Papers by Keyword: Hall-Petch Relationship

Abstract: This paper presents experimental results on the processing of complex concentrated alloy with a nominal composition of A_0.35CoCrFeNi. The alloy was produced by vacuum induction melting and tilt casting. The microstructure of the as-cast CCA consists of dendritic and interdendritic regions homogenized by heat treatment at 1360 °C. After rotary swaging at room temperature, the microstructure is characterized by an abundance of dislocations and continuously intersecting slip bands. Annealing experiments were carried out in the temperature range of 1150 °C – 1300 °C and different holding times to determine the parameters of grain growth kinetics. Phase and chemical analysis were investigated using XRD and EDS methods. The activation energy of recrystallization in the studied composition was 458 kJ mol^-1. The influence of grain size on room temperature mechanical properties and tensile properties was determined. The hardening coefficients k_h and k_σ, calculated using the Hall-Petch relation, were 277.5 HV µm^-1/2 and 655 MPa µm^-1/2, indicating the effectiveness of grain boundary hardening in the studied single-phase CCA.

Abstract: It is now well established that the grain size is the fundamental microstructural feature of all polycrystalline materials. In practice, a very wide range of grain sizes will be needed in order to fully evaluate the effect of grain size on the mechanical properties of metals. For many years this was a significant limitation because it was not possible to use conventional thermomechanical processing to produce materials with submicrometer or nanometer grain sizes. Recently, this problem has been addressed by developing alternative processing techniques based on the application of severe plastic deformation. This overview demonstrates that, although the flow stress increases with decreasing grain size at low temperatures and decreases with decreasing grain size at high temperatures, this clear dichotomy in behavior may be adequately explained by using a single theoretical flow mechanism based on the occurrence of grain boundary sliding.

Abstract: During the early stages of the plastic deformation of a polycrystal, dislocations can pile-up against grain boundaries. Experimental results on large-grained materials have provided excellent verification of this phenomenon. Such a pile-up may activate dislocation slip in the neighbouring grain. Whether this occurs depends on the misorientation between the grains and the resolved shear stresses in the affected grain. Several approximate criteria have been proposed to predict the occurrence of this mechanism. Here, the problem will be assessed directly by calculating the Peach-Köhler force produced by a single dislocation pile-up in one grain on all the possible slip systems in the neighbouring grain, in combination with the effect of the applied external stress as obtained through calculation of the Schmid factor. It will be seen that the problem is significantly more complex than what is generally assumed in basic explanations of the Hall-Petch effect: highly localised stress concentrations are generated for certain misorientations, which are capable of punching out small dislocation loops which may then propagate into the neighbouring grain.

Abstract: The influence of solution treatment on microstructure evolution and mechanical behavior of Cu-20Ni-20Mn alloy was investigated by optical microscopy (OM), X-ray diffraction (XRD) and hardness test. The results revealed that both solution temperature and holding time had effect on the grain growth behavior. The grain growth activation energy was determined by grain size of Cu-20Ni-20Mn alloy for different heat treatment temperatures and periods. With increasing temperature of solution treatment, the second phase is gradually dissolved into the Cu-rich matrix, and the lattice parameter of the matrix solution treated at 1173K for 0.5 h was about 3.668 Å. The hardness of the solution-treated alloy was lower than that of hot forging, and the hardness value decreased with the increase of solution temperature, which may be attributed to grain size. The hardening ability, corresponding to the Hall-Petch relationship, decreased linearly with D^-1/2.

Abstract: Microstructural transformations and mechanical properties of a low carbon martensite stainless bearing steel treated with different heat treatment parameters and cryogenic treatment (-82°C) were investigated. The function of microstructural transformations on strengthening and toughening process was quantitatively characterized as well. These analyses were performed by the optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and electron back scattering diffraction (EBSD) technique. The obtained results show that with execution of cryogenic treatment and tempering, the tensile strength increase owing to the reduction of retained austenite and fine carbides precipitating respectively. The effect of martensitic microstructure on yield strength increment can be regarded as packet size and block width which conform to Hall-Petch relationship. Meanwhile, the results suggest that the block width is the key structural controlling unit when analyzing the strength-structure relationship of lath martensite in low carbon martensite stainless bearing steel. In addition, packet size can be related to toughness controlling as well because of the same size as cleavage plane.

