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The present work has investigated whether the twin boundaries are to be included or not in the Hall-Petch equation for polycrystalline metals. Pure copper, Cu-3.5at%Al, Cu-6.8at%Al and Cu-14.8at%Al alloys, which have the average grain sizes from 8.4 to 176.0μm and the different ratio of annealing twins, were produced and pulled in tension at temperatures from 77 to 973K under a strain rate of 1.2 × 10-4s-1. The distribution of dislocations in the surface grains was also observed by using the etch pit technique. 0.1% proof stress including the twins for each specimen at room temperature conforms well to the Hall-Petch relation. The halves of intersecting stress σ0 derived from the straight lines are approximately equal to the critical resolved shear stress of each single crystal, i. e. CRSS, so far reported. It is found that the proportional relation between the σ0/2 value and the square root of aluminum concentration is in good agreement for the single crystals and the polycrystalline specimens. The temperature dependence on the σ0/2 values for Cu-14.8at%Al alloys is similar to the variations for the CRSS of single crystals, especially at the region of low and elevated temperatures. From the Hall-Petch parameters at 293K, the unlocking stress of edge dislocations from the solute atoms is found to be 3.5 times larger than the CRSS of single crystals. This CRSS is thought to be the unlocking stress of screw dislocations which come out onto the crystal surface. It is considered that the role of the twin boundaries is almost equivalent to the grain boundaries, because the multiplicative dislocations pile up against the twin and grain boundaries during the small deformation.

In order to clarify the grain size dependence on the fatigue behavior of polycrystalline metals including twins from the view point of dislocation theory, pure copper, Cu-3.5at%Al, Cu-6.8at%Al and Cu-14.8at%Al alloys having the average grain sizes ranging from 9 to 122μm were prepared by a strain-annealing method. Fatigue tests at room temperature for each specimen were performed under the constant stress amplitude over a life range of 104 to 107cycles. The fatigue properties at elevated temperatures for Cu-14.8at%Al alloys were confirmed up to 773K, and also the distribution of dislocations close to fatigue cracks was observed by using the etch pit technique. The grain size dependence on the fatigue strength for pure copper and Cu-3.5at%Al alloys disappears beyond 106cycles, whereas the fatigue lives of Cu-6.8at%Al and Cu-14.8at%Al alloys uniformly increase with grain refining over all ranges. Relationship between the fatigue strength at 107cycles and aluminum concentration for Cu-Al alloys of fcc solid solution can be approximately represented by a straight line as well as the shear stress. The fatigue strength of Cu-14.8at%Al alloys markedly decreases with increasing temperature and becomes almost half of that for room temperature at 773K. The results based on the etch pit observation indicate that the dislocations multiplied under cyclic loading pile up against the grain and twin boundaries, where large cracks along both boundaries form and propagate due to the stress concentration.

The present paper describes the variations of the mechanical properties of Al-Zn-Mg-Cu alloy single crystals with various aging times. The cylindrical single crystals of 7475 aluminum alloys were produced at 1033 K by the Bridgeman method, where the composition of initial material and the shape of carbon mold were modified for the growth of quaternary crystals. The specimens proper for tensile and hardness testing were obtained from the single crystal rod homogenized at 823 K for 900 ks using spark-cutting method. Subsequently, they were aged at 393 K for 3.6, 86.4, 900 and 2880 ks after quenching in ice water from 773 K. In the stress-strain curves of the alloy single crystals, the increase of yield stress and the decrease of elongation with an increase of aging time are seen together with a decrease in the rate of work hardening. Moreover, some serrations occur especially in the final plastic stage on the curves. Hardness of the alloy single crystals agrees with those of polycrystals due to the occurrence of multiple slips during loading. However, the alloy single crystals exhibit a marked decrease of yield strength in comparison with the polycrystals, which results from a single glide occurring in the single crystals. If we compare the increase of critical resolved shear stress (CRSS) derived from two kinds of theories of precipitation with the experimental value, both of them are roughly in agreement except for the specimen aged for 3.6 ks. The obtained results indicate that the maximum strength of Al-Zn-Mg-Cu alloy single crystals is achieved in the transitional process between the mechanism of particle cutting and the Orowan mechanism. [doi:10.2320/matertrans.MER2008178]

In order to make clear the micro-yielding mechanisms of polycrystalline metals including twins, the movement of dislocations in the surface grains of Cu-6.8at%Al alloy and pure Mg polycrystals during the early stages of deformation was directly observed by using etch pit technique. The fresh dislocations multiply from the Frank-Read sources within the grains, and pile up against the twin and grain boundaries of two kinds of specimens. The pile-up dislocations on the primary and/or secondary slip planes are also confirmed in Cu-6.8at%Al alloys. Especially during the compressive loading for pure Mg, the occurrence of deformation twins is remarkable with an increase of strain rate, whereas the distribution of fresh dislocations tends to decrease in the surface grains. The present results suggest that the effect of twin boundaries on micro-yielding is almost equivalent for that of grain boundaries, which act as barriers to moving dislocations even in the pre-yield deformation.

