Kinetics of discontinuous coarsening of cellular precipitate in a Cu-15 wt% in alloy

The morphology and growth kinetics of the cellular precipitate and discontinuously coarsened cellular precipitate have been studied in the temperature range 573–731 K by utilizing optical and scanning electron microscopy and X-ray diffraction. In order to avoid precipitation of the Widmanstatten precipitate phase, which has a retarding effect on the rate of growth of primary cells, isothermal aging of the alloy was preferred. The Cu-In alloy was observed to decompose completely by cellular precipitation reaction into a lamellar structure consisting of alternate lamellae of the α and δ phases at all aging temperatures. The fine lamellar structure of the primary cells decomposed into a coarse lamellar structure consisting of the same two phases by a discontinuous coarsening or secondary reaction. Lattice parameter measurements indicated that whereas the depleted matrix was richer in solute than the equilibrium solvus during the primary reaction, it was very close to the equilibrium solvus during the secondary reaction. Analysis of the growth kinetics both of the primary and secondary cellular reaction indicated that the transformations are controlled by diffusion through the cell boundaries.     

X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu

Al-Si eutectic growth has been studied in-situ for the first time using X-ray video microscopy during directional solidification (DS) in unmodified and Sr-modified Al-Si-Cu alloys. In the unmodified alloys, Si is found to grow predominantly with needle-like tip morphologies, leading a highly irregular progressing eutectic interface with subsequent nucleation and growth of Al from the Si surfaces. In the Sr-modified alloys, the eutectic reaction is strongly suppressed, occurring with low nucleation frequency at undercoolings in the range 10 K to 18 K. In order to transport Cu rejected at the eutectic front back into the melt, the modified eutectic colonies attain meso-scale interface perturbations that eventually evolve into equiaxed composite-structure cells. The eutectic front also attains short-range microscale interface perturbations consistent with the characteristics of a fibrous Si growth. Evidence was found in support of Si nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form at the Si-particle interfaces.     

Kinetics of discontinuous coarsening of cellular precipitate in a Ni-8.5 at.% Sn alloy

The morphology and growth kinetics of the cellular precipitate and discontinuous coarsening of the cellular precipitate have been studied in the temperature range 815–995 K by utilizing optical microscopy and X-ray diffraction. The Ni-Sn alloy was observed to decompose completely by cellular precipitation reaction into a lamellar structure consisting of alternate lamellae of the α and β phases at all aging temperatures. The fine lamellar structure of the primary cells decomposed into a coarse lamellar structure in two stages. In the first stage, the secondary cells with larger interlamellar spacing decomposed the primary cells. In the second stage, the tertiary cells with a much larger inter-lamellar spacing decomposed the secondary cells. The tertiary cells also decomposed the primary cells. The primary cell growth data were analysed by using theories of Petermann and Hornbogen, Turnbull and Cahn. The secondary and tertiary cell growth data were analysed by assuming that these also follow the theory of Petermann and Hornbogen of primary cell growth. From the diffusivity values, it has been concluded that the growth of primary, secondary and tertiary cells occurs by diffusion of tin along the migrating grain boundaries.     

The genesis of the cellular precipitation reaction

The morphology of cellular precipitation in a Cu-9.5 at. pet In alloy has been investigated by light and electron microscopy. Both cellular and general precipitation were observed to occur simultaneously in quenched and aged alloys while only cellular precipitation was observed to occur in isothermally aged alloys. Because of the presence of wide, solute rich, precipitate free zones in the vicinity of grain boundaries in the quenched and aged alloys, the early development of cellular precipitation was found to be identical for both types of heat treatment. From light and electron microscopy observations of the early stages of cellular precipitation a mechanism for the formation of cells was developed. At the start of aging, the unoccupied grain boundary begins to migrate under the influence of grain boundary migration forces as if it were in a single phase alloy. As the boundary migrates, solute segregates along it to form allotriomorphs which pin the boundary. The boundary continues to migrate and bows between the simultaneously forming allotriomorphs. With further aging, the allotriomorphs lengthen following the bowing boundary and the allotriomorphs become the initial precipitate lamellae of the developing cell as a steady-state lamellar structure develops. Assuming that the critical step in the development of a cell is the ability of the boundary to bow between the initial allotriomorphs, a criterion for the occurrence of cellular precipitation was developed. R. A. FOURNELLE, formerly Graduate Student, University of Missouri-Rolla This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.     

