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Through Ni-carbonyl CVD process, bulk Ni material (hereafter referred as CVD Ni) with bimodal grain structures, i.e. UF grains and large grains with UF/nano twins, can be produced. In tensile test, this material demonstrated a very good combination of strength and ductility as well as a special strain hardening behavior. Upon investigating the material's hardening behavior, a critical threshold strain, ec, of 0.02 was identified for the distinct change in the strain hardening behavior. Prior to this strain level, the strain hardening rate of the CVD Ni was found always higher than that of its counterpart coarse-grained (CG) Ni; but it dropped, after this strain, to the same level as the CG material. Furthermore, through the analysis of results from Tension Unloading Reloading-in-Tension (TURT) tests, this specific strain value was also found to be significant in the change of the hysteresis loop width as well as the back stress strain hardening responsible for the material's Bauschinger behavior. Microstructural evolution examination suggests that most of the twin boundaries (TBs) initially acted as obstacles to dislocation movement providing strong dislocation back stress and hence contributing to the strong hardening behavior. From the strain level of 0.02 and onwards, however, massive detwinning was detected which is responsible for the drop in the hardening rate. Finally, dislocation cell structures, that were typically evolved from entanglements of dislocation inside CG Ni, were also found inside the detwinned large grains of CVD Ni in the high strain regime, which yielded similar strain hardening behavior as in the CG Ni material.

The cyclic deformation response, CSSC behavior and microstructural evolution of oxygen free high conductivity polycrystalline copper samples were investigated under pure compression fatigue conditions. The results were compared with the observations made under symmetrical and pulsating tension fatigue. It was found that the cyclic deformation response, cyclic stress–strain curve and microstructural evolution of copper for pure compression fatigue condition are substantially different from those for push–pull fatigue. Unlike the symmetrical fatigue, rapid hardening and cyclic creep were observed in all tested compression fatigue conditions. To track surface micro-structural changes, optical and scanning electron microscopy (SEM), and atomic force measurement (AFM) were utilized. Dislocation structures were also examined using a transmission electron microscope (TEM) for different peak conditions after different numbers of cycles. The observations showed that cyclic plastic deformation accommodation by the material was not majorly contributed by the dislocation activities as evidenced by the very moderate slip activities on the surface as well as the lack of persistent slip band dislocation structures. Instead, cyclic creep was found to be the major form of plastic strain accommodation, which was found to be introduced though grain boundary activities, i.e. grain boundary extrusions.

The evolution of microstructure upon different annealing conditions in the Ni-carbonyl chemical vapor deposited (CVD) Ni material was investigated systematically in the present study by differential scanning calorimetry (DSC), optical microscopy and transmission electron microscopy (TEM) with both ex situ and hot-stage in situ annealing approaches. TEM observations reveal that the as-deposited CVD nickel possesses a bi-modal grain structure, with large columnar grains embedded in a nano-grained matrix. The large columnar grains are found to be full of ultrafine- and nano-growth twins; there are also scattered fivefold symmetry grains as large as 3–5 μm. Microstructure observations upon annealing show that grain growth did not occur until annealing at 400 °C or higher, and the nano-grain matrix grew into large equaxial grains above 600 °C. A major microstructural evolution phenomenon, the detwinning process was observed at 400 °C or higher temperatures, and twin boundary receding in the detwinning process was found to have a number of mechanisms all involving dislocation activities. Upon annealing, the ultrafine- and nano-twins were found to transform into dislocation cell structures and this phenomenon is considered to be driven by the excess free energy associated with the high density of grown-in twin boundaries. The large sized, fivefold twinned grains found in the as-processed bulk deposited Ni are found to be thermally stable up to 600 °C. Surface dragging effect to twin and grain boundaries is detected during the hot-stage in situ TEM observation.

