Showing papers on "Grain boundary strengthening published in 1976"

The Effects of Strain on the Microstructures, Fabrics, and Deformation Mechanisms in Quartzites

Abstract: The oaxes fabrics and the intracrystalline microstructures of naturally deformed quartz indicate that dislocation deformation mechanisms are important natural deformation processes. Two mechanisms, dynamic recovery and cyclic dynamic recrystallization, can lead to steady state flow which is necessary if large strains are to be attained. Cold working followed by cataclasis can also produce a steady state flow, but is limited to low temperature, high stress environments. Available data indicates that the c-axes preferred orientations and the optical strain features develop progressively as the strain increases. Recrystallization occurs by the continual rotation of sub-grains or by the development of small strain free grains. The new grains develop preferentially in the most misorientated areas of the deformed host grains, especially along deformation bands and in grain mantles. They have c axes orientations similar to the range in orientation of the host grain in deformation bands and mantles Grain growth is inhibited if there is a rapid increase in the dislocation density in the new grains. Grain refinement then accompanies recrystallization and produces a quartz mylonite. The new grains may be subjected to further phases of deformation and grain refinement. Deformation maps for quartz show that Coble creep replaces dislocation creep as the main deformation process when the grain size is less than 100 pm. The change is accompanied by strain softening and the maps imply that fine grained quartz mylonites are superplastic.

The role of microstructure on the strength and toughness of fully pearlitic steels

Abstract: An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.

Grain boundary cavitation under various states of applied stress

Abstract: Submicrometre grain boundary cavities are produced in Nimonic 80A when plastic deformation in any of three different stress states is followed by a short anneal. Tension, torsion and compression specimens were plastically strained in a systematic manner and then annealed for 2 h at 750 °C. Detailed quantitative observations with a 1 MV microscope showed that the number of cavities per unit volume was a function of the shear strain and independent of the stress state. Furthermore the measurements revealed the surprising result that most cavities were on those grain boundaries which were parallel to the maximum principal stress axis. However, Preferential cavity growth occurred during subsequent tensile creep and cavities on these parallel boundaries either remained constant in size or diminished while those on boundaries which were orthogonal to the applied stress axis grew relatively quickly, thus producing the usual appearance of cavitated tensile samples. Plastic strain was more detrimental to torsional creep ductility when the direction of torque between plastic deformation and creep was reversed which is in accordance with the anisotropic cavitated boundary distribution. The results are compatible with the hypothesis that cavities are produced by grain interior slip and stabilized by plastic deformation induced internal tensile stresses.

Grain Size Effects in Hydrogen-Assisted Cracking

Abstract: There is conflicting evidence in the literature with respect to the effect of grain size on hydrogen embrittlement. Differences may arise because of the degree of segregation in different grain size materials, because of different structures obtained in the effort to produce varying grain sizes, or because of the grain-size dependency of diffusion and growth processes. An extremely dirty heat of 4340 steel with 0.07 S and 0.015 P was investigated so that any tramp element segregation or hydrogen recombination poison effects would be present. Measurements were obtained on cathodically-charged samples with average grain sizes of 20, 50, 90 and 140 μm. In general, tramp element effects were not controlling. For those cases where the grain diameter was significantly larger than the plastic zone, increased grain size improved resistance. This was reflected by a slight increase in threshold stress intensity and an inverse grain-size squared dependence of crack velocity. Although the data are consistent with a pressure tensor hydrogen-assisted migration model, they could also be interpreted in terms of high austenitizing temperatures promoting retained austenite.

Segregation and the strength of grain boundaries

Abstract: A quantitative theory is presented which explains how solute segregation alters grain boundary strength. A simple treatment shows that, because of an incorrect assumption, previous theories in which segregation was thought to cause low temperature brittleness by lowering the grain boundary work of fracture are incorrect and that it is the brittle fracture stress that is altered. The decrease in grain boundary brittle fracture stress is shown to be proportional to the level of segregation and also to the excess size of the segregant atoms over those of the matrix. From the tem­perature dependence of the yield stress in a given alloy, it is then simple to calculate the impact transition temperature shift caused by the segrega­tion of various species. An evaluation for the temper brittle steel, AISI 3340, gives a good numerical correlation with the published transi­tion temperature shifts for the segregation of P, Sn, Sb, As, Si, Ge and Bi. The theory also explains why C, B and Be are remedial to the embrittle­ment of pure iron.