A transmission electron microscopy study of copper precipitation in the cementite phase of hypereutectoid alloy steels

The precipitation of copper has been detected and studied in three of the main decomposition products of austenite: allotriomorphic grain-boundary cementite, pearlitic cementite, and Widmanstätten cementite plates. The investigation has been carried out on two high-alloy hypereutectoid steels containing copper contents of 1.0 and 2.5 wt pct. The main advantage of these high-alloy steels is that the parent austenite phase remains stable upon cooling to room temperature, thus preserving the parent phase and the parent/product interfaces in the microstructure for subsequent examination. Transmission electron microscopy (TEM) revealed that the copper precipitation occurs in proeutectoid allotriomorphic grain-boundary cementite in association with the transformation interface. The copper particles were dispersed in the form of rows (or sheets) within the allotriomorphs of cementite. Evidence for copper precipitate particles nucleated at structural features imaged at the growth interface was also obtained. Copper precipitation was found to occur in both the ferrite and cementite lamellae of pearlite, and again, examination of partially decomposed structures revealed copper particles nucleated at the austenite/pearlite transformation interface. In addition, copper particles were also observed at the ferrite/cementite interface of pearlite. Copper precipitation observed in Widmanstätten cementite plates revealed a precipitate-free midrib region in the plates and a higher concentration of copper particles toward the broad faces of the plate. Copper particles were also found located at coarse linear interface defects at the broad faces of the plate.     

Influence of Crystallography on Ferrite Nucleation at Austenite Grain-Boundary Faces,Edges, and Corners in a Co-15Fe Alloy

The nucleation of ferrite precipitates at austenite grain faces, edges (triple lines), and corners (quadruple points) was studied in a Co-15Fe alloy in which the matrix phase was retained upon cooling to room temperature by serial sectioning coupled with electron backscatter diffraction analysis. Nearly half of the edges and corners were vacant at an undercooling of 60 K from the γ/(α + γ) boundary where the precipitation occurred significantly at grain faces. A significant proportion of precipitates had Kurdjumov–Sachs (K–S) and to a lesser extent Nishiyama–Wassermann (N–W) orientation relationships with more than one grain at all boundary sites. Vacant edges and corners were readily observed, of which the misorientations of matrix grain boundaries would permit a precipitate to have a specific orientation relationship with multiple grains. Small differences in the nucleation activation energy among the grain faces, edges, and corners may lend support to a view proposed from experiments of nucleation in Fe-C base alloys that ferrite nuclei are more or less surrounded by low-energy facets of α/γ phase boundary.     

Crystallography and Morphology of Niobium Carbide in As-Cast HP-Niobium Reformer Tubes

The microstructures of two as-cast heats of niobium-modified HP stainless steels were characterized. Particular attention was paid to the interdendritic niobium-rich carbides formed during solidification of these alloys. At low magnifications, these precipitates are grouped in colonies of similar lamellae. Higher magnifications revealed that the lamellae actually obtain two distinct morphologies. The type I morphology exhibits broad planar interfaces with a smooth platelike shape. Type II lamellae have undulating interfaces and an overall reticulated shape. To provide further insight into the origin of these two different morphologies, the microstructure and crystallography of each have been studied in detail using high resolution scanning electron microscopy, transmission electron microscopy, various electron diffraction methods (electron backscatter diffraction (EBSD), selected area diffraction (SAD), and convergent beam electron diffraction (CBED)), and energy dispersive X-ray spectroscopy.     

A high resolution transmission electron microscope study of early stage precipitation on dislocation lines in Al-Zn-Mg

The weak beam transmission electron microscopy technique was employed to study the early stages of precipitation on dislocation lines in Al-3.87 wt pct Zn-1.79 wt pct Mg. The heterogeneous precipitation sequence was found to follow the homogeneous sequence in this alloy. The interaction between the initial coherent precipitate particles and the strain fields of the catalyzing dislocations produced “gaps” of background intensity at precipitate locations along the otherwise continuous weak beam images of the dislocation lines. A simple model was developed to relate a distribution of measured weak beam gap lengths to a particle size distribution at a given aging treatment. In this manner the growth kinetics of the initial precipitate phase was observed; it was found that the precipitation followed the Cottrell-Bilbyt ^2/3law, suggesting that matrix dislocations may assist the growth of heterogeneous precipitates in a manner analogous to grain boundary “collector plates.” Weak beam microscopy was found to be superior to standard bright field microscopy for the current study. Particles too small to be visible in bright field were revealed in weak beam. Weak beam observations also indicated that the coherent precipitate particles were positioned asymmetrically about the dislocation cores.     

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.     

Cellular Precipitation at a 17-7 PH Stainless Steel Interphase Interface During Low-Temperature Nitridation

Cellular precipitation of Cr-rich nitrides was observed at an austenite–ferrite interface in 17-7 PH stainless steel after low-temperature nitridation. Fine-scale lamellar rocksalt-structured nitride (MN_1?x , M: randomly distributed Fe, Cr, and Al) was identified at the interfaces between austenite and ferrite by local-electrode atom-probe tomography and transmission electron microscopy. The small size and spacing of the nitride lamellae reflect the low mobility of substitutional atoms under the conditions of low-temperature nitridation. Nitrides of the same structure were formed within the ferrite grain as extremely small particles. The face-centered cubic nitride precipitates in the Bain orientation relationship with the ferrite.     