The effect of sub-grain on the yield stress of pure copper single crystals with the [253] orientation was investigated by using the etch pit technique. The single crystal plates were successfully prepared from the seed crystals, which were produced at the melting temperature of 1473 K by the Bridgeman method. The present investigation confirmed the Hall-Petch relation concerning the effect of sub-grain boundaries on the macroscopic yielding of pure copper. The result derived from the extrapolation of the relationship of critical resolved shear stress (CRSS) and the initial dislocation density and sub-grain size is in good agreement with the evaluation in high purity copper single crystals of low dislocation density.

Slip deformation in Cu-9at.% Al symmetric type bicrystal models subjected to tensile loading is investigated by a finite element crystal plasticity analysis code. Accumulation of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) are studied in detail. Results of the analysis show asymmetric non-uniform deformation and accumulation of GNDs on the primary and secondary slip systems with activation of secondary slip system near the grain boundary. Mechanism of asymmetric non-uniform deformation with GNDs accumulated near the grain boundary in the Cu-9at.% Al symmetric type bicrystal models is discussed from the viewpoint of the effects of the elastic anisotropy of Cu-9at.% Al and the heterogeneity of initial statically stored dislocations density.

The sliding behavior of α/β, β/γ, and γ/α interphase boundaries of CuZnSn alloy diffusion couples produced by solid-to-solid diffusion bonding was investigated. Particular attention was paid to differences in hardness between two component phases. The slidings at these three types of interphase boundaries showed so-called slide softening , in which the sliding rates increased with sliding displacement at relatively low temperatures of 723–873 K. The sliding rate of interphase boundaries consisting of component phases with dissimilar hardness tends to be higher than those with similar hardness. Macroscopic sliding accompanied a local plastic deformation in the softer phase adjacent to the interphase boundaries. The present results support the mechanism that an interphase boundary sliding plays an important role in the structural superplasticity of fine-grained materials consisting of a hard and a soft phases.

To analyze the rate-controlling mechanism of Luders deformation found in the [001] oriented Fe-30%Cr alloy single crystals, the activation volume and the activation enthalpy were obtained by a strain-rate and temperature change method during the deformation from 293K to 511K and were found to be 94-176b3 and 0.8-1.4eV, respectively. Aging at 473K was made after extension by 23.5%. The Luders band didn't move on further straining and the second Luders band started from opposite side of the chuck. It was considered that the Luders front was aged and locked by some solute atoms. The line intensity of carbon atom fairly increased around the Luders front after aging. The interaction of moving dislocations and carbon atom is considered to be the most likely rate-controlling mechanism for the deformation. The activation distance was calculated to be 4.1-6.2 times larger than carbon diameter and would be reasonable value considering the moving dislocations overcome carbon atoms.

Cu-33.0 Zn-4.3 Sn mass% alloy polycrystals were made and quenched from 1103 K. After the tensile tests, it was found that the shape memory effect below 268 K and the superelasticity above 278 K, and about 4% maximum pseudoelastic strain was observed. The superelastic strain and shape memory strain are complementary and the total superelastic and shape memory recoveries are normally close to 100%. The new alloy of Cu-34.9Zn-3.4Sn-2.2 Pb mass% having excellent hot work ability and corrosion resistance also showed shape memory effect at low temperature and can be used as the superplastic and corrosion resistant shape memory elements. (orig.)

In order to make clear the crystallographic interpretation of each factor composing the Hall-Petch relational equation, experiments were performed by using two kinds of polycrystalline pure aluminum, which were annealed at 573 K and followed by quenching from 773 K. The In-Situ observations of the appearance or morphological change of slip bands were continuously carried out in the surface grains from elastic strain range to yielding. The main results obtained indicate that the Hall-Petch rule is proper for slip deformation in a polycrystalline aluminum which consists of the heterogeneous grain sizes. The Hall-Petch slope decreases with increasing of the number of active slip planes in the slipped surface grains just after yielding. Moreover, the friction stress after yielding shows a lower value than the yield stress of the single crystal, but the quench-harding increment in a polycrystalline aluminum is consistent with that of the single crystal.

