The dendritic growth of γ′ precipitates and grain

The growth pattern of γ precipitates in the grains and at the grain boundaries has been investigated in a Ni-24Co-4Al-4Ti-5Cr-5Mo (weight percent) alloy of very small lattice misfit between the precipitate and the matrix phases under varying heat-treatment conditions. When aged at temperatures lower than the solvus temperature (T _s = 1150 °C) by more than 30 °C after direct cooling from the solution-treatment temperature, the nucleation density is high. In this condition, the supersaturation is quickly removed because of the overlapping diffusion fields and the precipitates undergo Ostwald ripening from the early stage. The precipitates then have an equilibrium shape of spheres in the grains and truncated spheres at nearly straight grain boundaries. The precipitates at the grain boundaries are coherent with one of the grains, and their number density is not much larger than that in the grains, apparently because of a large contact angle (about 150 deg) with the grain boundary. Quenching the alloy after the solution treatment and aging at any temperature also produce high precipitate number density and equilibrium shapes. When aged at temperatures just belowT _s (above 1140 °C), the nucleation density is low, the precipitates grow dendritically in the grains, and the grain boundaries become serrated. The observed dendritic growth characteristics do not quantitatively agree with the predictions of Mullins and Sekerka theory, but the discrepancy may be due to the uncertainties in both the observed and calculated quantities. By deeply etching the matrix, it is shown that the grain boundary serration is produced by the precipitates growing preferentially in the direction of the incoherent boundary because of the rapid solute diffusion along the grain boundary. The dendritic growth and grain boundary serration can be obtained also by slowly cooling through the temperature range just belowT _s.     

Morphology of NiFe2O4 precipitation in NiO

The morphology of NiFe₂O₄ precipitates in a NiO matrix has been studied using TEM. The spinel precipitates had a dendritic morphology when a polycrystalline sample of Fe-doped NiO was cooled at a constant rate from above the solvus temperature. They nucleate as octahedra bounded by {111} interfaces which then grow by a ledge mechanism until they reach a critical size and this shape becomes unstable. The precipitates then adopt a dendritic morphology characterized by branches in the 〈001〉 directions. The branches are bounded by {111} and {011} facets. HREM showed that during the initial stages of growth the interface is composed of a series of closely spaced ledges which are typically one spinel lattice-spacing high. Later, the ledge spacing decreases resulting in large flat {111} and {011} facets. Secondary branches were observed to form under large supersaturations of Fe when the precipitate spacing was large. The shape of the precipitates during the early stages of dendritic growth are very similar to those predicted by a Monte Carlo simulation which incorporates diffusional growth and anisotropic interface kinetics. The dendritic morphology is associated with the small lattice misfit, the large diffusion rate in Fe-doped NiO and the large undercooling necessary for nucleation. The diffusion rate is particularly large due to the concentration of cation vacancies being approximately half of the iron concentration.     

Precipitation on low-angle grain boundaries in Al-Zn-Mg

Weak beam microscopy was used to examine the precipitation of the 17 phase (MgZn₂) on low angle grain boundaries which had formed in an Al-2.84 wt pct Zn-1.95 wt pct Mg alloy. The low angle boundaries observed in the partially recovered structure can be separated into two categories, planar and nonplanar. The planar boundaries form either lozenge or hexagonal configurations. The nonplanar boundary dislocation intersections form a “chair”-shaped figure. The geometry of the dislocation strain fields in the boundaries controls the precipitate growth and the size, shape, and position of the precipitates on each boundary type. The lozenge boundary can have both equiaxed and lath shaped precipitates growing upon it. The equiaxed precipitates form only at four dislocation junctions because at this position the strain field distribution is symmetric. The lath precipitates are restricted to grow along dislocations in the boundary whose line direction is coincident with the long axis of the precipitate since in these pinned dislocation segments there are high line tension forces present to resist dislocation bowing. The “chair” boundary contains only lath precipitates, which begin to grow from alternate three dislocation intersections and continue growth along the dislocation line. The contrast observed bordering the lath on each side is produced by lattice dislocations. A proposed explanation for the above observations involves assuming that the dislocations in the chair boundary dissociate into Lomer-Cottrell locks which have constricted-extended node pairs. A lath can begin growth from the constricted node forcing the stair rod dislocation to dissociate. The reaction products of the stair rod can combine with the partials in the Lomer-Cottrell lock to form the observed lattice 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.     

