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A solid with larger sound speeds exhibits higher lattice thermal conductivity (k_{lat}). Diamond is a prominent instance where its mean sound speed is 14400 m s-1 and k_{lat} is 2300 W m-1 K-1. Here, we report an extreme exception that CuP2 has quite large mean sound speeds of 4155 m s-1, comparable to GaAs, but the single crystals show a very low lattice thermal conductivity of about 4 W m-1 K-1 at room temperature, one order of magnitude smaller than GaAs. To understand such a puzzling thermal transport behavior, we have thoroughly investigated the atomic structure and lattice dynamics by combining neutron scattering techniques with first-principles simulations. Cu atoms form dimers sandwiched in between the layered P atomic networks and the dimers vibrate as a rattling mode with frequency around 11 meV. This mode is manifested to be remarkably anharmonic and strongly scatters acoustic phonons to achieve the low k_{lat}. Such a dimer rattling behavior in layered structures might offer an unprecedented strategy for suppressing thermal conduction without involving atomic disorder., four figures and one table in the main text

Supplemental material, DS_10.1177_0022034520908533 for Global, Regional, and National Levels and Trends in Burden of Oral Conditions from 1990 to 2017: A Systematic Analysis for the Global Burden of Disease 2017 Study by E. Bernabe, W. Marcenes, C.R. Hernandez, J. Bailey, L.G. Abreu, V. Alipour, S. Amini, J. Arabloo, Z. Arefi, A. Arora, M.A. Ayanore, T.W. Bärnighausen, A. Bijani, D.Y. Cho, D.T. Chu, C.S. Crowe, G.T. Demoz, D.G. Demsie, Z.S. Dibaji Forooshani, M. Du, M. El Tantawi, F. Fischer, M.O. Folayan, N.D. Futran, Y.C.D. Geramo, A. Haj-Mirzaian, N. Hariyani, A. Hasanzadeh, S. Hassanipour, S.I. Hay, M.K. Hole, S. Hostiuc, M.D. Ilic, S.L. James, R. Kalhor, L. Kemmer, M. Keramati, Y.S. Khader, S. Kisa, A. Kisa, A. Koyanagi, R. Lalloo, Q. Le Nguyen, S.D. London, N.D. Manohar, B.B. Massenburg, M.R. Mathur, H.G. Meles, T. Mestrovic, A. Mohammadian-Hafshejani, R. Mohammadpourhodki, A.H. Mokdad, S.D. Morrison, J. Nazari, T.H. Nguyen, C.T. Nguyen, M.R. Nixon, T.O. Olagunju, K. Pakshir, M. Pathak, N. Rabiee, A. Rafiei, K. Ramezanzadeh, M.J. Rios-Blancas, E.M. Roro, S. Sabour, A.M. Samy, M. Sawhney, F. Schwendicke, F. Shaahmadi, M.A. Shaikh, C. Stein, M.R. Tovani-Palone, B.X. Tran, B. Unnikrishnan, G.T. Vu, A. Vukovic, T.S.S. Warouw, Z. Zaidi, Z.J. Zhang and N.J. Kassebaum in Journal of Dental Research

The high-cycle fatigue (HCF) behaviors of wrought Al alloys are vitally important for their industrial application. To optimize the fatigue property of the wrought Al alloy, the effects of aging conditions on the fatigue properties of 7N01, 7075 and 2024 Al alloys were studied. The results show that the overaged samples have the lowest tensile strength among the three aging states, but their fatigue strength is the highest. Further investigations on the dislocation morphologies indicates that the dislocations are easy to accumulate and annihilate at the second phase boundaries for the overaged state, which reduces the fatigue damage degree at the grain boundaries. Besides, the discontinuous distribution of the grain boundary precipitates and the increased width of the precipitate-free zone also contributed to the improved fatigue property of the overaged state. Those results should be significantly important for the optimization of their fatigue properties and the industrial application of wrought Al alloys.