Abstract: Combined methods to obtain ultra-fine grain (UFG) α-brass samples are proposed. Severe plastic deformation followed by recrystallization was conducted, where multiple rolling and equal channel angular pressing (ECAP) were employed. Recrystallization was accomplished by heat-treatment after the severe plastic deformation, and the grain size after the severe plastic deformation was decreased. By multiple rolling, plates with thickness of 0.1 mm and grain size of 1.0 μm were obtained. By ECAP process, square bar with cross-section of 6 mm × 6 mm and minimum grain size of 4.1 μm was obtained. The 0.2 % proof strength, ultimate tensile strength, and fatigue limit were increased with the value of inverse square root of grain size (Hall-Petch relationship). Then, the 0.2 % proof strength of UFG brass was tenfold, the ultimate tensile strength and the fatigue limit were two fold increases from the conventional α-brass. Because of the high strength, the scatter of fatigue strength of UFG brass was large, which reflects the sensitivity to defects in material.

Abstract: Two Al alloys (AA1570 and AA6061) in the solutionized state have been processed by HPT at room temperature to achieve a homogeneous UFG structure. After HPT, the grain size was found to have a mean value about 100 nm for both alloys. Measured yield stress values of HPT-produced UFG alloys being plotted in terms of the Hall-Petch relationship were found to exceed the plot predictions for the range of ultrafine grain size. For both alloys, Atom Probe Tomography measurements allowed to reveal segregation of solute elements along grain boundaries. The origin of the extremely high strength of the alloys nanostructured by HPT is discussed with a special attention to the influence of such segregations on the emission and the mobility of dislocations.

Abstract: The Fe-25Cr-1N alloy produced by solution nitriding possesses extremely high yield strength owing to the solid solution strengthening by nitrogen. However, it was found that the steel exhibited an insufficient elongation because of the brittle intergranular fracture caused during the uniform tensile deformation. This is due to the marked stress concentration at grain boundaries, which is derived from the grain coarsening caused during long time solution nitriding and the development of planar dislocation structure characteristic of high nitrogen austenitic steels. The most effective way to reduce the stress concentration at grain boundary during deformation should be grain refinement. In this study, grain refinement was attempted by the two-step heat treatment for the Fe-25Cr-1N(-Mn) alloy, and then the mechanical properties were investigated by means of tensile tests and fatigue tests. The two-step heat treatment resulted in the grain refinement of austenite to 20 microns in diameter. The intergranular fracture was greatly suppressed from 70% (as-solution-nitrided) to 10% (grain-refined) in area fraction by the grain refinement. In addition, elongation was markedly increased with local necking. The yield stress and tensile strength were also increased, and thus, the fatigue limit is also raised by more than 30%.

Abstract: Full-dense bulk Pb(Zr,Ti)O3 (PZT) ceramics with various grain sizes were prepared by aerosol deposition (AD) employing various firing temperature from 800 to 1200°C. It was confirmed that the Hall-Petch relationship is obeyed in AD-processed PZT ceramics over the almost whole grain size range, which is indicating that only grain size could be the determining factor of hardness in AD since density is almost constant against the grain size. As compared with conventional sintering technique, AD-processed PZT ceramics exhibited higher hardness and higher or at least equivalent electrical properties over the whole grain size range investigated, which is indicating that the piezoelectric ceramics with improved mechanical properties can be realized by AD without any sacrifice in electrical properties.

Abstract: Tensile properties of twinning induced plasticity (TWIP) steels (31%Mn-3%Al-3%Si-Fe) with various mean grain sizes ranging from ultrafine grain size (1.1μm) to conventional one (35.5μm) at a wide range of strain rates from 10-3sec-1 to 103sec-1 were studied. The ultrafine grained TWIP steel exhibits a large work hardening and keeps an adequate elongation at any strain rate. The strength held to the Hall-Petch relationship at each strain rate and the Hall-Petch slopes do not change largely.