Discontinuous Coarsening Reaction Research Articles

Development and performance of INCONEL® alloy 740H® seam-welded piping

INCONEL® alloy 740H® is an age-hardenable nickel-based alloy approved for pressure vessels and piping within ASME Section I, VIII, and B31.1. Currently, the code applies a weld strength reduction factor (WSRF) of 0.7 to the allowable stresses for longitudinal seam welds in the time dependent creep regime. In this work, a full-scale seam weld was successfully produced using typical industrial practices. The component was solution heat-treated and aged after fabrication to improve the performance of the weldment. Tensile, bend, impact, and low-cycle fatigue tests showed the component met all the specification requirements and elevated temperature properties were within base metal expectations with failures predominately in the weld metal. Long-term creep tests, including large samples more representative of the entire weldment, were fabricated and tested to times in excess of 10,000 h at multiple temperatures. Analysis of the creep data supports a WSRF of 0.9 for the solution annealed + aged weldments in contrast to the WSRF of 0.7 currently applied to welded + aged weldments. Metallurgical analysis shows that the solution annealing causes recrystallization of the weld metal and reduces the chemical compositional and microstructural gradients in the weldments resulting in acceptable short-term performance. In long-term creep, evidence for coarsened zones in the weldments due to discontinuous coarsening reactions were identified as the mechanism leading to accelerated creep damage formation in the weld metal and sample failure.

Comparison of discontinuous precipitation behavior of Cu-15Ni-8Sn alloy with and without Co addition

The microstructure evolution, phase interface relationships, and corresponding hardness changes of discontinuous precipitation in Cu-15Ni-8Sn alloy during the aging process were investigated. The samples were isothermally aged for 15 h at 500–700 °C to yield discontinuous precipitates with varying lamellar spacings. The results showed that discontinuous precipitation with a lamellar structure is composed of the α-Cu phase and γ-DO3 phase, which has an incoherence interface and satisfies the K-S orientation relationship. With the increase of aging temperature, the lamellar spacing of the discontinuous precipitation increases, the length of lamellar shortens, and the lamellar nature weakens. Simultaneously, the discontinuous coarsening reaction is accelerated. The addition of the trace Co element can inhibit the formation of discontinuous precipitation and discontinuous coarsening phase. Co enrichment was found in the γ-DO3 phase, which can alter the lamellar spacing and then affect the hardness of the Cu-15Ni-0.5Co alloy. In addition, the hardness of discontinuous precipitation in the Cu-15Ni-8Sn- (0.5Co) alloy is negatively correlated with the lamellar spacing and can follow the Hall-Petch type relation.

Characterization of Discontinuous Coarsening Reaction Products in INCONEL® Alloy 740H® Fusion Welds

Characterization of γ′ coarsened zones (CZs) in alloy 740H fusion welds via a variety of electron microscopy techniques was conducted. The effects of solute partitioning during nonequilibrium solidification on the amount of strengthening precipitates along the grain boundaries were evaluated via electron-probe microanalysis and scanning electron microscopy. Electron backscatter diffraction was used to present evidence for the preferential growth of CZs toward regions of lower γ′ content, even if growth in that direction increases grain boundary area. Scanning electron microscopy and image analysis were used to quantify the propensity for CZs to develop along certain segments of the grain boundaries, as governed by the local variations in γ′ content. Scanning transmission electron microscopy with X-ray energy-dispersive spectrometry (XEDS) was used to assess the compositions of the matrix and precipitate phases within the CZs and to quantify the segregation of alloying components to the reaction front. Thermodynamic and kinetic modeling were used to compare calculated and experimental compositions. The work presented here provides new insight into the progression of the discontinuous coarsening (DC) reaction in a complex engineering alloy.