The strengthening mechanisms and the strength degradation behaviors of the high-strength Al–Fe wire were investigated in this study. The dispersed nano-scale precipitates, the texture and the fine grains in the radial section of the Al–Fe wire are the three main strengthening factors, i.e., precipitation strengthening, texture strengthening and grain-boundary strengthening. Besides, during the annealing process, the strength degradation behaviors of the Al–Fe wires due to the coarsening of precipitates and grains as well as the texture evolution were quantitatively calculated. Finally, the strength degradation mechanisms were divided into three stages by the annealing temperatures. In stage I (0–90 °C), the slight decrease of strength is attributed to the recovery of defects including vacancies and dislocations. In stage II (90–200 °C), the loss of strength is mainly caused by the coarsening of precipitates leading to a decrease of precipitation strengthening. In stage III (200–300 °C), the grain growth is responsible for the strength degradation of the Al–Fe wire. Besides, the thermo-stability of grain increases to 150 °C in the Al–Fe wire due to the precipitates formed at the grain boundaries.

In order to optimize the fatigue strength of 7075 Al alloy, the hardness, tensile and fatigue tests as well as the microstructure observation under different aging conditions were investigated. The result indicates that, with the aging time increasing, the precipitation spacing first decreases and then increases, which leads to the hardness and tensile strength first increasing and then decreasing. With the increase of yield strength, the fatigue strength increases first and then decreases, resulting in an optimization of fatigue strength. Especially, the specimen aged at 120 °C for 48 h with strength and elongation in middle of the three specimens exhibits the highest fatigue strength of 165 MPa. Considering the effect of yield strength on fatigue strength and fatigue damage, the causes for the parabolic variation of fatigue strength were analyzed.

The strength and plasticity of metallic materials usually exhibit a trade-off relation. This study reports a simultaneous improvement in the ultimate tensile strength (UTS) and uniform elongation (UE) of Ti-24Nb-4Zr-8Sn (Ti2448) fabricated by selective laser melting (SLM), relative to those produced via forging. Detailed microstructural characterization reveals that the outstanding tensile property may result from the bi-model structure that forms during the rapid cooling associated with SLM. Coarse grains are surrounded by fine grains within the melt pool, which causes a back stress during tension. The back stress provides additional strain-hardening capacity, which postpones the initiation of necking and then leads to the simultaneous improvement of the strength and plasticity (SISP) of the Ti2448 alloy. Furthermore, the tensile property of the SLM-fabricated sample is anisotropic which is strongly related to the irregular shape of the melt pool. Keywords: Selective laser melting, Strength and plasticity, Strain hardening, Bi-model structure

Synchronous improvement of strength and plasticity (SISP) is realized for the first time in a precipitated-phase strengthened Ni-Co based superalloy through reducing stacking fault energy (SFE). Microstructure characterizations indicate that, with lowering SFE, the dislocation slip mode transforms from wavy to planar slip, meanwhile, the twinnability is also improved. Both the planar slip mode and the increased twinnability enhance the strain-hardening capability of the superalloy, which postpones the initiation of necking. Thus, the SISP in the double-phase superalloy is achieved.

In this study, a criterion for optimizing the fatigue strength of metallic materials is proposed, and verified by many experimental results. The strategy for optimizing the fatigue strength is to continuously increase the ultimate tensile strength (UTS) under the condition of keeping certain work-hardening abilities. It is found that the relationship between the true UTS and true uniform elongation can be described by two straight lines, and the optimal fatigue strength can be obtained at the point where the two straight lines intersect with each other. This study has great guiding significance for economizing time and money to optimize the fatigue strength of metallic materials.

Two Fleischer cross slip modes of a dissociated screw dislocation 〈1 1 0〉/2 are quantitatively investigated in silver by molecular dynamics simulations. It is shown that contrary to previous studies where the acute angle mode is always preferred, the obtuse angle mode is also obtained. Our theoretical analysis demonstrates that the transition of the preferred mode in simulations arises from the dissociation width between the conjugate partials on the primary plane. This implies that all the interactions of dislocations have to be considered to describe the process of cross slip.