Cyclic deformation response of Ti-5553 alloy with β–α bimodal structure is systematically studied through total strain controlled fatigue tests. Results on the relationship between mechanical response and microstructure evolution are presented in this report. It was found that cyclic hardening/softening behavior of the alloy depended strongly on the applied strain amplitude. At low strain levels, the material showed moderate hardening behavior at the early stage of cycling and then behaved elastically; whereas at high strain amplitudes, the alloy showed softening behavior from the beginning to the end of cycling. In the intermediate strain range, moderate hardening at the beginning followed by softening was detected. Transmission electron microscopy investigation revealed that such special macroscopic responses were due to the microstructure heterogeneity in the material. The activation and participation in the cyclic deformation of different constituents, namely the soft primary α p phase, the higher strength transformed β phase and the fine embedded α s precipitates in the β matrix played different roles at different strain levels and at different stages of cycling. Thus, the aforementioned specific cyclic hardening/softening behavior was introduced. Specifically, dislocation activities including activation of multiple slip systems and annihilation of pre-existing dislocations in the primary α p phase were found to play a very important role in the overall cyclic deformation behavior.

A systematic investigation of the cyclic deformation behavior of OFHC copper processed by 7 cycles of accumulative roll bonding has been carried out. The ARB copper shows a unique composite natured microstructure with components of various grain sizes. Upon cyclic deformation, a general cyclic softening trend was observed. Due to the composite natured of the ARB copper microstructure, several micro-mechanisms have been observed to have activated for cyclic strain accommodation. These mechanisms were found to be activated in sequence rather than concurrently. The first of the mechanisms to activate is believed to be slip found in the pre-existing large equiaxed grains. Following the exhaustion of the above, the pre-existing shear bands were re-activated and further developed. It is believed that the strain incompatibility between the grains within the shear band and those outside was partially responsible for the coarsening of grains along the shear bands. The cyclic softening observed is believed to be related to the grain coarsening along the path of strain localization. However, due to the limited ductility and the abundance of crack nucleation sites, the cyclic lifespan of this ARB material was found not long enough for the extensive grain coarsening necessary in the development of a macro shear band. In an attempt to understand the behavior observed, the consideration of using a composite model is also discussed.

The results of mechanical testing on ultra-fine grained aluminum processed by accumulative roll-bonding (ARB) were analyzed with TEM observation and in accordance of the microstructural evolution upon annealing. It was found that rapid grain growth, with the corresponding decrease in strength, did not occur until the annealing temperature of 200 °C or higher. The oxide rolled into the material near the bonding interfaces was seen to act as an obstruction for grain boundary migration across said interfaces. More interestingly, the strain near the interface due to the surface preparation technique used during ARB was found to form discontinuous segregates consisting of smaller grains formed during annealing or even ARB processing of higher number of cycles. Such phenomenon is attributed to recovery or polygonization due to the strain incurred. This study has also demonstrated that yield point phenomenon may be observed in a commercially pure fcc metal when the grain size is within a certain range.

The Ohno continuous casting (OCC) process has been used to generate 62% Bi–Sn alloy wires approximately 2 mm in diameter with eutectic morphology at speeds between 30 and 387 mm/min. The microstructure was examined and tensile tests performed for wires cast at various speeds. The deformation behavior was investigated by observing the surface structure of wires at various tensile deformation stages. It was found that the eutectic morphology was of a quasi complex-regular structure for wires cast at higher speeds, whereas at lower speeds it contained a remnant of Chinese script type structure. It was found that bismuth fracture begins before inter-phase cracking in the initial stages and proceeds concurrently with inter-phase cracks in the latter part of the deformation process, allowing the accommodation of large strain. For wires cast at higher speeds, the inter-phase cracks play a leading role in wire fracture, whereas for wires cast at lower speeds, the bismuth fracture exerts a leading role in crack propagation and fracture. This difference was manifested in the fracture surface and elongation value.

Transmission electron microscopy observations of the recovered slugs of electroformed copper liner materials that had undergone high-strain-rate deformation show the existence of a wide range of crystal defects, including vacancy clusters and porosity. Cellular structures formed by tangled dislocations and subgrain boundaries consisting of dislocation arrays were also detected. Electron backscattering Kikuchi pattern technique analysis reveals that the fibrous texture observed in the as-formed copper liners of shaped charges disappeared after explosive detonation deformation. In a specimen that had been plastically deformed at a normal strain rate (4×10 −4 s −1 ), a high density of dislocations was observed within grains. These experimental results indicate that dynamic recovery and recrystallization play an important role during high-strain-rate deformation by virtue of a temperature increase in the deformation process, whereas the conventional slip mechanism operates during deformation at the normal strain rates.

