Texture evolution induced by strain path changes in low carbon steel sheets

Two-stage sequences of simple shear performed on low carbon steel sheets were supplemented by texture measurements. A detailed analysis of the texture evolution during the simple shear sequences was realized by studying the orientation distribution functions. The calculation of the orientation stability map (using the classical viscoplastic Taylor model) allows explanation of the observed texture development: the initial texture is shown to be partly stable and hence the measured textures differ from the usually reported ones. The influence of the texture evolution on the shape of the stress-strain curves, as well as on the remaining symmetries of the material, is also discussed. In particular, it is demonstrated that both intragranular and geometrical contributions have to be considered to explain the shape of the stress-strain curves after a strain path change.     

Effect of grain boundaries on isothermal solidification during transient liquid phase brazing

The influence of liquid penetration at grain boundary regions on the rate of advance of the solid-liquid interface during isothermal solidification of transient liquid phase (TLP) brazed nickel joints has been examined. The test samples used in this study were Ohno-cast nickel with a grain size of >4 mm and a fine-grained nickel with a grain size of around 40 μm. Both Ni-base materials had the same chemical composition. The rate of isothermal solidification was greater when fine-grained nickel was employed during TLP brazing using Ni-11 wt pct P filler metal at 1200 °C. Liquid penetration at grain boundaries accelerates the isothermal solidification process by increasing the effective solid-liquid interfacial area and increasing the rate of solute diffusion into the base material. An analysis of electron channeling patterns has confirmed that random high-angle boundaries have a greater influence on the rate of isothermal solidification than ordered boundaries including small-angle or twin boundaries. Formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto. Formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto     

回火温度对Q960钢小角度晶界的影响

设计了一种低合金高强度的Q960工程机械用钢.在同种成分、同种轧制和淬火工艺下,研究回火温度对Q960钢小角度晶界的影响机理.经分析得出,随着回火温度的升高,小角度晶界的频率逐渐降低且实验钢的抗拉强度和屈服强度也相应地降低,而屈强比逐渐升高.不同回火温度下小角度晶界频率的峰值均出现在取向差5°附近,即取向差较小.取向差小使得小角度晶界比较稳定,原子迁移率小.通过计算键级积分(BOI)得出,B元素对Q960钢的小角度晶界有加强作用.     

The embrittlement and de-embrittlement of grain boundaries in an Fe-Mn-Ni alloy due to grain boundary segregation of Mn

A ductile-brittle-ductile (DBD) transition behavior in an age-hardenable Fe-8Mn-7Ni alloy has been analyzed in light of segregation and desegregation of alloying elements at prior austenite grain boundaries. The DBD transition in the alloy can be distinguished by two C-type curves: one corresponding to the start of zero tensile elongation and the other to the finish. The activation energies for ductile-to-brittle and brittle-to-ductile transitions are in close agreement with that for age hardening. Manganese content at the prior austenite grain boundaries was analyzed by Auger electron spectroscopy, and intergranular fracture strength at the brittle fracture region showed inverse trends with Mn concentration at the grain boundaries. All these observations strongly suggest that manganese segregation and its desegregation are responsible for the DBD transition of this alloy. Formerly Graduate Student, Department of Metallurgical Engineering, Seoul National University     

Chromium depletion and martensite formation at grain boundaries in sensitised austenitic stainless steel

A microstructural and compositional investigation of grain boundary precipitation and martensite formation in sensitised 304 stainless steel has been conducted. Grain boundary depletion of chromium has been quantified in terms of sensitisation time, temperature and boundary type by energy dispersive X-ray microanalysis in the transmission electron microscope. Chromium depleted profiles measured in grain boundary vicinities are sometimes asymmetrical and correlate with the expected profiles generated by growth of semicoherent and incoherent carbide interfaces. The depletion of chromium promotes martensite formation within near-grain boundary regions and this transformation has been directly studied by in situ cold stage microscopy down to − 150°C. Transformation occurs at the most severely depleted boundaries and initiation is favoured at slip band-boundary intersection points and along grain boundaries whose plane orientation matches that of the martensite habit plane. The preferential formation of grain boundary martensite could be an important factor in the stress corrosion and environment sensitive failure of this material.     