The new commercial Cu-Zn based alloys having excellent hot workability and shape memory effects have been developed. Higher than 200% elongation was observed in the hot tensile test with the alloy sample containing Cu-38Zn-1.9Sn-1.9mass%Pb at the temperature range from 773K to 873K with the initial strain rate of 8.3×10 -4 s -1 . The stress-strain curve showed the typical type of superplastic deformation. After heat treatment of the developed alloys at 1073K, they showed shape memory effects(SME). The transformation temperatures of samples were measured by electric resistance, and found to be Ms=119K, Mf=96K, As=105K, Af=129K. From the width of hysteresis about 15K, the formation of thermoelastic martensite has been confirmed.

Stress–strain response of cyclic superelastic deformation was examined in Cu–20at.%Al–10at.%Mn shape memory alloy single crystals. As the superelastic deformation was repeated, the critical stress for the onset of stress-induced transformation decreased at an almost constant rate to zero by a finite number of cycles N c . By further cycles residual martensite was accumulated until all the stress-induced martensite was left after unloading. The decrease of the stress per cycle and the number N c depended on the direction of applied stress, i.e. tension, compression and tension–compression, and also the crystal orientation. These N c ’s can be related by a linear S–N curve; log N c decreased linearly with increasing log Δ σ , where Δ σ is the hysteresis-width in superelastic stress–strain curve. The observed fatigue behaviour can be understood in terms of internal stress of dislocation substructure formed during the cycles.

Fatigue crack initiation and propagation were studied on compact tension specimens of pure copper single crystals having the [211], [221], [210] and [321] tensile axes. Two stages of crack propagation were discriminated by the different dependence of the crack length on the number of cycles : a first stage of accelerating growth and a second stage of steady-state motion. It is found that the presence of these stages can be explained semi-quantitatively within the framework of the theory of shear decohesion at the crack tip. The tensile orientation strongly affects the crack initiation and propagation in the early stage.

The effect of a boundary node on intergranular fracture at intermediate temperatures is discussed from creep tests employing a Cu−9 at.% Al alloy tricrystal. In situ observation to measure the amount of grain-boundary sliding is made during the creep tests. At temperatures between 773 and 923 K under an applied stress of 35 or 40 MPa, cracks to cause intergranular fracture appear in bicrystal and tricrystal specimens after the occurrence of a critical amount of grain-boundary sliding. The boundary node lowers the rate of the grain-boundary sliding and thereby retards the intergranular fracture. This is different from the effect of the boundary node at higher temperatures, where the boundary node becomes a preferential site for crack formation.

The microscopic deformation mechanism of KCl single crystals under the indentation with a steel ball was studied under the load range of 0.5N-5.0N in the temperature range 20°C-300°C. Indentations were made on the {100} plane, and using the etch pit detecting technique the dislocation structures were investigated on the indented surface and its cross section.The results obtained are as follows:(1) The dislocation structure formed on a {100} plane at room temperature consists of a high-dislocation-density range and a wing-shaped area of comparatively low density extending along the ‹110› and ‹100› directions.(2) Dislocation activities on the {110}90 and {110}45 planes are more active under the testing temperature at 100°C and 200°C than at room temperature. At these testing temperatures, the deformation is mainly caused by the slip on the {110}90 plane under comparatively high loads, while the deformation under the lower loads is caused in the pattern that the slips on the {110}90 and {110}45 planes have almost equal effect on the formation of the dislocation structure.(3) At 300°C, the slip activities on the slip system besides the {110} ‹110› are observed. Dislocation structures with a pattern of circle in the neighborhood of indent, and with a wing-shaped pattern along the ‹110› direction in the outer range, are observed.(4) On the cross section, dislocation activities on the {110}45 plane are detected at all testing temperatures. It is confirmed that the secondary slip activities occur on the {100} plane at 300°C.

Scratching tests were carried out on the (111) face of Cu-14.7 at.%Al single crystals and bicrystals with the use of pyramidal indentor. In the scratching on the single crystals three kinds of scratching directions, [112], [112] and [110] were chosen. In the case of bicrystals, the [110] and [110] directions were adopted. After scratching the distribution pattern of the slipped traces and the surface profiles across the scratched track were examined. In addition, the dislocation distributions inside the crystals were revealed by successively removing thin layers and developing etch pits on the exposed surface.The conclusions obtained are as follows:(1) The slipped traces on either side of the scratched track are produced more extensively than those around the indented terminal.(2) The swells of the material were caused in front of the scratched track by slips on the {111} faces which diverge downwards. So the width of the scratched track appears narrower than that in the indented terminal.(3) In the scratching which acrosses the grain boundary, the deformation in the surface layer of the crystal is obstructed due to the existence of grain boundary. But the deformation inside the crystal is little influenced by the grain boundary.