In order to understand the mechanism of sag behavior of engineering materials, systematic symmetrical and asymmetrical cyclic loading tests, rather than stress relaxation and the Bauschinger effect test, were carried out with two model materials, namely industrial pure iron and spheroidized mid-carbon steel, under different pre-strain conditions. It was found that the applied mean stress had a significant effect on the amount of cumulative cyclic creep strain and it could also strongly extend the secondary creep stage. It was also found that the Bauschinger effect caused by pre-strain would become negligible in cyclic deformation after a certain, but small number of cycles. This indicates that Bauschinger effect test cannot thoroughly reflect the nature of sagging behavior. Finally, both increase in the pre-strain level, and decrease in the cyclic peak stress level resulted in reduced amount of energy that could be recovered by cyclic deformation from the total energy accommodated during pre-plastic straining. On the other hand, it is such unrecovered energy that would make the material enter a stable stage of dislocation substructure earlier during cycling.

Deformation processes of nanocrystalline (6–40 nm) nickel produced by an electrodeposition technique were studied. First, the results of unidirectional tensile tests were discussed with respect to the deviation from the Hall-Petch relationship. It was suggested that such a mechanical behavior exhibited by nanocrystalline materials could be described by a composite model proposed previously. Further experimental work on static and dynamic creep tests under the load control condition showed that nanocrystalline nickel electrodeposits exhibited a significant room temperature creep behavior. It appeared that grain boundary sliding and diffusive matter transport within the intercrystalline region played an important role in terms of deformation mechanisms of nanocrystalline materials. The contributions of dynamic creep to stress-strain behavior and, in turn, to the assessment of the Hall-Petch relationship for nanocrystalline materials are discussed.

In this study, the microstructures and mechanical properties of underaged and peak-aged 6061 Al Al 2 O 3 particulate composites were investigated. In the underaged composites, small spherical zones were observed and, in the peak-aged composites, needle-shaped Mg 2 Si precipitates were observed throughout the matrix. The stress-strain responses of the 6061 Al matrix composites were observed to be similar to those of commercially pure Al matrix composites reinforced with 10 vol.% Al 2 O 3 although 6061 Al matrix composites were found to be stronger. Unlike 7xxx Al matrix composites in which the flow stress fell off at high strain rates, the flow stress of 6061 Al matrix composites increased rapidly at high strain rates. The work hardenability of the underaged composites was found to be greater than that of the peak-aged composites. Dislocations were observed to be distributed rather homogeneously irrespective of the matrix microstructure. Some Al 2 O 3 particulates were observed to crack when the flow stress exceeded the critical flow stress, namely 415 MPa. Pores frequently observed near cracks in Al 2 O 3 of the composites suggest that cracks may initiate and grow easily in the presence of pores.

Cyclic creep and fracture behaviour of SAE spring steel 5160 under two prestrain conditions were studied. The cyclic creep rate, cyclic plastic strain range, total plastic strain range and cyclic creep life of the material were all found to depend strongly on the prestrain condition. It was also found that, after a certain number of cycles, cyclic hardening would reach a saturation state, followed by cyclic softening which would last until the final failure. However, a higher prestrain delayed the appearance of cyclic softening. Prestraining was also found to extend considerably the third stage of cyclic creep and thus increase the elongation to fracture of the material to the extent observed in a uniaxial tension test. Fracture surface study exhibited a mixed mode failure process and also a transition from quasi-brittle fracture at the lower prestrain, consisting of both intergranular and transgranular features, to ductile fracture at the higher prestrain, which is mainly made of dimples and is similar to that found in a uniaxial tension test. However, in the ductile fracture area, some transgranular facets and fatigue striation regions could still be observed. Finally, when the prestrain reached a certain value, compressive cyclic creep was detected and this is attributed to a strong Bauschinger effect.