The elastic strain energy of growth ledges on coherent and partially coherent precipitates

The formation rate of growth ledges on a faceted precipitate strongly affects the growth kinetics and the shape of the precipitate. An Eshelby-type model is used to compare the strain energy associated with the nucleation of a ledge on different facet planes of a body-centered cubic (bcc) precipitate in face-centered cubic (fcc) matrix. Ledge nucleation is only likely at facet areas where the interaction energy between the ledge and the precipitate is negative. The strain energy for ledge formation is not symmetric on any of the facet planes, but it is symmetric about the center of the precipitate. For coherent precipitates comparable to those observed in the Ni-Cr system, ledges form with the lowest strain energy on the broad facet of the precipitate implying that precipitate thickening should occur faster than lengthening and widening. A procedure for modifying the Eshelby model is suggested in order to allow strain-energy calculations of partially coherent precipitates. The strain energy for ledge formation on at least one type of partially coherent lath is lowest for a ledge located on the facet perpendicular to the crystallographic invariant line (IL). This situation favors precipitate lengthening in the invariant line direction.     

Interphase boundary structure with irrational orientation relationship formed in grain boundary precipitation

Interphase boundary structure of bcc precipitates formed at fcc-matrix grain boundaries in a Ni-43 mass pct Cr alloy has been studied experimentally using transmission electron microscopy (TEM) and theoretically analyzed using a geometrical model (near-coincidence site (NCS) lattice model). On both sides of the grain boundary, regardless of the presence of near-rational orientation relationship Kurdjumov-Sachs(K-S) or Nishiyama-Wassermann(N-W), precipitate(bcc)/matrix(fcc) interphase boundaries exhibit planar facets that often contain ledges and regularly aligned, line defects. The analysis indicates that the facet planes correspond to the planes of higher NCS densities for either of the near-rational orientation relationships or the irrational orientation relationships. Similar results are obtained for the precipitate/matrix interface, which lost its original orientation relationship through matrix recrystallization. It is suggested that some partial coherency is expected even for interfaces with an irrational orientation relationship formed during grain boundary precipitation. 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.     

Crystallography of grain boundary proeutectoid ferrite

A study has been made of the crystallography of proeutectoid ferrite precipitated at high angle austenite grain boundaries in an Fe-0.47 pct C alloy, isothermally transformed above the eutectoid temperature. Using the Kossel X-ray microdiffraction technique, ferrite orientations have been determined in relation to the orientations of both matrix grains at the grain boundary; the austenite orientations were derived indirectly. It has been observed that the ferrite, irrespective of morphology, possessed an orientation relationship with respect to at least one matrix grain which approximated to the Kurdjumov-Sachs and, to a lesser extent, the Nishiyama relationships. At several of the boundaries the ferrite was allowed to possess an orientation relationship with both matrix grains and it has been shown that the ferrite orientation at these boundaries was often influenced by the orientation of both matrix grains. Several instances have been observed in which preferential growth occurred into the austenite grain with which the precipitate did not have a specific orientation relationship. The results have been compared with the work of Ryder, Pitsch and Mehl,5 and factors governing the observation of Widmanstatten sideplates have been discussed.     

Crystallography of grain boundary proeutectoid ferrite

A study has been made of the crystallography of proeutectoid ferrite precipitated at high angle austenite grain boundaries in an Fe-0.47 pct C alloy, isothermally transformed above the eutectoid temperature. Using the Kossel X-ray microdiffraction technique, ferrite orientations have been determined in relation to the orientations of both matrix grains at the grain boundary; the austenite orientations were derived indirectly. It has been observed that the ferrite, irrespective of morphology, possessed an orientation relationship with respect to at least one matrix grain which approximated to the Kurdjumov-Sachs and, to a lesser extent, the Nishiyama relationships. At several of the boundaries the ferrite was allowed to possess an orientation relationship with both matrix grains and it has been shown that the ferrite orientation at these boundaries was often influenced by the orientation of both matrix grains. Several instances have been observed in which preferential growth occurred into the austenite grain with which the precipitate did not have a specific orientation relationship. The results have been compared with the work of Ryder, Pitsch and Mehl,5 and factors governing the observation of Widmanstatten sideplates have been discussed.     