Surface strengthening is an important method which can effectively improve the fatigue property of metallic materials. In order to improve the fatigue property, for the first time, a new developed surface spinning strengthening-II (3S-II) method and the traditional shot peening (SP) treatment were applied to modify the microstructure and fatigue performance in the surface layer of 50CrMnMoVNb spring steel. It is interesting to find that the fatigue lives of the samples treated by the 3S-II method show obvious polarization characteristics (either quite higher or slightly lower than those treated by SP method) at the high stress amplitude, which does not follow the traditional life distribution rules and cannot be evaluated by the S-N curve. Moreover, the fatigue lives of the samples with the fatigue crack initiated from the surface defects are extremely lower than those of the samples whose fatigue crack initiated from the subsurface. Therefore, a fatigue risk factor, "Rf", was proposed and developed based on the competition between the applied stress and the material property to evaluate the fatigue cracking risk of the surface strengthened 50CrMnMoVNb spring steel, especially the fatigue risk of the samples influenced by the mixed factors. The calculation results show that the position where Rf max appears is exactly the position of fatigue crack initiation. As a result, "Rf" can be used to quantitatively evaluate the fatigue risk of metallic material under the influence of a variety of factors, including surface strengthening, surface roughness and residual compressive stress and so on.

The evolution of microstructure in the drawing process of commercially pure aluminum wire (CPAW) does not only depend on the nature of materials, but also on the stress profile. In this study, the effect of stress profile on the texture evolution of the CPAW was systematically investigated by combining the numerical simulation and the microstructure observation. The results show that the tensile stress at the wire center promotes the formation of texture, whereas the shear stress nearby the rim makes little contribution to the texture formation. Therefore, the texture at the wire center is stronger than that in the surface layer, which also results in a higher microhardness at the center of the CPAW under axial loading.

In previous work, bond-order potentials (BOPs) have been used to establish a general yield criterion, different from the Schmid law, for the glide of 〈1 1 1〉/2 screw dislocations on {1 1 0} planes in body-centered cubic metals, while their applications are restricted to the 2D simulations at 0 K because of their low computational efficiency. On the other hand, embedded-atom-method (EAM) potentials have been generally employed for long dislocation segments at finite temperatures, but there is no study to clarify whether or not they can reproduce the yield criteria revealed by the BOPs. In this work, systematic atomistic simulations with an EAM potential have been performed to calculate the behaviors of 〈1 1 1〉/2 screw dislocations in α-iron under different loading conditions. We find that at 0 K, the simulation results can be well explained by the general yield criterion when the glide is restricted to {1 1 0} planes. Under uniaxial loadings, the activated slip systems are consistent with the experimental observations. At finite temperatures, as a preliminary attempt, the influence of the non-glide stress on the slip planes is presented, which could not be rationalized by the yield criterion at 0 K because extra effect from temperatures has come into the picture.

The surface strengthening behaviors of cold-rolled (CR) Cu, fully recrystallized (FR) Cu and partially recrystallized (PR) Cu with heterogeneous microstructures were investigated by using the surface spinning strengthening (3S) method. It was found that the heterogeneous microstructure significantly affects surface strengthening behaviors as the fine grains have good plastic deformation ability, the ultrafine grains can bear a high stress, and the whole hardened layer can be divided into grain refinement and deformation zones. The maximum microhardness is 173 HV across all Cu because all the grains were refined to the nano-scale with an average size of 40 nm in the topmost layer of the grain refinement zone. The fine grains in the deformation zone of the PR Cu deform plastically to generate a thick hardened layer. In addition, the surface strengthening behaviors and mechanisms are discussed in detail.

The effects of electropulsing treatment (EPT) and annealing on the tensile behaviors of twinning-induced plasticity (TWIP) steel after interrupted tension are compared. It is found that EPT effectively restores the tensile property of specimens with macro voids completely, while annealing could not. The selective heating and thermal compressive stress induced by EPT around voids make them readily to be healed while keeping the matrix unchanged. The healing effect of voids improves the strain-hardening capability of TWIP steels and leads to the recovery of strength and elongation of the EPT specimen, relative to those annealed counterparts.