Mechanical Alloying of IN-738

Abstract: Mechanical alloying has been applied to produce a dispersion-strengthened superalloy IN-738 containing 1.5 wt pct Y2O3. Annealing of extrusion bars above the recrystallization temperature of 1160°C can be described by three stages of recrystallization:finegrain; isotropic coarse-grain; and fibrous coarse grain growth. A maximum grain length of 550 μm and a maximum grain aspect ratio of 4.8 have been obtained for an alloy, which had been extruded at 1100°C and annealed at 1280°C and 1270°C for 3 h, respectively. The three stages of grain growth are explained in terms of recovery, differences in nucleation rate and dispersoid concentration in the two normal directions and release in stored cold work. Secondary recrystallization can be excluded as a mechanism for fibrous grain coarsening. Dispersion-strengthened IN-738, heat treated to a coarse elongated grain structure, has both high intermediate temperature strength and high elevated temperature strength. The creep strength at 1000°C exceeds that of cast or directionally solidified IN-738 after 300 h service life. The failure mechanism at elevated temperature is intergranular fracture along transverse grain boundaries, nucleated by cavities that form during grain boundary sliding. Nucleation of voids is retarded in the creep specimens due to diffusional accommodation of grain boundary sliding. A depletion of surface zones of chromium, aluminum and titanium contributes to initiation of creep failure at 1000°C.

The effect of grain size on the shock-loading response of 304-type stainless steel

Abstract: The residual microstructure and mechanical response of shock-loaded stainless steel (AISI-304) of four different grain sizes—23, 55, 85 and 187 µm-was investigated. In addition to mechanical twinning and planar dislocation arrays, transformation to both ɛ and α martensite occurred in all shock-loaded specimens but became more extensive with decreasing grain size. In comparison to the Hall-Petch behavior of yield and early flow stress observed for the material after 5.2 pet cold rolling, the strengthening efficiency of shock loading decreased with increasing grain size. Shock loading enhanced the strain-induced transformation to α martensite during subsequent tensile deformation.

Some observations on grain boundaries in copper-bismuth alloys

Abstract: An electron microscope study of copper-bismuth alloys has shown a high incidence of faceting of the grain boundaries. This is not observed in pure copper, suggesting the bismuth has a large effect on the low energy configurations of the boundary plane.

The creation and accommodation of extrinsic dislocations at grain boundaries

Abstract: This paper considers the observation of extrinsic grain boundary dislocations in a commercial austenitic steel. It is shown that the stability of these non-equilibrium defects is a sensitive function of the precise thermo-mechanical history of the specimen material and is often controlled by the presence of active pinning centres in the grain boundary (e.g. solute and/or precipitate)

Effect of internal boundaries on the yield strengths of spheroidized steels

Abstract: The combined effects of cementite particles, grain boundaries, and subgrain boundaries on the room temperature yielding behavior of spheroidized, plain carbon steels were investigated. Spheroidization by austenitizing and quenching, followed by annealing at temperatures just under theA 1 temperature, produced a subgrain-connected cementite particle distribution. The subgrain size, λl,p, stabilized by the particles governs the yield stress via the relationσ y = 9.5 + 1.33 λl,p -1/2, kgf/mm2 In contrast, austenitizing and quenching, followed by thermal cycling about theA 1 temperature, produced microstructures with a large fraction of intraboundary, subgrain-free cementite particles. The lower yield stress of these steels could not be accounted for by either the Orowan or the Ansell-Lenel theory. The yield stress is predominantly controlled by the ferrite grain size, λg, via the relationσ y = 12.4 + 1.87 λg -1/2, kgf/mm2 The intraboundary particles contribute only a small strain-hardening term which increases the value of the friction stress (12.4 kgf/mm2) over that associated with grain boundary strengthening alone (8.8 ± 0.8 kgf/mm2).