Crystallographic and mechanistic aspects of growth by shear and by diffusional processes

Growth by shear and by diffusional processes, both taking place predominantly by means of ledge mechanisms, are reviewed for the purpose of distinguishing critically between them at the atomic, microscopic, and macroscopic levels. At the atomic level, diffusional growth is described as individual, poorly coordinated, thermally activated jumps occurring in the manner of biased random walk, whereas growth by shear is taken to be tightly coordinated “glide” of atoms to sites in the product phase which are “predestined” to within the radius of a shuffle. Obedience to the invariant plane strain (IPS) surface relief effect and the transformation crystallography prescribed by the phenomenological theory of martensite is shown to be an unsatisfactory means of distinguishing between these two fundamentally different atomic growth mechanisms. In substitutional alloys, continuous differences in compositionand in long-range order (LRO) from the earliest stages of growth onward are concluded to be the most useful phenomenological approach to achieving differentiation. At a more fundamental level, however, the details of interphase boundary structure are the primary determinant of the operative mechanism (when the driving force for growth is sufficient to permit either to occur). In the presence of a stacking sequence change across the boundary, terraces of ledges are immobile irrespective of their structural details during diffusional growth. Kinks on the risers of superledges are probably the primary sites for diffusional transfer of atoms across interphase boundaries. In martensitic transformations, on the other hand, terraces containing edge dislocations in glide orientation or pure screw dislocations are mobile and accomplish the lattice invariant deformation (LID), though probably only after being overrun by a transformation dislocation. Risers associated with transformation dislocations are also mobile and cause the crystal structure change during growth by shear. The successes achieved by the invariant line (IL) component of the phenomenological theory of martensite in predicting precipitate needle growth directions and precipitate plate habit planes (Dahmen and co-workers) are here ascribed to the rate of ledge formation usually being a minimum at an interface containing an IL, primarily because nuclei formed sympathetically at this boundary orientation are likely to have the highest edge energies. Since martensite plate broad faces also contain the IL, the ability of the phenomenological theory to predict the habit plane and the orientation relationships of both precipitate and martensite plates is no longer surprising. The IPS relief effect at a free surface can be generated by precipitate plates when growth ledges are generated predominantly on only one broad face and only one of several crystallographically equivalent Burgers vectors of growth ledges is operative. Both pReferences probably result from larger reductions in transformation strain energy for the particular geometry with which a given plate intercepts the free surface. Precipitate morphology often differs significantly from that of martensite even if precipitates are plate-shaped and can readily differ very greatly. Whereas martensite morphology is determined by the need to minimize shear strain energy, that of precipitates derives from the more flexible base of the interphase boundary orientation-dependence of the reciprocal of the average intergrowth ledge spacing, as modified by both the orientation-dependence of interkink spacing on growth ledge risers and the spacing/ height ratio dependence of diffusion field overlap upon growth kinetics. 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.     

The role of ledges in the proeutectoid ferrite and proeutectoid cementite reactions in steel

The influence of interphase boundary ledges on the growth and morphology of proeutectoid ferrite and proeutectoid cementite precipitates in steel is examined. After reviewing current theoretical treatments of growth by the ledge mechanism, investigations that clearly document the presence and motion of ledges with thermionic emission electron microscopy (THEEM) and transmission electron microscopy (TEM) are reviewed. A fundamental distinction is made between two types of ledges: (1) mobile growth ledges whose lateral migration displaces the inter-phase boundary and (2) misfit-compensating structural ledges. Both types of ledges strongly affect the apparent habit plane and aspect ratio of precipitate plates. Agreement between measured growth rates of proeutectoid ferrite and cementite (plates and allotriomorphs) and predicted growth kinetics assuming volume diffusion-controlled migration of ledge-free disordered boundaries is shown to be consistently poor. Physically realistic growth models should incorporate the ledge mechanism. More accurate comparisons of the growth models with experimental data will need to account for observed ledge heights, interledge spacings, and ledge velocities. In this vein, the sluggish growth kinetics of cementite allotriomorphs observed in an Fe-C alloy are shown to be quantitatively consistent with a strong increase in interledge spacing with time. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