Effects of changes in strain path on work hardening in cubic metals

Effects of abrupt changes in strain path on work hardening in stretching 1200 aluminum, OFHC copper, 70-30 brass, a low-carbon ferritic steel, and 310 and 304 austenitic steels have been investigated. Tests were made with first-stage extension in uniaxial and equibiaxial tension. Second-stage stretching was in uniaxial tension. It is shown that all of the above materials have some susceptibility to transient reductions in work-hardening rate,dσ/dε, after changes in strain path. Some of the material and process variables which can influence the form and magnitude of the transients are identified. Within the group of materials tested, stacking fault energy (SFE) appears to be the most generally influential material variable. The magnitudes of prestrain required to give significant reductions indσ/dε·1/σ after strain path changes are higher in the low SFE alloys. Changes in strain path are accompanied by a reduction in the effective extension at the first onset of strain localization, when the reduction indσ/dε is sufficient to causedσ/dε·1/σ to fall below unity. Within the ranges of prestrain explored, the maximum reductions indσ/dε·1/σ were in the range of 0.5 to 1.0. Thus, none of the changes in strain path investigated caused a reduction in the limit of effective uniform elongation, until the prestrain was sufficient to reducedσ/dε·1/σ in monotonic deformation to less than a value in the range of 1.5 to 2.0. For these reasons, the possible reductions in effective uniform elongation were much more severe in the high SFE materials than those in the low SFE alloys. S. PLATIAS, formerly Postgraduate Student, School of Metallurgy and Materials, University of Birmingham P.S. BATE, formerly Research Fellow, School of Metallurgy and Materials, University of Birmingham     

The effect of extrinsic grain boundary dislocations with unrelaxed and relaxed cores on the state of random boundaries in an austenitic steel

A transmission electron microscopic investigation employing in situ heating of thin foils has been carried out to determine the effect of extrinsic grain boundary dislocations (EGBDs) with unrelaxed as well as relaxed cores on the spreading temperature (T_d) of EGBDs on random grain boundaries in an austenitic stainless steel. The results show that while T_d is not measurably affected by the presence of unrelaxed EGBDs up to a density of a least 5.6 × 10⁷/m, its value is observed to decrease with increasing density of EGBDs with relaxed cores. These changes in T_d are discussed in terms of the possible changes in the structure and the energy of random grain boundaries which may be caused by the presence of these two types of EGBDs. Formerly with the Metallurgical Science Laboratory, The University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada,     

Dissociation of grain boundaries induced by changes of composition,the ejection of dislocations from grain boundaries,and the nucleation of diffusion induced grain boundary migration

We present various observations of the dissociation of large and small angle grain boundaries in copper during the alloying of the specimens with zinc. In all large angle cases, this dissociation takes the form of the large angle boundary reacting to form another large angle boundary and a small angle boundary. This process may be of great importance in understanding the nucleation of diffusion induced grain boundary migration. Small angle boundaries can undergo complete disintegration upon the addition of zinc, and the magnitude of the stress involved in such a disintegration is estimated.     

Effects of strain path changes on the formability of sheet metals

The effects of a change in strain path on the deformation characteristics of aluminum-killed steel and 2036-T4 aluminum sheets have been studied. These sheets were pre-strained various amounts in balanced biaxial tension and the resulting uniaxial proper-ties and forming limits for other loading paths were determined. In comparison to uni-axial prestrain the steel was found to suffer a more rapid loss in uniform strain upon the strain path change from biaxial to uniaxial. In contrast, the uniform strain in aluminum does not drop as rapidly after the same change. In keeping with this behavior, the form-ing limit diagram of steel is found to decrease with prestrain at a much faster rate than that of aluminum. Such effects can be explained in terms of the transition flow behavior of the metals occurring upon the path change. Thus, the path change produces strain soften-ing and premature failure in steel, while causing additional strain hardening and consequent flow stabilization in aluminum. AMIT K. GHOSH, formerly with General Motors Research Laboratories     

Diffusion in an ensemble of intersecting grain boundaries forming a triple junction

Grain boundary diffusion in an ensemble of three intersecting grain boundaries forming a triple junction is described in the framework of quasi-steady Fisher’s model. Two configurations, which differ in the number of grain boundaries adjacent to the surface with a diffusant source and in the tilt angle to the surface, are considered. Analytical expressions for the diffusant concentration distribution along each grain boundary that constitutes the triple junction and for the point of the triple junction are derived with the proviso of equal diffusion fluxes at the triple point. The expressions for the diffusant concentration distribution along the grain boundaries include not only diffusion constants (grain-boundary and bulk diffusion coefficients) but also structural characteristics of the ensemble of grain boundaries (the depth of the triple junction point and the angle between the grains forming the triple junction). It is shown that, if the coefficients of grain boundary diffusion are equal for all boundaries making the ensemble and for an equilibrium angle of 120° in a polycrystal, the diffusive mass transport rate in the triple junction zone is lower than that in a single grain boundary irrespective of the configuration of grain boundaries.     