Tensile tests and microstructure observations were conducted for two types of twinning-induced plasticity steels, Fe-22Mn-0.6C and Fe-30Mn-3Si-3Al (wt pct), from 293 K to 443 K. With increasing temperature, Fe-22Mn-0.6C steel exhibited enhanced mechanical properties and stable twinning capability, but Fe-30Mn-3Si-3Al steel displayed a decline on its mechanical properties and twinning capability. Mechanisms for the different mechanical responses were analyzed by assessing the dynamic strain aging effect.

As a significant scientific problem directly impacting on the long-term safety of engineering materials and facilities, the improvement of fatigue strength under fully-reversed cycling was comprehensively explored in this study. Advantageous material characteristics for the improvement of fatigue strength were summarized from the achievements of the previous researches, followed by a new attempt to combine them in material design. As the model material, α-Cu-Al alloys with clean ultrafine-grains as well as large proportions of twin boundaries were thus produced, which exhibited a notable fatigue strength improvement (up to 155% higher than the coarse-grained counterparts and 40% higher than the counterparts produced by severe plastic deformation). Furthermore, a general principle briefly summarized as localized fatigue damage reduction was proposed based on the analysis of the optimizing methods including microstructure optimization and composition optimization. Accordingly, several recommended features to obtain such high fatigue strength materials were finally listed for further anti-fatigue design, such as uniform grains with small size and stable boundaries; low initial dislocation density, and proper alloying composition.

The effect of nitrogen addition on the mechanical properties of Fe–22Mn–0.6C (wt%) twinning-induced plasticity steel was studied. It was found that the stacking fault energies of the two steels were comparable, and the twinned grain fractions of FeMnC and FeMnC-N steels were similar before the true strain of 0.5. With increasing the strain to 0.7, the fraction of secondary twinned grain rose to support the further strong work-hardening rate of FeMnC-N steel. Moreover, the nitrogen addition suppressed the dynamic strain aging, which can trigger early shear fracture in FeMnC steel. Therefore, the work-hardening rate was kept increasing and the frequency of plastic instability was suppressed by nitrogen addition. As a result, the ultimate tensile strength and uniform elongation simultaneously increased in FeMnC-N steel.

The tensile, high-cycle fatigue (HCF) and fatigue crack growth (FCG) rate tests of Fe-30Mn-0.9C and Fe-30Mn-0.3C twinning-induced plasticity (TWIP) steels were performed. Meanwhile, the corresponding surface damages, fatigue fracture morphologies and microstructure evolutions were also investigated. It is detected that both the fatigue strength and FCG resistance of Fe-30Mn-0.9C steel are higher than those of Fe-30Mn-0.3C steel, because Fe-30Mn-0.9C steel possesses higher yield strength, plasticity and slip planarity than Fe-30Mn-0.3C steel. Furthermore, it is proposed that, for the Fe-Mn-C TWIP steel, increasing the C content to enhance the short range order (SRO) would lead to the synchronous increment in the fatigue strength and FCG resistance. This study may have a certain guiding significance for the selection of materials to against fatigue fracture.

Although the Charpy impact test has been routine for decades to assess the ductile or brittle nature of materials, the impact toughness, which is strongly sample-thickness dependent, is not an intrinsic property. By re-examining the energy absorption during fracturing of relatively high strength alloys, here we find a remarkably good linear relation between the impact energies and the fracture surface areas of samples with different thickness, and the slope essentially renders the intrinsic impact toughness. The new findings, which also provide a scaling law to well predict the thickness effect on the traditional impact toughness, may have broad applications for precisely determining the ductile-to-brittle transition temperature of small-dimensional devices, selecting materials according to their toughness at the thickness in usage, and evaluating the intrinsic toughness of emerging high strength materials with limited achievable size.