Undercooling and solidification of Al-50 at. pct Si Alloy by electromagnetic levitation

Electromagnetic levitation is applied to achieve containerless solidification of 10-mm-diameter droplets of Al-50 at. pct Si. A maximum undercooling of 320 K is obtained. Phase morphologies on the droplet surfaces and on the deeply etched sections of the samples solidified at different undercoolings are examined by scanning electron microscopy. The primary silicon shows well-developed faceted dendrites at a small undercooling, but a fine granular form at a large undercooling. Stratified deposits of aluminum are found within the primary silicon plates, arising from solute pileup during growth. The microstructural refinement at a large undercooling has its origins in solute restriction of crystal growth and in fragmentation of the primary silicon dendrites. The form of the Al-Si eutectic is also found to be changed into an anomalous form at a large undercooling.     

Inhibitors of sterol synthesis. Chemical synthesis of 7 alpha-ethyl and 16 alpha-ethyl derivatives of delta 8(14)-15-oxygenated sterols and their effects on 3-hydroxy-3-methylglutaryl coenzyme A reductase in CHO-K1 cells

Three-dimensional data representing biological structures can be derived using several methods, including serial section reconstruction, optical sectioning, and tomography. The investigation, comprehension, and communication of structural relationships to others is greatly facilitated by computer-based visualization procedures. We describe SYNU, a suite of programs developed for interactive investigation of three-dimensional structure and for the production of high-quality three-dimensional images and animations. We illustrate the capabilities of SYNU in applications to biological data obtained by confocal light microscopy, serial section, and high-resolution electron microscopy from investigations at the cellular, subcellular, and molecular levels.     

Discontinuous precipitation and coarsening in Al-Zn alloys

The morphology and kinetics of discontinuous precipitation (DP) and discontinuous coarsening (DC) in solution treated and isothermally aged Al-Zn alloys containing 39.3 and 59.3 at.% Zn have been investigated at temperatures ranging from 323 to 523 K by light microscopy, electron microscopy and X-ray diffraction. At all aging temperatures the supersaturated α solid solution was observed to decompose rapidly by DP into a lamellar mixture of solute depleted α phase and β phase precipitate. DP occurred so rapidly in the 59.5 at.% Zn alloy that the heat of transformation raised the temperature of the alloy significantly. With further aging a slower DC reaction transformed the lamellar DP into a coarser lamellar structure of the same two phases; however, the composition of the α phase of the DC was closer to the equilibrium solvus composition than that of the DP. With still further aging a second, much slower DC reaction was observed to decompose the lamellar product of the first DC reaction in the 59.5 at.% Zn alloy into a still coarser lamellar structure. Analysis of the kinetics of both the DP and DC reactions showed them to be controlled by boundary diffusion in the advancing reaction interface. Reaction front migration rates for both DP and DC increased markedly with increasing Zn content. This increase seems to be associated partially with an increase in boundary diffusivity with increasing Zn content.     

Light and electron microscopic localization of acetylcholinesterase activity in the rat renal nerves

Acetylcholinesterase activity is shown in the renal nerves of the rat with the technique of Karnovsky and Roots. By light microscopy, the acetylcholinesterase-positive nerves are seen in association with blood vessels, including the glomerular arterioles, and occasionally with renal tubules. By electron microscopy the precipitate appears extracellularly around axons and varicosities. DFP inhibits the deposition of precipitate. Previous demonstration by serial section electron microscopy in the rat revealed that all nerves around the glomerular arterioles contain small dense-cored vesicles characteristic of adrenergic nerves, indicating that the acetylcholinesterase-positive nerves demonstrated here are likely to be adrenergic nerves containing acetylcholinesterase.     

A fine γ′+α cellular structure in Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C and its influence on high-temperature tensile properties

Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C alloy, which reveals an excellent combination of high strength and good elongation endowed by formation of homogeneously dispersed fine γ′ precipitates in the matrix during aging at 823 K, has been investigated by means of transmission electron and optical microscopies, electron diffractions, and tensile tests. The influence of unique γ′+α cellular products on the mechanical properties has also been studied. Because of low elastic mismatch between the austenitic γ matrix and isomorphic γ′ precipitate phases, the homogeneously distributed precipitate particles, which formed at the early stage of aging, were observed to persist even after long-term aging. After very lengthy aging, the fine γ′ phase particles were changed to coarser γ′ lamellae at the grain boundary reaction front, which were alternately arranged with fine α lamellae that were estimated to have been transformed from the austenite-stabilizing-solute(Ni, C)-depleted γ lamellae. The fine duplex γ′+α cellular product did not affect deleteriously the room-temperature tensile properties of the alloy. However, the cellular structure was observed to cause the grain boundary embrittlement of the aged alloy at elevated temperatures higher than 681 K.     