Microstructural Evolution of INCONEL® Alloy 740H® Fusion Welds During Creep

Electron microscopy techniques have been used to investigate the cause of premature creep failure in the fusion zone of INCONEL® Alloy 740H® (INCONEL and 740H are registered trademarks of Special Metals Corporation) welds. The reduced creep rupture lives of all-weld-metal and cross-weld creep specimens (relative to base metal specimens) have been attributed to the presence of large grain boundary regions that were denuded in fine γ′ but contained coarse, elongated particles. Investigation of creep rupture specimens has revealed four factors that influence the formation of these coarsened zones, and the large particles found within them have been identified as γ′. Comparisons of the microstructural characteristics of these zones to the characteristics that are typical of denuded zones formed by a variety of mechanisms identified in the literature have been made. It is concluded that the mechanism of γ′-denuded zone formation in alloy 740H is discontinuous coarsening of the γ′ phase. The discontinuous reaction is catalyzed by the grain boundary migration and sliding which occur during creep and likely promoted by the inhomogeneous weld metal microstructure that results from solute segregation during solidification. The increased susceptibility to the formation of the observed γ′-denuded zones in the weld metal as compared to the base metal is discussed in the context of differences in the contributions to the driving force for the discontinuous coarsening reaction.

Kinetics of Lamellar Decomposition Reactions in U-Nb Alloys

Discontinuous precipitation (DP) and discontinuous coarsening (DC) reactions have been observed in numerous alloy systems [1]. DP has been observed in the U-Nb system [2, 3, 4, 5]. The U-Nb phase diagram (Fig. 1) exhibits a continuous γ-BCC solid solution at high temperatures and a two-phase mixture of a-orthorhombic and γ-BCC below the 647°C monotectoid isotherm. The DP reaction occurs during continuous cooling and isothermal aging over 300-647°C. No metallographic evidence of a DC reaction in U-Nb has been published, although this is suggested from x-ray observations of distinct changes in the Nb content of the γ phase upon prolonged holding after the DP reaction [2, 3, 6]. This study will provide direct evidence for a DC reaction. Discontinuous and other aging reactions [7] are undesirable in U-Nb alloys, since they degrade corrosion resistance [5], ductility [8], and the shape-memory effect [9]. Hence, an improved understanding of the kinetics of these discontinuous phase transformations in U-Nb alloys is of practical interest.

Discontinuous coarsening behavior of Ni2MnAl intermetallic compound during isothermal aging treatment of Fe-Mn-Ni-Al alloys

The discontinuous reaction of the Ni(2)MnAl intermetallic phase was investigated during the aging of a solution-treated Fe-8.3Mn-8.2Ni-4.2Al alloy. During aging, Ni(2)MnAl lamellae formed at the prior austenite grain boundaries and twin boundaries and grew into the neighboring grains. The presence of continuously precipitated fine Ni(2)MnAl particles before the growth of the discontinuously precipitated lamellae was confirmed by dark-field transmission electron microscopy, and it was concluded that the present reaction is a type of discontinuous coarsening process, alpha' + Ni(2)MnAl (continuous precipitation) --> alpha + Ni(2)MnAl (discontinuous coarsening). The chemical driving force and the reduction of the total coherent strain energy were suggested to be the driving force for the discontinuous coarsening reaction.

Nitriding behavior of Fe–4wt%V and Fe–2wt%V alloys

The nitriding behavior of Fe–4wt%V and Fe–2wt%V alloys was investigated by exposing these alloys to a gas mixture of ammonia and hydrogen at 580 °C. The microstructure of the nitrided specimens was analyzed employing light and scanning electron microscopy, hardness measurements, and X-ray diffraction. To determine the elemental composition of the nitrided zone electron probe microanalysis (EPMA) was applied. The nitrided zone of the Fe–4wt%V specimens comprised many grains exhibiting a lamellar morphology characterized by a lamellar spacing of 20–40 nm. Such a lamellar microstructure was absent in the nitrided zone of the Fe–2wt%V specimens. These morphological results, together with the obtained hardness and X-ray diffraction data, demonstrate that a discontinuous coarsening reaction occurs in nitrided Fe–4wt%V alloy. Such a reaction was not observed before for nitrided Fe–V alloys. EPMA showed the uptake of excess nitrogen in the nitrided zone. The discontinuously transformed regions contain less nitrogen than the not transformed regions. The effect of these phenomena on the nitriding kinetics was analyzed and discussed.

