Solid State Phenomena Vols. 172-174

Abstract: A generic model for nucleation of oriented second-phase in alloys in the vicinity of cracksand dislocations, is considered. The model employs the Ginsburg-Landau approach, which accountsfor the elasticity of crystalline solid and the interaction of structure/composition with the elastic fieldin the vicinity of the defect and in the crystalline bulk. We examine the nature of the structural phasetransition and construct its phase diagram.

Abstract: Natural formation of the unusual bcc modification for Co was demonstrated to occur upon precipitation of Co in supersaturated Au₉₀Co₁₀ alloy, applying high resolution transmission electron microscopy. The bcc Co phase precipitates as thin (0.5 – 1 nm) plates along {100} habit planes, exhibiting the orientation relationship: {100}_Au,fcc//{100}_Co,bcc and <001>_Au,fcc//<011>_Co,bcc. Prolonged annealing induces a Bain type transformation from bcc Co into fcc Co. The precipitation kinetics of Co in supersaturated solid solutions of Au₉₀Co₁₀ was investigated by DSC upon isothermal annealing at temperatures in the range 567 K to 612 K and upon isochronal annealing at heating rates in the range 5 to 40 K·min^‑1. Results of the fitting of a modular model of transformation kinetics to, simultaneously, all isothermal DSC runs and to, simultaneously, all isochronal DSC runs, could be discussed in terms of the different extents of the effects of quenched‑in defects on isothermally and isochronally conducted transformations.

Abstract: The partitioning of alloying elements between a and b phases was measured at different steps of the isothermal transformation at 710 and 610°C in Ti 17 alloy using EDX analyzer in the TEM. In addition, the transformation crystallography was determined. No differences in substitution elements were observed for a same transformation temperature in the a phase, however the composition varied with the transformation temperature. For the partial transformed specimens, gradient in composition were obtained. Results are compared to calculated compositions using ThermoCalc software and Saunders database.

Abstract: Nickel sulphide (NiS) can form inclusions in tempered glass which lead to fracture due to a phase transformation with a volume change of about 4%. A heat treatment, aiming to provoke this phase transformation, is currently used in industry to reduce this effect. In order to propose more efficient treatments, a complete study going from identification of the transformation mechanisms to the modelling of the transformation was carried out. Depending on stoichiometry and temperature, two mechanisms for the transformation (partitioned or partitionless) have been evidenced by detailed microstructural and calorimetric studies leading to a fruitful parallel with the Fe-C phase transformations which provides the basis for further modelling of the kinetics. An in situ follow up of the transformation by optical microscopy has given information (like nucleation rate and interface migration velocity) necessary to build the kinetic models. This modelling is based on Zener and Zener-Hillert kinetics models for interface velocity and described the transformation under isothermal treatment and anisothermal conditions.

Abstract: For lightening the hot forged automotive components such as connecting rods, crank shafts etc. the increase in their yield strength is an important technical issue. Recent developments indicate that it is a promising way to increase the yield strength of the components using the ferrite-pearlite microstructure strengthened by precipitation hardening of VC. In this study, the influence of alloying elements, cooling rate and aging temperature on the precipitation hardening behavior of V containing middle carbon steels was investigated. The precipitation hardening is very sensitive to cooling rate and aging temperature. The addition of Si reduced the sensitivity of the cooling rate. The deformation in the austenite region slightly decreases the precipitation hardening. From a detailed analysis, it was found out that the precipitation hardening is strongly influenced by the γ→α transformation behavior, which indicates that the interphase precipitation plays a significant role for the precipitation hardening.

Abstract: Recent modifications in chemical composition have been applied commercially to high alloy tool steels, using different combinations of Cr, Si and Mo contents. Several reports have been published in the literature about the effects of such modifications on mechanical properties and tool performance, but only a few of these studies were concerned with the effects on secondary carbide formation. In previous papers, improvements in toughness and tempering resistance that were found in a 5% Cr tool steel (type H11 with lower Si contents) have been attributed to particular distributions of Cr-rich M₇C₃ particles. Although M₇C₃ carbides have been studied extensively in low alloy steels, some important differences have now been observed by the present authors for high alloy tool steels, especially regarding the effects of Si and Cr. The present work is concerned with the formation of Cr-rich M₇C₃ as well as Fe-rich M₃C particles in modified H11 tool steels, discussing the precipitation sequence and particle distributions developed during tempering within the martensite microstructure. By means of transmission electron microscopy, the effect of Si on M₃C cementite formation has been found to be responsible for a substantial change in the distribution of the M₇C₃ carbide phase, leading to a concentration of these particles at high energy interfaces in interlath and interpackage regions.