The purpose of this investigation was to elucidate the effect of grain boundaries on yield drop caused by short-range order in α-CuAl alloys. Bicrystals with Σ3 {111} boundaries (coherent twin boundaries) of Cu-3 at.% Al and Cu-9 at.% Al alloys were grown by the Bridgman method. Bicrystal and single crystal specimens with different orientations were cut from the bicrystals. Difference in deformation behavior at room temperature among the specimens were compared by tensile tests. When the grain boundaries in the bicrystal specimens were obstacles to the motion of dislocations on the primary slip planes, the yield drop observed in α-CuAl alloy single crystals vanished or diminished. Changes in flow stresses due to the Σ3 {111} boundaries were shown. The contribution of the Σ3 {111} boundaries to the strength of the alloys and its aluminum concentration dependence were discussed.

In order to elucidate the mechnism of friction and wear, it is necessary to have a knowledge on deformation in materials produced by invasion of sharp projections or wear particles.In this study, the cleaved faces of KCl single crystal were indented with a steel ball or a Vickers diamond indentor. The deformation and hardness around the indentation were examined from macroscopic viewpoints, and the dislocation patterns were observed from microscopic one. The results obtained are as follows:(1) The indents formed on the indented surfaces with a ball or a pyramid were not perfectly circular nor square in shape due to the crystal anisotropy.(2) The appearances of the pile-up and sink-in around indents were clarified.(3) The strain hardening around the indent was more active in the ‹110› direction than in the ‹100›.(4) The deformed area on {100} face with a ball or pyramidal indentor consisted of a high dislocation density range and a wing shaped area of comparatively low density extended along the ‹110› and ‹100› directions.(5) The hardness and the dislocation density were related each other.(6) The etch pit patterns in the cleaved cross section were observed on {110}45 faces.

The indentation test is probably the simplest method of estimating the strength of material under the frictional condition that the asperity tips are pressed into other body and that wear debris are scratched between two bodies. With the advancement of techniques of revealing dislocations in crystals, it has been thought possible to obtain a right understanding of the deformation in the material with a single crystalline structure caused by indentation.In the present study, KCl single crystal was indented with a steel ball to examine the dislocation structures associated with indentation at various loads. The slip systems and the range of deformed area were investigated in detail on the indented surface and on its cross section by using a dislocation etching technique.The results obtained are as follows:(1) The deformed area on the indented surface consists of a circular high dislocation density area and the area of comparatively low dislocation density shaped like wings and extended in the ‹110› and ‹100› directions.(2) Dislocation patterns on the indented surface and on its cross section consist of etch pit rows that belong to the {110}90 and {110}45 planes, respectively.(3) The plastic deformation range is affected by the diameter of indentor under comparatively high loads, but not under exceedingly low loads.

Copper Σ3{111} and Σ9{221} symmetrical ‹110› tilt bicrystals and their component single crystals were tensile tested at room temperature. By the stress-strain curves obtained and the observation of slip lines, the effect of grain boundary on plastic deformation was discussed. The effect of grain boundary on the deformation behavior depended on the crystallographic relationship between the grain boundary planes and the primary slip planes in the two component single crystals. When both of the primary slip planes in the component single crystals were parallel to the grain boundary, the difference of the stress-strain curves between the bicrystal and the component single crystal was not found. On the other hand, when the primary slip planes in the bicrystal were not parallel to the grain boundary, the grain boundary raised the flow stress of the bicrystal above that of the component single crystal. In case of the bicrystals in which the multiple slips take place in the component single crystals, no effect of the grain boundary on the deformation behavior of the bicrystal was observed.

In the present study the repeated scratch tests were made on the (111) face of the αCu-Al single crystals in the ‹211› and ‹110› directions with a unidirectional or reciprocating motion. After the (111) face had been scratched, the face was electropolished to remove a thin layer of suitable thickness. The slip systems and the etch pit distributions were observed on the exposed surfaces.The results obtained are as follows:(1) The deformations produced by scratch in the [211] direction are found to be more pronounced than those in the [211] scratch. The active slip systems on both sides of the scratched track differ from those observed under the track.(2) The scratched tracks and the slip traces on the scratched faces and the dislocation distributions inside of the material show that the deformation produced by the unidirectional scratch is more remarkable than that produced by the reciprocating scratchs in both cases of ‹211› and ‹110› directions.