Refining grain size of a TiAl alloy by cyclic heat treatment through discontinuous coarsening

Recently, we reported a novel approach for grain refinement of TiAl alloys by cyclic heat treatment (CHT) only. Different from our previous studies involving fast cooling, in this study, an alternative CHT route is provided for grain refinement under relatively slow cooling. It is particularly shown that a coarse fully lamellar structure can be refined to a fine one with a size of ∼30 μm. The underlying mechanism for the refinement involves discontinuous coarsening reaction during CHT.

Characteristics of the continuous coarsening and discontinuous coarsening of spinodally decomposed Cu-Ni-Fe alloy

The spinodal decomposition and coarsening reaction of a 45Cu-30Ni-25Fe alloy aged with and without prior 75% cold rolling have been studied by Vickers hardness tester, X-ray diffraction, optical microscope, and scanning and transmission electron microscopes. Spinodal decomposition took place in the alloys aged in the temperature range of 600–950°C without prior deformation, and then spinodal structure in these alloys would coarsen only continuously. Spinodal decomposition and continuous coarsening reaction of spinodal structure took place in the alloys aged at lower temperature with prior deformation. This process was corresponding to recovery of spinodal structure in the deformed alloys. Discontinuous coarsening reaction of spinodal structure would also take place in the alloys aged at higher temperature with prior deformation. This process was corresponding to recrystallization of spinodal structure in the deformed alloys.

A Comparative Study of the Kinetics of Interface Diffusion Controlled Transformations In Fe-Zn Alloys

A comparative study of the kinetics of discontinuous precipitation, coarsening and dissolution, diffusion induced grain boundary migration and grain growth has been carried out in Fe-Zn alloys. Except for cells of the discontinuous coarsening reaction, rates of migration of the cell boundaries of precipitation and dissolution and diffusion induced grain boundary migration (DIGM) rates lie within an order of magnitude of each other. The rate of cell boundary migration during discontinuous coarsening is two orders of magnitude smaller than those for the four transformation processes mentioned above. The kinetics (of minimum rate) of grain growth which occurs after long annealing times is comparable to the discontinuous precipitation and coarsening in the Fe-Zn alloy. The kinetics (of maximum rate) of grain growth, which corresponds to short annealing times, are comparable to the discontinuous dissolution and DIGM. The diffusivity values lie within an order of magnitude of the boundary diffusivity during grain growth when considered at the same temperature (extrapolated). Although the driving force differs by four orders of magnitude from as low as 0.08 Jmol−1 for grain growth to 1420 Jmol−1 for DIGM at 923 °K, the small difference in the diffusivity (kDbδ) predicts that the conduit for solute transport is the same for the above transformations.On a effectué une étude comparative de la cinétique de précipitation discontinue, de grossissement et de dissolution, de migration de joints de grain induite par diffusion (DIGM) et de croissance de grain, d'alliages de Fe-Zn. À l'exception des cellules de réaction de grossissement discontinu, le taux de migration des joints de cellules de précipitation, de dissolution ainsi que le taux de migration de joints de grain induite par diffusion se situent à un ordre de grandeur l'un de l'autre. Le taux de migration du joint de la cellule lors du grossissement discontinu est de deux ordres de grandeur plus petit que celui des quatre procédés de transformation mentionnés ci-dessus. La cinétique (taux minimum) de croissance de grain qui se produit après une longue durée de recuit est comparable à la précipitation discontinue et au grossissement de l'alliage Fe-Zn. La cinétique (taux maximum) de croissance de grain qui correspond à de courtes périodes de recuit est comparable à la dissolution discontinue et à “DIGM”. Les valeurs de diffusivité se situent à un ordre de grandeur de la diffusivité du joint lors de la croissance de grain lorsque considérée à la même température (extrapolée). Bien que la force directrice diffère par quatre ordres de grandeur, allant de 0.08 Jmol−1 pour la croissance de grain jusqu'à 1420 Jmol−1 à 923 °K pour “DGIM”, la petite différence de diffusivité (kDbδ) prédit que le conduit pour le transport de soluté est le même pour les transformations ci-dessus.

