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Showing papers on "Grain boundary strengthening published in 1986"


Journal ArticleDOI
TL;DR: In this paper, the influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas medium high-pressure deformation apparatus.
Abstract: The influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas-medium high-pressure deformation apparatus. The specimens used were hot-pressed, dense olivine aggregates with controlled grain size ranging from a few to 70 μm, with or without added water. Mechanical tests were made at 1573 K and 300 MPa confining pressure and at strain rates of 10−3 to 10−6 s−1. The results reveal two distinct mechanisms of deformation, depending on stress level and grain size. At relatively high stress and large grain size, the strain rate is proportional to about the cube power of the stress and is nearly independent of grain size. In this regime, microstructural observations gave evidence of intragranular deformation involving dislocation motion. At low stress and small grain size, the strain rate depends almost linearly on stress and decreases markedly with increase in grain size. In the latter regime, little evidence was found for intragranular deformation. These observations suggest that the deformation mechanism in the grain size insensitive regime is dislocation creep, while that in the grain size sensitive regime is diffusion creep. In both regimes, water was found to enhance the creep rate. The absence of grain size sensitivity in the dislocation creep regime and comparison with single-crystal data indicate that the water weakening effect is mainly an intragranular process. However, the existence also of a water weakening effect in the diffusion creep regime indicates that water also enhances diffusion. The extrapolation of the present results to coarser grain sizes indicates that the transition from dislocation to diffusion creep occurs at 0.1–1 MPa for 10-mm grain size. Therefore it is suggested that this transition may occur in the upper mantle and that, in both regimes, the presence of trace amounts of water will result in significantly lower creep strength than under strictly “dry” conditions.

814 citations


Journal ArticleDOI
TL;DR: In this article, the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized.
Abstract: Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

306 citations


Journal ArticleDOI
TL;DR: Hall-petch strengthening has been observed for the microhardness of electrodeposited nickel material extending to a finest grain diameter of approximately 12 nm as mentioned in this paper, which is the smallest grain size known.

292 citations



Journal ArticleDOI
Erland M. Schulson1, Timothy P. Weihs1, Ian Baker1, H.J. Frost1, J.A. Horton1 
TL;DR: In this article, it was shown that the addition of 750 ppm by weight (0.35 at%) of boron to stoichiometric Ni3Al reduces the effectiveness with which grain boundaries strengthen the alloy.

198 citations


Journal ArticleDOI
TL;DR: In this paper, the mechanical properties of aluminium polycrystals exhibiting differences in grain boundary (GB) properties have been studied by means of the Hall-Petch analysis and the results obtained indicate that th...
Abstract: Mechanical properties of aluminium polycrystals exhibiting differences in grain boundary (GB) properties have been studied by means of the Hall-Petch analysis. The results obtained indicate that th...

187 citations



Journal ArticleDOI
TL;DR: In this paper, the slip band at the tip of a small fatigue crack interacting with grain boundaries is modelled for four cases: a slip band not reaching the grain boundary, a slip-band blocked by the boundary, slip band propagated into an adjacent grain, and then blocked by a second grain boundary.

170 citations


Journal ArticleDOI
TL;DR: In this paper, a theory of grain growth was developed in which textural effects were taken into account by introduction of orientation dependent grain boundary energies and mobilities, and it was shown that grain growth leads to pronounced texture changes which are accompanied by characteristic changes of the scattering of the grain size distribution, which then not even approximately follows a t 1 2 - law.

159 citations


Journal ArticleDOI
TL;DR: In this article, a dynamic calculation involving the formation of a non-equilibrium crack in the carbide is used to explain the experimental observations, which indicates a grain size dependence.

156 citations


Journal ArticleDOI
TL;DR: In this article, the role of dislocations and grain boundaries in martensite nucleation was investigated and the authors concluded that grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei.
Abstract: In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature (Ms) 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 Ms in fine-grained specimens.

Journal ArticleDOI
TL;DR: In this article, the kinetics of grain growth in a Zn-22% Al alloy during static annealing and superplastic deformation (SPD) were studied.

Journal ArticleDOI
TL;DR: In this paper, the authors have performed atomistic simulations on three (001) symmetric tilt grain boundaries: 5(210), (310), and 13(320), and they have shown that the Al-rich grain boundaries have the highest grain boundary energies.

