of cold work. The fine periodic arrays coarsen and eventually form nuclei for the ordered fct phase. The coarsening of the periodic arrays prohibits the growth of the discontinuous precipitate early in the process, so only a small volume fraction of discontinuous precipitate is formed at the grain boundaries. Aging subsequent to cold work results in ordered, fct precipitates heterogeneously nucleated on dislocations. At 500°C no precipitate is observed in the absence of cold work. When aging is preceded by cold work, the ordered fct phase appears as heterogeneously nucleated Widmanstatten laths. No grain boundary nucleated colonies are observed at this temperature.