On interactions amongst trace and alloying elements at the grain boundaries of a low alloy steel

Interactions amongst segregants at the grain boundaries of a low alloy steel have been studied using the technique of Auger electron spectroscopy in the temperature range of 750–900 K. Segregation isotherms delineate three kinetically distinct regimes. The first of these is characterized by rapid cosegregation of Cr and N with desegregation of C followed by segregation of V which occurs at relatively higher temperatures. The second stage involves rapid segregation of P which initially attains a maximum coverage and then subsequently decreases with time. The third regime noted at higher temperatures shows slow segregation of S, Sn and Sb. This regime, however, exhibits high sensitivity of site competition to temperature.     

Cavitation at grain and phase boundaries during superplastic flow of an aluminum bronze

Cavities have been observed to form at grain and phase boundaries under certain strain rate conditions during superplastic tensile deformation of a Cu-9.5 pct Al-4 pct Fe aluminum-bronze. The cavities form preferentially at α-β interfaces or triple junctions involving both phases. The process of cavitation is associated with grain boundary sliding and cavity nucleation probably occurs at points of stress concentration in the sliding interfaces. The ductility is not markedly impaired by the cavities because the high strain-rate sensitivity of the material inhibits the interlinkage of cavities at high strains. A range of strains and strain rates for superplastic forming processes has been determined at which the volume fraction of cavities present was tolerable.     

Roles of dislocations and grain boundaries in martensite nucleation

In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature (M_s) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations, work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation with decreasing grain size is the main factor to depress M_s in fine-grained specimens.     

The influence of grain junctions and boundaries on superplastic deformation

The crystallography of the grain boundary/grain junction network has been analysed in an aluminium—lithium alloy after 1100% superplastic deformation. The grain junctions were classified as “I-lines” or “U-lines” as defined by an extension to the O-lattice model. Overall there were 38% I-lines in the sample population which compares to a predicted 4% for random generation. It was shown that the grain orientation distribution alone was not responsible for the high proportion of I-lines. The occurrence of intergranular cavities in the microstructure was associated with high angle boundaries and U-lines. Cavities arrested at I-lines, which highlights the structural differences between the two junction types. With regard to the mechanisms of superplastic flow, it was argued that the two recognised competing mechanisms for grain accommodation, diffusion creep and dislocation creep, are associated with U- and I-lines respectively, which accounts for the observed proportions of these defects.     

Striation and faceting of grain boundaries in nickel due to sulfur and other elements

As is the case for free surfaces, the grain boundaries of metals may show striation and faceting as a result of the effect of certain elements. This is true for nickel containing sulfur, tellurium and bismuth, and for copper containing bismuth. In all of the cases, the phenomenon occurs only when the grain boundary concentration of the element concerned approaches saturation, and material transport is taking place towards or away from the grain boundaries. Thus, intergranular striation and faceting are telltale signs of a non-equilibrium segregation. Formerly Research Associate, Département de Métallurgie, Ecole Nationale Supérieure des Mines de Saint-Etienne Formerly Professor, Département de Métallurgie, Ecole Nationale Supérieure des Mines de Saint-Etienne.     

Low energy grain boundaries in silver and copper close to the melting point

Crystallographic parameters of low energy grain boundaries in silver and copper in the temperature range 0.8-0.996 T_m have been studied. The investigations have been carried out by sintering single crystalline spheres to single crystalline plates having surfaces parallel to low index planes. In the above temperature range the most important energy cusps were found to be associated with [110] tilt boundaries. This result indicates that the parallelity of close packed rows of atoms from both grains surfaces leads to low energy grain boundary structures. The distribution of misorientation angles for low energy [1 1 0] tilt boundaries is almost continuous at 0.8 T_m. At higher temperatures the spheres select some orientations as more preferred than others. This set of orientations is smaller than predicted according to recent geometrical criteria for low energy boundaries. It can be interpreted in terms of grain boundary faceting in such a way that it contains compact structural units of low energy. There seems to be no connection between the type of structural units in the grain boundary and the present crystallographic criteria. This is the reason why these criteria fail to select between nonsymmetrical boundaries of higher and lower energy.