Abstract: Demands for medium carbon steels with high strength used for forging parts in automobile have been increasing. V addition to such steels leads to interphase boundary precipitation (IBP) of VC and thus an increase of strength. However, mechanism and strengthening effect of IBP have not been clarified in detail. In this study, precipitation of VC accompanying ferrite and pearlite transformations and its effect on hardness have been examined in medium carbon steels microalloyed with 0.3%V. Specimens transformed in a temperature range between 873 and 973K consist of pearlite and small amount of proeutectoid ferrite. Hardness increase by the V addition becomes larger by lowering transformation temperature at these temperatures. Meanwhile the alloying effect of V on the hardness remarkably decreases at 823K where bainite transformation takes place partly. TEM characterization has revealed that VC are precipitated in both of proeutectoid and pearlitic ferrites in the manner of fine rows parallel to the austenite / ferrite interphase boundary. The size of VC decreases and its number density increases by lowering transformation temperature, corresponding to the larger hardness increase. Orientation relationship analyses between ferrite and austenite in the V-added specimen based of EBSD measurements reveals that proeutectoid ferrite grows preferentially towards an austenite grain with which ferrite does not hold a specific orientation relationship, indicating that classical ledge mechanism does not play a role for interphase boundary precipitation of VC in this alloy.

Abstract: The use of martensitic stainless steels is commonly due to high mechanical properties requirements. To obtain these high values from the industrial material (whose microstructure consists in ferrite and M₂₃C₆ carbides), a suitable heat treatment, consisting in an austenitization of the steel at a temperature higher than A3 point, followed by a fast quenching, is necessary. For economic reasons, the shortest the heat treatment time, the better it will be. Therefore, one essential point, to reduce austenitization time, is to obtain a final product made of ferrite and carbides, with the lowest carbides size as possible: the lowest they will be, the shortest time the transformation ferrite + carbides --> austenite will take. The formation of these carbides occurs during the batch annealing of the steel, at low temperature. To study the influence of carbides size on the austenitization kinetics of a 1.4006 grade martensitic stainless steel, several batch annealings were made at different temperatures. Carbides sizes were measured by electronic microscopy and austenitization kinetics were measured by dilatometry. Small carbides size logically induces fastest austenitization kinetics. The austenization occurs in three stages: a fast one which corresponds to the dissolution of the smallest carbides leading to a homogeneous repartition of carbon, a chromium gradient into ferrite and thus an austenitization until reaching Cr-rich ferrite. The second one is limited by the diffusion of chromium, inducing a slower transformation. The apparent third stage is an artefact of the second one, as it corresponds to an expansion of the austenite due to the diffusion of carbide, and an apparent increase of the transformation kinetics.

Abstract: The precipitation process in an aged Cu-1.9wt%Ni-0.3wt%Be alloy has been examined by high-resolution transmission electron microscopy. The precipitation sequence found is: Guinier- Preston (G.P.) zones → γ'' → γ' → stable γ. The disk-shaped G.P. zones and the disk-shaped γ'', γ' and γ precipitated phases are composed of monolayers of Be atoms on {100}_α of the Cu matrix and alternative Be and Ni matrix layers parallel to {100}_α. The γ'' phases consisting of two to eight Be-layers has a body-centered tetragonal (bct) lattice with a=b=0.24 nm and c=0.28 nm. The γ' or γ phase is bct with a=b=0.24 nm and c=0.26 nm or a=b=0.26 nm and c=0.27 nm. The γ'', γ' or γ phase aligns with the matrix according to the Bain orientation relationship. The growth kinetics of disk-shaped γ precipitates on aging at 500°C has been also investigated. The {001}_α habit planes of the γ precipitates migrate by a ledge mechanism. The average thickness of the γ disks increases with aging time t as t^1/2. An analysis of experimental data using a kinetic model yields the diffusivity of solute in the Cu matrix, which is in agreement with the reported diffusivity of Ni in Cu.

Abstract: Three types of austenitic stainless steels JK2, JJ1 and JN1 were isothermally aged at temperatures from 873 to 1173 K for 10 to 1000 minutes in order to study the microstructural evolution. In general, the kinetics of precipitation for JN1 steel was faster than that of JJ1 steel, because of its higher interstitial solute content. The TTP diagrams showed that the intergranular precipitation of Cr₂₃C₆ and Cr₂N preceeded to the intragranular precipitation of Cr₂N and h phase in JN1 and JJ1 steels, respectively.