Cyclic deformation behavior of double-slip oriented Cu single crystals with a stress axis in the [034] direction was investigated under plastic strain control mode for a shear strain amplitude range of 1 x 10(-4) to 5 x 10(-3). Dislocation structures in the tested samples were observed using a transmission electronic microscope. It has been found that the effect of the operation of critical slip in these [034] crystals on cyclic responses and dislocation structures is nearly the same as that of increase in strain amplitude. The nucleation stress and number of cycles for PSB formation at each specific strain amplitude in the double-slip oriented crystals were found to be both considerably lower than those observed in single-slip oriented crystals. This observation is in a good agreement with the Kuhlmann-Wilsdorf and Laird analysis, in that the formation of PSBs is associated with glide behavior on the secondary slip system. A dislocation ''cord'' structure has also been observed and is believed to be caused by the operation of the cross-slip system during cyclic deformation, Labyrinth wall structures were found to form with increase in strain amplitude by the operation of critical slip and cross-slip systems. However, the formation of labyrinth structure was suppressed by the coplanar slip at high strain amplitudes.

Elastic-plastic finite element simulations have been carried out to determine the effect of the presence of single particles and particle clusters ahead of the crack tip on the stress distribution at the crack tip. The resulting knowledge about stress distribution has been used to provide insight regarding the predominant factors that influence crack propagation. This study indicates that the presence of particle(s) relatively far away from the crack tip increases the crack opening stress at the crack tip. The crack opening stress is, however, subdued as the crack approaches the particle(s). Particle clustering influences the stress distribution by suppressing plastic deformation inside the cluster and hence plastic deformation occurs predominantly outside the cluster. Experimental observation of slip bands and crack propagation characteristics in SiC-reinforced aluminum A356 confirms this fact. It is also found that particle clustering creates higher interface shear stress and von-Mises stress regions in the particle away from crack tip, while the highest interface normal stress region exists at the interface of the particle along the crack line. Therefore, depending upon the stress that predominates, the crack can either deviate and go around the cluster region or go straight through the cluster region.

A study has been made of the cyclic stress-strain response of 70-30 alpha brass single crystals oriented for easy glide. The cyclic hardening rate of the material was found to change in an alternating fashion. Surface observation revealed that (i) under a constant plastic strain amplitude slip band intensity may change at different numbers of cycles, indicating a plastic strain transfer behavior; and that (ii) cyclic strain is accomodated by the whole gauge length. The cyclic stress-strain curve obtained has three regions: a low hardening rate region, a very high hardening rate regime and a short plateau.

Single-cycle tests, i.e. tension-unloading-compression (TUC) and compression-unloading-tension (CUT) tests, have been carried out along the axial direction of a modified Zr-2.5wt.%Nb pressure tube material to study the effect of direction of the applied stress and plastic prestrain on the Bauschinger effect. In both the tension-first and compression-first tests the material exhibits a very large Bauschinger effect. Two different methods were used to determine the back stress: one incorporated the effect of residual stresses and the other isolated the contribution of residual stresses. Calculation of back stresses using the two respective methods indicated that the large Bauschinger effect is attributable mainly to the residual stresses; these residual stresses are introduced during the preloading half-cycle as a result of the highly anisotropic mechanical properties of the zirconium grains. In the tension-first tests, as expected, the reverse (compressive) flow stress σ f c always decreases as the amount of prestrain is increased. However, for compression-first tests the reverse (tensile) flow stress σ f t only decreases initially with increasing prestrain; thereafter, for higher prestrains, σ f t is observed to increase with increasing prestrain. On the basis of research carried out by various workers on the deformation mechanisms operating in zirconium alloys, it is apparent that twinning must have occurred during the compression part of the cycle, resulting in grain reorientation such that the reoriented grain is more difficult to deform.

The characteristics of short fatigue crack growth from a sharp notch and the behavior of the J-R curve of 10% Al2O3 particulate-reinforced Al(2014) alloy were studied through three-point bend tests. A new single-specimen method for determining the J-R curve was used which employs a cyclic-monotonic alternate loading procedure. The results obtained using this method were similar to those obtained from conventional testing. By considering the results of optical observations, scanning electron microscope fractography and microhardness measurements, it was found that there are several regimes of crack growth behavior, and each regime shows distinct crack-extension mechanisms.