Crystal-Growth Transition and Homogenous Nucleation Undercooling of Bismuth

The cross-sectional and surface morphologies of highly undercooled bismuth samples are investigated by optical microscopy and scanning electron microscopy. It is found that the grain morphology can be classified into three types. When the undercooling is less than 49 K (49 °C), flaky grains with pronounced edges and faces are arranged parallel to each other, showing the feature of lateral growth. When the undercooling is over 95 K (95 °C), refined equiaxial grains with several smooth bulges on the surface of each grain are randomly arranged, showing the feature of continuous growth. In the undercooling region from 49 K to 95 K (49 °C to 95 °C), the features of both lateral and continuous growth are observed. The microstructures within the sample grains obtained at different undercooling regions are dissimilar, but they all show features of anisotropic growth. Based on the critical growth-transition undercoolings, direct expressions that express the relationship between the solid-liquid interface energy and temperature are determined. Homogenous nucleation undercooling is also predicted according to the solid-liquid interface energy obtained from the critical growth-transition undercooling. The predicted results of homogenous nucleation undercooling for bismuth are in good agreement with the experimental results.     

Precipitation Kinetics of Cr2N in High Nitrogen Austenitic Stainless Steel

The precipitation behavior of Cr2 N during isothermal aging in the temperature range from 700 ℃ to 950 ℃ in Fe-18Cr-12Mn-0.48N （in mass percent） high nitrogen austenitic stainless steel, including morphology and content of precipitate, was investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The isothermal precipitation kinetics curve of Cr2 N and the corresponding precipitation activation energy were obtained. The results show that Cr2N phase precipitates in a cellular way and its morphology is transformed from initial granular precipitates to lamellar ones in the cell with increasing aging time. The nose temperature of Cr2 N precipitation is about 800 ℃, with a corresponding incubation period of 30 min, and the ceiling temperature of Cr2N precipitation is 950℃. The diffusionactivation energy of Cr2 N precipitation is 296 kJ/mol.     

In-situ observation of unsteady peritectic growth modes

A transparent non-facetted/non-facetted peritectic system was used for in-situ studies of peritectic growth morphologies above and below the limit of constitutional undercooling. For a hypo-peritectic alloy with near peritectic composition an oscillatory growth mode was found for velocities in the cellular/dendritic regime. It is demonstrated that the co-existence of two distinct cellular/dendritic solid/liquid interfaces which try to reach stable growth conditions causes this oscillations. Additionally, it is shown that on reducing the withdrawal speed the two distinct interfaces arrange into an isothermal coupled peritectic growth mode.     

A high-resolution transmission electron microscopy study of the precipitation process in a dilute Ti-N alloy

The precipitation processes in dilute nitrogen alloys of titanium have been examined in detail by conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The alloy Ti-2 at. pct N on quenching from its high-temperatureβ phase field has been found to undergo early stages of decomposition. The supersaturated solid solution (α″-hcp) on decomposition gives rise to an intimately mixed, irresolvable product microstructure. The associated strong tweed contrast presents difficulties in understanding the characteristic features of the process. Therefore, HREM has been carried out with a view to getting a clear picture of the decomposition process. Studies on the quenched samples of the alloy suggest the formation of solute-rich zones of a few atom layers thick, randomly distributed throughout the matrix. On aging, these zones grow to a size beyond which the precipitate/matrix interfaces appear to become incoherent and theα′ (tetragonal) product phase is seen distinctly. The structural details, the crystallography of the precipitation process, and the sequence of precipitation reaction in the system are illustrated.     

Precipitation Kinetics of Cr2N in High Nitrogen Austenitic Stainless Steel

 The precipitation behavior of Cr2N during isothermal aging in the temperature range from 700 ℃ to 950 ℃ in Fe 18Cr 12Mn 048N (in mass percent) high nitrogen austenitic stainless steel, including morphology and content of precipitate, was investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The isothermal precipitation kinetics curve of Cr2N and the corresponding precipitation activation energy were obtained. The results show that Cr2N phase precipitates in a cellular way and its morphology is transformed from initial granular precipitates to lamellar ones in the cell with increasing aging time. The nose temperature of Cr2N precipitation is about 800 ℃, with a corresponding incubation period of 30 min, and the ceiling temperature of Cr2N precipitation is 950 ℃. The diffusion activation energy of Cr2N precipitation is 296 kJ/mol.