Journal ArticleDOI
TL;DR: In this article, the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries was analyzed in terms of the influence.
Abstract: The nucleation kinetics of proeutectoid ferrite allotriomorphs at austenite grain boundaries in Fe-0.5 at. Pct C-3 at. Pct X alloys, where X is successively Mn, Ni, Co, and Si and in an Fe-0.8 at. Pct C-2.5 at. Pct Mo alloy have been measured using previously developed experimental techniques. The results were analyzed in terms of the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries. Classical nucleation theory was employed in conjunction with the pillbox model of the critical nucleus applied during the predecessor study of ferrite nucleation kinetics at grain boundaries in Fe-C alloys. The free energy change associated with nucleation was evaluated from both the Hillert-Staffanson and the Central Atoms Models of interstitial-substitutional solid solutions. The grain boundary concentrations of X determined with a Scanning Auger Microprobe were utilized to calculate the reduction in the austenite grain boundary energy produced by the segregation of alloying elements. Analysis of these data in terms of nucleation theory indicates that much of the influence of X upon ferrite nucleation rate derives from effects upon the volume-free energy change,i.e., upon alterations in the path of theγ/(α + γ) phase boundary. Additional effects arise from reductions in austenite grain boundary energy, with austenite-forming alloying elements being more effective in this regard than ferrite-formers. By difference, the remaining influence of the alloy elements studied evidently results from their ability to diminish the energies of the austenite: ferrite boundaries enclosing the critical nucleus. The role of nucleation kinetics in the formation of a bay in the TTT diagram of Fe-C-Mo alloys is also considered.

Journal ArticleDOI
TL;DR: Grain boundary cavitation in Type 304 stainless steel under creep loading was investigated in this article, where the correlation between cavity density and boundary structure was characterized by high voltage electron microscopy and Kikuchi patterns.

Journal ArticleDOI
TL;DR: In this paper, a simple model for the driving force for grain boundary migration based on dislocation density contrasts, as controlled by intra-grain strains and grain orientations, is proposed and tested, with an 80% success rate for the mobile grain boundaries studied.


Journal ArticleDOI
TL;DR: In this article, the slow plastic flow properties of a series of binary B2 FeAl intermetallics at elevated temperatures, and ranging in aluminum content from 39.8 to 48.7 at.

Journal ArticleDOI
TL;DR: In this paper, a microstructural and compositional investigation of grain boundary precipitation and martensite formation in sensitised 304 stainless steel has been conducted, where the depletion of chromium promotes martensites formation within near-grain boundary regions and this transformation has been directly studied by in situ cold stage microscopy down to − 150°C.

Journal ArticleDOI
TL;DR: In this article, the densities of dislocations trapped in coherent twin boundaries may be used to provide a direct and quantitative comparison of the extent of intragranular slip in the three regions of behaviour associated with superplasticity.

Journal ArticleDOI
TL;DR: In this article, high purity polycrystalline iron of two different grain sizes was deformed at room temperature in tension to strains of up to 0.27, and the deformation mechanisms operative at selected strain levels identified by examining both the bulk microstructure by transmission electron microscopy and features developed on prepolished surfaces using optical microscopy.

Journal ArticleDOI
TL;DR: Grain size dependences of Vickers microhardness and fracture toughness in fully dense Al 2O3 and Y2O3 ceramics without additives were examined in this article, where the difference in grain size dependence of fracture toughness for Al 2 O 3 and Y 2O 3 was caused by the formation of microcracks due to the local microstresses produced by thermal expansion anisotropy, the crystal structure of Y 2 O3 being cubic and that of Al2 O 3 being hexagonal.

Journal ArticleDOI
V. Randle1, B. Ralph1
TL;DR: In this article, electron micrographs have been used to establish the microstructural parameters characterising the γ′ dispersion and also the nature of the boundaryparticle interactions, which are interpreted in terms of models proposed by Zener and by Ashby and colleagues.

Journal ArticleDOI
TL;DR: In this article, the effect of second-phase martensite on cleavage crack propagation was studied to define the effective grain size of dual-phase steels which controls the ductile-to-brittle transition temperature.

Journal ArticleDOI
TL;DR: In this article, the authors analyzed intergranular creep crack growth in metals at high temperatures by assuming that the crack advances when cavities coalesce on grain boundary facets approximately normal to the maximum principal tensile stress.
Abstract: Intergranular creep crack growth in metals at high temperatures is analysed by assuming that the crack advances when cavities coalesce on grain boundary facets approximately normal to the maximum principal tensile stress. The analyses are based on a material model that describes the nucleation and growth of grain boundary cavities, accounting for diffusive growth as well as growth by dislocation creep of the surrouding grains, and also incorporating the effect of grain boundary sliding. Plane strain center cracked panels are analysed by a numerical method that fully accounts for the development of damage in every point of the specimen, and the solutions are compared with crack growth rates predicted by a simple model based on the singular stress fields around the tip of a sharp crack. The development of crack growth rates and the general crack growth patterns predicted by this material model are determined for a range of material parameters, including cases where failure occurs at small strains as well as cases where failure occurs at large strains.