The effect of undercooling on the cellular precipitation reaction in Cu-3 Pct Ti

The effect of undercooling on the morphology of the cellular precipitation reaction in Cu-3 Pct Ti is examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and serial sectioning experiments. The reaction front formed at small undercooling, which exhibits strong faceting of the precipitate growth interfaces, gradually changes with increasing undercooling to a smoothly curved reaction front with concave precipitate growth interfaces and convex grain boundary segments. This concealment of the faceted reaction front appears to be due to the rapid accumulation of growth ledges with increasing undercooling. This study also indicates that the cellular precipitation reaction, at small undercooling, is initiated by Widmanstätten precipitation. At larger undercoolings, a second mechanism is responsible for cellular genesis. Finally, contrary to accepted models of colony development, serial sectioning experiments show that nucleation of additional lamellae may occur at the faces of existing lamellae, from where they extend laterally to achieve the characteristic interlamellar spacing for that temperature.     

高强IF钢第二相粒子的应变诱导析出行为

在 Gleeble- 3500型热模拟试验机上,利用应力松弛试验研究了含磷高强IF钢第二相粒子的析出行为。结果表明,微合金元素析出钉扎住了位错与晶界,导致应力松弛曲线呈现出3个阶段的特征。试验用钢的PTT曲线呈现典型的“C”曲线形状,最快析出的鼻子点温度约为850 ℃,在此温度下,第二相粒子析出开始时间与结束时间分别为14和246 s,随着弛豫时间的增加,第二相粒子的析出数量逐渐增多,形貌逐渐粗化,析出粒子为Ti4C2S2 和TiC,主要呈圆形颗粒形貌。由于试验钢种采用磷强化,在析出的第二相粒子中存在棒形和长条形FeTiP,同时由于磷的晶界偏聚,所以FeTiP同样存在晶界析出的规律。     

The influence of applied stress and stress sense on grain boundary precipitate morphology in a nickel-base superalloy during creep

Creep induced instability of strengthening precipitates at grain boundaries is of general concern in the applications of many high temperature alloys. Having shown that the general validity of the existing mechanism for such an instability in nickel-base superalloys may be considered suspect, this paper reports and discusses the effects of both tensile and compressive creep on γ′ grain boundary precipitate morphology in an alloy consisting of γ′ (Ni₃Al) precipitates in a γ (nickel solid solution) matrix. We find that the uniform distribution of γ′ precipitates is altered by the application of uniaxial creep stress, with the stress-induced precipitate morphology depending strongly on stress sense. Tensile creep results in the dissolution of γ′ precipitates at grain boundaries aligned more or less transverse to the stress axis, with an accompanying increase in volume fraction of γ′ precipitates at grain boundaries oriented parallel to, or almost parallel to the stress axis. In contrast, the reverse change in morphology occurs during compressive creep. The observed morphology changes and their dependence on stress sense are shown to be consistent with the flow of chromium atoms from grain boundaries that are under normal compression towards grain boundaries that are under normal tension. The results conclusively demonstrate that Herring-Nabarro type diffusion in multiphase, polycrystalline alloys can cause chemical changes in grain boundary regions which, in the extreme, result in phase changes at grain boundaries. The results and proposed mechanism are discussed in terms of the findings of other investigations.     

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.     

On the rotation of precipitate particles

An energetically unfavourable situation can develop when coherent particles are bypassed by migrating grain boundaries. This is due to the precipitates being exposed to incoherent interfaces in the new matrix. In this work, the rotation of precipitate particles to low energy, coherent orientations in the new matrix is shown to be one of several possible responses to this situation. A physical and kinetic model for the rotation is put forward and the results of calculations of rotation rate presented. It is shown that particle rotation is controlled by interfacial diffusion and depends upon alloy composition, time, temperature and particle size and shape. The possibility of particle rotation occurring during particle/boundary contact is also discussed. This is shown to depend upon the nature of the boundary type. Generally, boundaries moving under large driving pressures with high velocities (e.g. phase transformation interfaces), are less likely to allow rotation during particle/boundary contact than less mobile boundaries such as those in grain growth and recrystallization. Experimental results from SAD and TEM of a Ti stabilized austenitic stainless steel containing a dispersion of coherent TiC precipitates is also presented and supports the particle rotation model.     