Microanalytical study of the discontinuous precipitation reaction in a Co–13 at.% Al alloy

High spatial resolution energy dispersive X-ray (EDX) microanalysis has been used to determine the changes of the Al concentration accompanying the formation of α and β lamellae during discontinuous precipitation (DP) and discontinuous coarsening (DC) reactions in a Co–13 at.% Al alloy. The detected step-like change of the Al across the reaction front (RF) has confirmed that the solute redistribution during DP and DC is essentially limited to the RF. The values of the grain boundary diffusivity triple product, sδDb determined from the profiles of the Al concentration across the α lamellae yielded the convincing evidence that the DP reaction in the Co–13 at.% Al alloy is governed by the grain boundary diffusion along the moving reaction front. The EDX analysis has also shown that the Al profiles within the single α lamella can vary with increasing distance from the reaction front. This was caused either by the variation of the α lamella thickness preceding re-nucleation of the γ lamella or by the stop-and-go motion of the cell boundary. Two kinds of discontinuous coarsening (denoted as DC I and DC II) occurring simultaneously after termination of DP reaction have been identified. They differ from each other not only by the α lamellae thickness but also the residual saturation in Al content which suggests the various surface and chemical contributions to the total driving force of DC I and DC II.

Discontinuous coarsening of discontinuous precipitates in a Co–6 at.% Mo alloy

The morphology and growth kinetics of the discontinuous coarsening of discontinuous precipitates in a Co–6 at.% Mo alloy have been investigated at temperatures ranging from 773 to 998 K by optical and scanning electron microscopy and X-ray diffraction. At all aging temperatures the alloy was observed to decompose completely by discontinuous precipitation into a fine lamellar structure of b.c.t. Co rich solid solution and an intermetallic compound phase of Co 3Mo. This lamellar structure was then decomposed at all aging temperatures by a discontinuous coarsening reaction. This reaction occurred at a much slower rate than the primary discontinuous precipitation reaction and resulted in a much coarser lamellar structure of the same phases. Through the measurements of the growth rates and lamellar spacings for both the discontinuous precipitation and discontinuous coarsening reactions, the grain boundary diffusion coefficients ( S δ D b) were analyzed using the kinetic model of Petermann and Hornbogen. By comparison with the diffusion data in literature, it was concluded that the reactions were controlled by grain boundary diffusion in the reaction fronts of the discontinuous precipitates.

Discontinuous coarsening of primary [formula omitted] lamellae in surface layer of the Ti 58Al 42 intermetallic alloy

Some experimental description of discontinuous coarsening (DC) reaction observed in various ({alpha}{sub 2} + {gamma}) two-phase TiAl-based alloys can be found in previous papers. This paper aims at examining the DC reaction in the surface layer of the alloy, and investigating the reaction by thermodynamic analysis. The cells in small size (about 30--50{micro}m) with coarse {alpha}{sub 2}/{gamma} lamellae due to the discontinuous coarsening nucleate randomly within the fine primary lamellae of the alloy surface and extend into a limited thickness of the surface layer for the Ti{sub 58}Al{sub 42} alloy ({alpha}{sub 2}{sup ss}) treated at 1,000 C. The elastic strain energy in the surface layer of the specimen has great effect on the discontinuous coarsening reaction. With the further prolonging of time, the extension of DC cellular microstructures with coarse {alpha}{sub 2}/{gamma} lamellae into the interior of the specimen becomes extremely difficult, and the discontinuous coarsening reaction nearly stops.

フルラメラ組織を有するTiAl合金の不連続粗大化反応を利用した粒径制御

Discontinuous coarsening reaction have been used to obtain the microstructure of fine-grained lamellae in a Ti-46 mol%Al alloy, which is expected to improve ductility at room temperature. The process was very simple because the structure change was performed only by aging subsequent to quenching. The refinement mechanism was investigated by optical and scanning electron microscopy.The nuclei of discontinuous coarsening reaction were coarsely spaced lamellae which were generated at grain boundaries and inside grains of finely spaced lamellae. The nucleation of coarsely spaced lamellae during aging increased with increasing cooling rates from the liquid phase and the alpha single phase. When the aging temperature was below 1473 K, the discontinuous coarsening reaction occurred preferentially and the microstructure was refined. The refinement by this process was achieved via the following steps: (1) nucleation of coarsely spaced lamellae, (2) changing the growth direction and (3) suppression of the grain growth. Thermal stability of the fine-grained microstructure obtained by this process was quite high.