01 Jul 1986
TL;DR: In this paper, the effects of grain size of internal microfractures in polycrystalline ice were examined under uniaxial, constant load creep conditions at -5 C. The results indicated that the average crack size is approximately one-half the average grain diameter over the stated grain size range.
Abstract: : This work presents the results of a study to examine the effects of grain size of internal microfractures in polycrystalline ice. Laboratory-prepared specimens were tested under uniaxial, constant-load creep conditions at -5 C. Grain size ranged from 1.5 to 6.0 mm. This range of grain size, under an initial creep stress of 2.0 MPa, led to a significant change in the character of deformation. The finest-grained material displayed no internal cracking and typically experienced strains of 10 to the minus 2nd power at the minimum creep rate epsilon. The coarse-grained material experienced severe cracking and a drop in the strain at epsilon min to approximately 4x10 to the minus 3rd power. Extensive post-test optical analysis allowed estimation of the size distribution and number of microcracks in the tested material. These data led to the development of a relationship between the average crack size and the average grain size. Additionally, the crack size distribution, when normalized to the grain diameter, was very similar for all specimens tested. The results indicate that the average crack size is approximately one-half the average grain diameter over the stated grain size range. A dislocation pileup model is found to adequately predict the onset of internal cracking. The work employed acoustic emission techniques to monitor the fracturing rate occurred. Other topics covered in this report include creep behavior, crack healing, the effect of stress level on fracture size and the orientation of cracked grains. Theoretical aspects of the grain size effect on material behavior are also given.

Journal ArticleDOI
TL;DR: Grain boundary precipitation in a commercial alloy 7075 Al has been studied using transmission electron microscopy as mentioned in this paper, and the crystallography of the precipitates and the matrix is characterized in terms of the misorientation between the grains and the orientation of the grain boundary.

Journal ArticleDOI
TL;DR: The causes of embrittlement in several plain carbon-manganese and niobium-treated steels between 800 and 1200 °C have been investigated in this paper, where the size, distribution, and composition of grain boundary precipitates were measured on extraction replicas.
Abstract: The causes of embrittlement in several plain carbon-manganese and niobium-treated steels between 800 and 1200 °C have been investigated. Tensile ductility was measured as a function of temperature and strain rate. Percent elongation and reduction in area were used to characterize the temperature dependence and severity of the ductility loss. The size, distribution, and composition of grain boundary precipitates were measured on extraction replicas. Grain boundary segregation was measured by AES on samples that were deformed at 900 °C before being fractured under ultra-high vacuum at room temperature. Segregation of impurity residual elements and grain boundary precipitation are the primary factors responsible for the observed ductility loss. The embrittlement results in a low ductility fracture which is largely intergranular through the austenite grain boundaries. Segregation of Cu, Sn, and Sb was found on the fracture surfaces of the embrittled samples. High temperature deformation was necessary to produce segregation as no segregation was detected in undeformed samples. Grain boundary precipitation, particularly AIN but also Nb (C,N), contributed to the embrittlement when there was a relatively fine distribution of precipitates along the austenite grain boundaries. The most severe ductility loss occurred when grain boundary precipitation combined with Cu, Sn, and Sb segregation.

Journal ArticleDOI
TL;DR: It is shown that the analogy between these melting phenomena and related interfacial phase transitions, such as complete wetting in and surface-induced disorder in solid^,^ leads to new predictions for melting at boundaries.
Abstract: In a recent Letter, Broughton and ~ i l m e r ' studied melting at grain boundaries (GB) in a system of particles interacting via truncated Lennard-Jones forces. They observed the growth of a disordered, liquidlike film within the GB as the temperature, T, was raised towards the triple point with T q. In this Comment, I show that the analogy between these melting phenomena and related interfacial phase transitions, such as complete wetting in and surface-induced disorder in solid^,^ leads to new predictions for melting at boundaries. On any point of the crystal-liquid (CL) coexistence curve with pressure P and T = T m ( P ) , the GB with interfacial tension y ~ s may contain a droplet of the melt bounded by the CL interface with tension ~ C L . In equilibrium, the interfacial tensions must balance which implies yGB =2 ycLcos($), where is the contact angle. In a fluid context, this is known as Antonow's rule.5 When $=0, the droplet spreads out and the GB is wetted by the melt. In this case, a disordered, liquidlike film appears in the GB when the CL coexistence curve is approached from T < Tm ( P I . The Landau free energy per unit area for a film of thickness 1 is3*41697

Journal ArticleDOI
TL;DR: In this paper, a deformation mechanism (DM) map for distributed grain size was constructed for quartzite with realistic grain size distributions using two bounding models: homogeneous strain rate with local stress varying from grain to grain and homogeneous stress with local strain rate varying.
Abstract: Conventional deformation mechanism (DM) maps are constructed for single-valued grain size When diffusional creep is competitive with power law creep, such maps are a poor guide to rheology and microstructure development in distributed grain size materials because the strain rate dependence on grain size implies wide variations in local stress and/or strain rate We present DM maps for quartzite with realistic grain size distributions using two bounding models: (1) homogeneous strain rate with local stress varying from grain to grain and (2) homogeneous stress with local strain rate varying Distributed grain size systems are stronger at low stresses and have larger power law creep fields and larger regions where two or more mechanisms contribute significantly to the total strain rate Our DM maps do not incorporate grain shape and packing, but we review theoretical work on the effects of these variables and compare rheological predictions with our own for the development of idealized core-and-mantle microstructure