Creep deformation of TD--nickel chromium

Abstract: The creep behavior of thoria dispersed nickel-chromium (TD-NiCr) was examined at 1093°C (2000°F). Major emphasis was placed on 1) the effects of the material and the test related variables (grain size, temperature, stress, strain and strain rate) on the deformation characteristics, and 2) the evaluation of single crystal TD-NiCr material produced by a directional recrystallization technique. Creep activation enthalpies were found to increase with increasing grain size reaching maximum values for the single crystal TD-NiCr. Stress exponent of the steady state creep rate was also significantly higher for the single crystal material as compared to that determined for the polycrystalline TD-NiCr. The elevated temperature deformation of TD-NiCr was analyzed in terms of two parallel-concurrent processes: 1) diffusion controlled grain boundary sliding and 2) dislocation motion. The characteristics of the dislocation motion deformation mode (as observed in the single crystal TD-NiCr) suggest that strong particle-dislocation interactions are present. The relative contributions of dislocation motion and grain boundary sliding in TD-NiCr were estimated. In creep, grain boundary sliding was found to predominate for the small, equiaxed grain structures, whereas the dislocation deformation mode became significant for only the large grain TD-NiCr and the single crystal material.

Grain-boundary migration in metals fatigued at high temperatures

Abstract: THE migration of grain boundaries in polycrystalline metals fatigued at high homologous temperatures has been reported for FCC, BCC and CPH metals1–4. Migration results in a ‘diamond’ or orthogonal configuration of boundaries with segments aligned in the maximum shear stress directions. It has been suggested that this migration contributes to grainboundary failure by the absorption of point defects at migrating boundaries5 and/or by the development of the diamond configuration of boundaries, which puts these boundaries in a favourable arrangement for sliding1–4. In contrast to the information on cavity growth during high temperature deformation, there seems to be little quantitative information on grain-boundary migration in these conditions. We report here the results of measurements of grain-boundary migration on Al, Cu, Zr and Zircaloy-2 specimens fatigued at temperatures where grain-boundary cavitation occurs. These results indicate that in the initial 10% of the fatigue life the rate of boundary migration decreases with time and the amount of migration is ∝time2/3. The possible influence of boundary migration on cavitation is suggested to be limited to the initial part of the fatigue life.

Quantitative aspects of precipitation at grain boundaries in an austenitic stainless steel

Abstract: This paper shows, in a quantitative manner, how the precipitation of niobium carbide in an austenitic stainless steel is affected by varying the amount of deformation prior to ageing. In particular, the extrinsic dislocation content of grain boundaries is shown to govern the overall size distribution of grain-boundary precipitates developed during ageing.

The role of grain‐boundary structure in shock‐induced spallation of molybdenum

Abstract: Grain boundaries and grain‐boundary ledge structures are shown to have a controlling influence on intergranular spallation in molybdenum resulting from shock‐wave rarefraction interactions. The structure of grain boundaries and their relationship to spall fracture are examined in detail utilizing the techniques of transmission and scanning electron microscopy, and are shown to be conducive to microcrack nucleation, coalescence, and propagation along the boundary planes. Grain boundaries possessing a high degree of complex structure such as dislocations and ledges are shown to vary significantly in effective dislocation density from the grain matrices, and this phenomenon coupled with the ability of ledges to nucleate cracks appears to account for intergranular spallation in molybdenum shock loaded to pressures of 150 and 250 kbar.

The role of the grain boundaries in hot pressing silicon carbide

Abstract: Silicon carbide is difficult to sinter without the aid of additives and/or pressure. Additions of boron carbide, boron or alumina were made to obtain dense materials. This is explained in terms of surface free energy modifications taking place in the grain boundaries. An expression of the chemical potential over the grain boundaries is developed to show that there is no theoretical limitation for the sintering of covalent ceramics if the grain size of the starting powder is small enough.

Mechanical Behavior Model for Graphites

Abstract: A physically based statistical theory of fracture for polycrystalline bulk graphite is presented. It is based on the inherent weakness of graphitic grains normal to the a-b plane and a coincidence alignment of these planes which has been found to have a dominant influence on strength. These concepts lead to a self-consistent treatment of the tensile fracture of graphite on the basis of the physical parameters of grain size, grain orientation, grain cleavage stress, porosity, fracture toughness, and specimen volume. This interpretive analytical model for the fracture of graphite provides a method for predicting the mechanical and fracture behavior of graphite as a function of its microstructure. The concepts and logic are applicable to other material systems.