不連続粗大化反応により微細粒化させたフルラメラTiAlの力学的特性

不連続粗大化反応により微細粒化させたフルラメラTiAlの力学的特性

Kinetics of discontinuous coarsening of cellular precipitate in a Ni-8 at.% in alloy

The morphology and growth kinetics of the cellular precipitate and products of two discontinuous coarsening reactions have been studied in the temperature range 710–1025 K. The NiIn alloy was observed to decompose completely by cellular precipitation reaction into a lamellar structure consisting of alternate lamellae of α and φ 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 were also observed to decompose the primary cells. The primary cell growth data were analysed by using theories of Cahn, Hillert, Turnbull, and, Petermann and Hornbogen. The secondary and tertiary cell growth data were analysed by using theories of Livingston and Cahn and Petermann and Hornbogen. The driving force available for the transformation was calculated from the regular solution model. From the diffusivity values, it has been concluded that the growth of primary, secondary and tertiary cells occurs by diffusion of indium along the migrating grain boundaries.

The discontinuous coarsening reactions in the PbCd and the ZnCd lamellar eutectic alloys-I. The driving force, initiation and the morphology

The morphologies of the discontinuous coarsening of the PbCd and the ZnCd lamellar eutectic alloys have been investigated at temperatures ranging from 353 to 483 K by scanning electron microscopy. Two types of coarsened cell morphology were observed and the terminology used by Frebel and Duddek [ Mater. Sci. Engng 32, 17 (1978)], namely, Type I and Type II has been adopted to describe them. A simple geometrical model is suggested for the Type II cell growth. The driving forces and the initiation of the reaction are discussed and the effects of lamellar faults and mismatch surfaces in the morphology of the secondary cells is described. The manner in which the microstructural features develop during the discontinuous coarsening reactions has been analysed and compared with the existing data.

The discontinuous coarsening reactions in the PbCd and the ZnCd lamellar eutectic alloys—II. Growth kinetics

The growth kinetics of the discontinuous coarsening reactions in the PbCd and the ZnCd lamellar eutectic alloys have been studied at temperatures ranging from 353 to 489 K by scanning and transmission electron microscopy. In the PbCd system discontinuous coarsening were observed at all ageing temperatures whereas in the ZnCd system it was only observed between 396 and 433 K. The lamellar spacing of the discontinuous cells in the PbCd system were found to increase with increasing temperature, pass through a maximum and then decrease. The kinetics of the reaction are compared with the existing data. The analysis of the growth kinetics suggests that the reaction is controlled by grain boundary diffusion. The reported data on discontinuous coarsening are analysed by using Livingston and Cahn's [ Acta metall. 22, 495 (1974)] and Fournelle's [ Acta metall. 27, 1147 (1979)] models. The applicability of the models to different systems is discussed.

Discontinuous coarsening of high perfection lamellae in titanium aluminides

A discontinuous coarsening reaction, involving grain boundary migration, was observed to occur in an as-cast Ti-43at%Al alloy having a high perfection of α 2 + γ lamellae with average spacing of 0.10 μm. The high perfection lamellae were observed to grow continuously across the prior grain boundary into the adjacent grain, resulting in secondary lamellae with coarser interlamellar spacings. The critical temperature for the initiation of discontinuous coarsening was found to be 900°C for the as-cast alloy. Both regular and degenerate types of secondary lamellae were observed, with the initial stage of coarsening reaction generally starting with regular lamellae. The interlamellar spacings of the regular secondary lamellae are more than four times those of primary lamellae.

Kinetics of cellular precipitation and discontinuous coarsening of the cellular precipitate in CuSb alloys

The morphology and growth kinetics of the cellular precipitate and discontinuously coarsened cellular precipitate have been studied in the temperature range 525–648 K by utilizing optical microscopy and X-ray diffraction. The CuSb alloys were observed to decompose completely by cellular precipitation reaction into a lamellar structure consisting of α and ϵ 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 reaction. Lattice parameter measurements indicated that, whereas the depleted matrix was richer in solute than the equilibrium solvus composition during the primary reaction, it was very close to the equilibrium solvus during the secondary reaction. Analysis of the growth kinetics of both the primary and secondary cellular reaction indicated that the transformations are controlled by diffusion of Sb through the cell boundaries.