αCu-Al single crystals were scratched with a steel ball in various directions in order to clarify the mechanism of the deformation observed in abrasive wear of the finished surface. Microscopic observations of the slip traces and of the scratched track were made on the (111) face scratched in the [211], [211] and [011] directions.After the (111) face had been scratched, the face was electropolished to remove a thin layer of suitable thickness in order to observe the distribution of the etch pits and slip traces on the exposed surface. This in turn permitted the determination of the distribution of the dislocation density on the cross-section perpendicular to the (111) crystal face.The conclusions are summarized as follows:(1) The slip width and slip depth increase in proportion to the load.(2) The scratched track width and the slip depth are wider for scratches in the [211] direction than for scratches in the [211] direction.(3) The active slip systems in the scratch can be explained by taking into consideration the stresses produced in the material.(4) Beyond a certain depth, the effect of the frictional stress produced by the scratch disappears, and only the effect of the normal stress remains.

Indentation tests using a steel ball were carried out on the (001), (011) and (111) faces of αCu-Al single crystals in order to elucidate the mechanism of plastic deformation produced in the surface layer. The indentation configuration produced by the indentor and the slip trace distribution were studied in detail.In addition, the dislocation density around the indentation on the (111) and (211) faces was revealed by removing successively the thin surface layers and developing etch pits on the exposed surfaces.The results obtained are summarized as follows:(1) The slip tends to occur in the ‹110› direction on the (001) and (011) crystal faces.(2) In the case of indentation on the (111) face, the slip occurs on two sets of planes arranged in truncated triangular pyramids, one diverging and the other converging into the material.(3) The distributions of the dislocation density and the range of plastic deformation on the (111) and (211) faces were clarified.

Cu-14.1 at %Al polycrystals with three different grain sizes (76,113 and 157 μm) have been tested for the Portevin-LeChatlier effect under various conditions of temperature (−196 to 200°C) and strain rate (2.78 × 10−5 to 5.56 × 103 sec−1). In the above range of strain rate, serrated yielding was observed in the temperature range 60 to 160° C. The strain rate dependence of the onset of serrations is most probably due to the diffusion of vacancy-solute atom pairs, as indicated by the low value of the activation energy for migration (0.77 eV). The correlation governing the test variables at the onset of serrations appears to be:\(\dot \varepsilon \) = (const)e02.2 ± 0.2 μ−0.87 + 0.03, where\(\dot \varepsilon \), e0 and μ are strain rate, critical strain for the onset of serration and grain size, respectively. The onset of serrations is most probably due to dynamic strain ageing, although the possibility of short range ordering is not ruled out.

The effects of grain boundaries on stress corrosion cracking (SCC) of Cu-Zn and Cu-Al alloys have been studied employing various types of orientation-controlled bicrystals. It was found that SCC susceptibility in Cu-Zn alloys decreases in the following order; beta-phase crystal surface, (alpha/alpha)-grain boundary, alpha-phase crystal surface, diffusion-induced twin boundary in alpha phase, (beta/beta)-grain boundary or (alpha/beta)-interphase interface. It was also clarified that, in an alphaCu-Al alloy, the grain boundary shows much higher susceptibility than the grain interior. The susceptibilities of the symmetrical - and -tilt, and the - twist boundaries of the alphaCu-Al alloy to SCC are strongly dependent upon the misorientations. Sharp minima indicating great resistance to SCC exist at some angles, which correspond to relatively small sigma-values. Moreover, the susceptibility of a grain boundary to SCC is much enhanced by a stress field arising from pile-up dislocations in the immediate vicinity of the boundary. It becomes evident that the susceptibility to intergranular stress corrosion cracking is suddenly decreased when the grain boundary misorientation decreases below approximately 15°. This may result from the increased number of dislocations passing through the boundary with decreasing the misorientation angle, by which a stress concentration is avoidable.

It has been known that an aluminum single crystal having tensile orientation shows initial rapid hardening, its flow stress increases gradually until failure. However, many problems pertaining to the deformation mechanism of the oriented single crystal remain unsolved.In the present study, to clarify the deformation mode of aluminum single crystals having multiple slip orientations, tensile tests were performed at various temperatures on oriented single crystals.At room temperature, the oriented single crystal deformed only by fine multiple slips and the flow stress increased with an increase of strain until failure. On the other hand, the coarse wavy slips composed of {111} and {100} slips occurred at high temperatures, such as 473K.In the single crystal having a tensile orientation deviated from by a few degrees, the clustered slips were observed in addition to the fine multiple slips. It is concluded that the deformation mode and flow stress of single crystal are very sensitive to the tensile orientation in the vicinity of .