The anistropic mechanical behavior of a modified Zr-2.5wt.%Nb pressure tube material has been studied. Based upon the stress-strain relationships in the three principal orthogonal directions of the tube, namely axial, circumferential and radial directions, a series of constitutive equations in the form of power law and Ramberg-Osgood equations have been established. The use of the Ramberg-Osgood equation is more advantageous owing to the fact that a single equation can be used to describe the behavior of a material in the elastic region, the transition region, and the fully developed plastic region, while the power law equation can only be used to describe material behavior in the fully developed plastic region. A better Ramberg-Osgood curve fit is obtained when the parameters are calculated from ln-ln plots as compared with the three parameters method. To further demonstrate the effectiveness of the ln-ln plot method, it was used to obtain the parameters of the Ramberg-Osgood equations for CANDU reactor Zircaloy-2 calandria tubes (a calandria tube is placed concentric to the pressure tube in the nuclear reactor; the gap between these two tubes is maintained by spacers) and standard Zr-2.5Nb pressure tubes, in tension. However, it was found that there are instances where a single Ramberg-Osgood equation cannot adequately describe the full range of deformation when the material showed complicated strain hardening behavior. In such cases, two Ramberg-Osgood equations are used to describe the material behavior. In addition to the establishment of constitutive equations, an attempt has been made to explain the deformation mechanisms responsible for the highly anistropic behavior of modified Zr-2.5Nb pressure tube material based upon previous work of various researches on single crystal, textured polycrystalline zirconium, Zircaloy-2 and Zircaloy-4.

Fatigue fractography of cast particulate-SiC-reinforced Al(A356) metal matrix composites was carried out. In the low cycle regime, the features on the fracture surfaces, mainly distorted dimples and cracked and also debonded SiC particles, are similar to those found in uniaxial tension. However, in the high cycle regime, extensive slip behavior in the aluminum matrix and relative movement between the two sides of the fracture surface produced some special appearances, namely large flat aluminum matrix areas with characteristic line patterns and step-like fracture surface regions. Mechanisms for the formation of the above phenomena are proposed. In particular, the constraint exerted by reinforcing particles distributed along grain boundaries on the cyclic deformation as well as fatigue crack propagation in the corresponding aluminum grains is discussed. In all samples studied, it is found that fatigue cracks were usually initiated at processing defects, such as casting shrinkage and unwetted SiC particle clusters.

Asymmetry behavior between tension and compression, i.e. stresses in compression are larger than those in tension, has been observed frequently in cyclic deformation. In order to investigate this behavior, a parameter, the asymmetry factor, has been defined to measure the degree of asymmetry in copper single crystals. It is found that the asymmetry factors “scatter” widely during rapid hardening and converge to the commonly accepted imbalance of about 5% between tension and compression after the crystals reach saturation. The causes of these behaviors are explored by studying crystals of different orientation and model experimental systems. The asymmetry is explained in terms of several factors, but the dominant one in saturation appears to be deformation constraint associated with strain localization. At long to medium life the strain localization takes the form of persistent slip bands, but other forms of localization can affect the asymmetry behavior at short life.

Cyclic softening occurs when a prestrained metal is subsequently fatigued at strain amplitudes sufficient to produce microslip in the inherited dislocation structures. The dependence of this threshold on the nature and magnitude of the prestrain is reviewed along with the changes which occur in the cyclic mechanical properties and the dislocation structures. It is shown that cyclic softening is a recovery process induced by cyclic strain and that it leads to low energy dislocation structures. Speculation on the relation of cyclically softened structures to the Dickson wall model is offered. New results on the dislocation structures produced in high-low amplitude block tests suggest softened structures are self-organizing non-equilibrium structures, and the connection of these with low energy structures is indicated.

Masing behavior was investigated based upon the observed shapes of hysteresis loops in polycrystalline copper cycled with different loading methods. It was found that ramp-loaded specimens showed strong Masing behavior, whereas specimens tested in load control behaved much less in this aspect. To understand the above results, an elastic-perfect plastic multiphase model was given and friction stress and back stress analyses were carried out in support. The mechanism and results given in this letter are consistent with other results on cyclic deformation which contribute to understanding Masing behavior.