Evolution of Grain Boundary Precipitates in an Al-Cu-Li Alloy During Aging

The grain boundary microstructure of Al-Cu-Li alloy AA2050 was investigated for different isothermal aging times to rationalize intergranular corrosion (IGC) characteristics. In the underaged condition, the dominant grain boundary precipitates are fine T₁ (Al₂CuLi). Extended aging revealed that grain boundaries were decorated by large T₁ precipitates and S′ phase (Al₂CuMg), with S′ growth not dimensionally constrained. Such a transition in the precipitate type at grain boundaries is a unique feature of the Al-Cu-Li system.     

析出相对Monel K-500合金冲击韧性的影响

通过一系列实验研究了Monel K-500合金的冲击韧性。结果表明,合金经550~650℃时效后其冲击断口呈沿晶形貌;在750~850℃时效时,其冲击韧性与强度呈正相关关系。根据对合金在不同热处理状态的析出相形态及分布的研究,揭示了形成这些现象的原因。合金在550~650℃时效时,在晶内析出的γ’相呈球状均匀分布,而在部分晶界附近的γ’相呈条棒状或排成帘状垂直于晶界分布,由此导致时效态合金在断裂时,裂纹沿晶界扩展。在750~850℃时效会导致合金形成大量沿晶界网状分布的二次MC相,从而降低合金的冲击韧性。     

Pinning of Austenite Grain Boundaries by

The growth behavior of austenite grains in the presence of A1N precipitates varies with the temperature and time of anneal. To study this behavior, two iron alloys, (in weight percent) a 0.1 carbon base chemistry with 0.03A1/0.01N and 0.09A1/0.04N, respectively, were annealed between 1000 °C and 1200 °C for times of up to 180 minutes. Using optical microscopy, as many as 1000 austenite grains per heat-treatment condition were measured. Conditions of sup- pressed, abnormal, and uniform grain growth were observed. Using an extraction replica tech- nique, the size, shape, and distribution of the A1N particles were determined using transmission electron microscopy (TEM). The largest grain boundary curvatures calculated, using the Hellman- Hillert pinning model, were in close agreement with independent calculations of curvatures using the grain size data. The largest grains in the lognormal size distribution of austenite grains were found to be the ones with the potential to grow to abnormally large sizes.     

镁在高温合金中的晶界效应和作用机制

偏聚到晶界的镁原子可将原子错配度较小的其它溶质原子从晶界驱逐到晶内点阵或晶界相中，并可增强间隙原子向晶界的偏聚；镁原子可进入晶界相的单胞中，从而促使晶界相球化，并降低其稳定性。     

The effect of heat input on the microstructure and properties of nickel aluminum bronze laser clad with a consumable of composition Cu-9.0Al-4.6Ni-3.9Fe-1.2Mn

The effect of heat input in the laser cladding of nickel aluminum bronze was investigated. Nickel aluminum bronze castings were clad with a consumable of the composition Cu-9.0Al-4.6Ni-3.9Fe-1.2Mn and exposed to a variety of heat inputs from 42.5 to 595 J/mm. At the lowest heat input, the deposit microstructure was almost entirely martensitic. Increases in heat input caused the amount of α to increase. Depending upon heat input, the α was present as grain boundary allotriomorphs, secondary Widmanstätten α sideplates, and intragranular Widmanstätten α precipitates. The reheated zones were of lower hardness and, at all heat inputs, consisted of a mixture of grain boundary allotriomorphs and Widmanstätten α and martensite. Laser cladding improved the corrosion- and cavitation-erosion resistance of the surfaces but reduced their ductility. The properties of the clad surfaces depended on heat input.