Solar powered unmanned aerial vehicle (UAV), achieving a long time flight, has been drawn attention. The energy management is a dominate role to determine the comprehensive performance of the solar powered UAV. In this work, a three dimensional numerical model is built to study the heat and mass transfer in the solar powered UAV at the extreme condition. The effects of flight velocity, flight attitude, day and light on the heat and mass transfer in the solar powered UAV are investigated in details. Then, an improved energy management strategy is proposed to improve the efficiency of the energy utilization for UAV. Results show that the temperature in the solar powered UAV is affected by the flight altitude. When the flight altitude varies from 11 km to 20 km, the temperature of the battery has approximately decreased by 5 K. The improved thermal management system can keep the battery temperature at about 220 K. The minimum temperature of the battery approximately increases by 50 K compared with the traditional structure. The heat transfer in the solar powered UAV is strengthened with the improved structure. Besides, the outer surfaces of the UAV are also influenced by the improved structure with the minimum temperature of the solar panel increased by 3 K. The data obtained from the analysis of the simulation results are helpful to the optimal design of global energy management.

The gradient layer induced by surface mechanical strengthening attracts plenty of attention as it can effectively improve the service performance of metallic materials, but how to design a suitable gradient layer and evaluate gradient layer properties need to be investigated further. In this work, the 316 stainless steel and TC4 alloy processed by surface spinning strengthening (3S) were investigated, including the work-hardening capacity before the 3S treatment and gradient layer properties after the 3S treatment. Results show that 316 stainless steel has a higher work-hardening capacity compared to the TC4 alloy before the 3S treatment; the maximum microhardness increment ratio, thickness of gradient layer and surface strengthening energy are also higher, and surface strengthening exponent is lower in the 316 stainless steel compared to the TC4 alloy after the 3S treatment. In addition, the exponential relation between the maximum microhardness increment ratio and work-hardening exponent, the exponential relation between the thickness of gradient layer and work-hardening exponent, the power function relation between the surface strengthening exponent and work-hardening exponent, and the linear relation between the surface strengthening energy and surface strengthening exponent are studied, respectively. Based on the above results and analysis, the microhardness distributions of gradient layer were discussed based on the work-hardening capacity, and the effects of the work-hardening capacity of the as-received metallic materials on the gradient layer properties of the surface strengthened metallic materials were summarized.

We report that materials with similar tensile properties can also exhibit quite different low-cycle fatigue (LCF) performances. Experimental results demonstrate that the LCF properties of twinning induced plasticity (TWIP) steels are naturally dominated by microscopic deformation mechanisms (mainly dislocation slip mode), which slightly influences the initial work hardening. However, such slight difference in the initial work hardening (the butterfly effect), corresponding to different damage mechanisms, accumulates and enlarges cycle by cycle during fatigue, finally leading to wide variations in cyclic stress response and fatigue life.

Pure molybdenum (PM) and La2O3 dispersion strengthened Mo alloy (ODS-Mo) were prepared by powder metallurgy through two different mixing method (solid-solid or SS mixing and solid-liquid or SL mixing). After annealed at 1050 °C for 1 h, the PM, SS-Mo and SL-Mo were compared in microstructure and mechanical properties. The microstructural examinations showed that the PM was mostly recrystallized with rather coarse grains, but the ODS-Mo alloys remained fine elongated grains. This discrepancy is due to a higher recrystallization temperature held in the ODS-Mo alloys. The uniaxial testing results showed that the ODS-Mo had higher tensile mechanical properties compared to the PM, which are attributed to the remarkable strengthening and ductilizing effect induced by the La2O3 particles. The low cycle fatigue (LCF) testing results revealed that the PM experienced cyclic hardening behaviors, while the ODS-Mo alloys exhibited cyclic softening behaviors. The Basquin-Manson-Coffin analyses results demonstrated that the ODS-Mo possessed a higher fatigue ductility and longer fatigue life than the PM. Two kinds of cracks, i.e., cleavage cracks and intergranular cracks, were experimentally observed to coexist on the fracture surface. In particular in the SL-Mo alloy, the two crack interacted and propagated forward until final fracture, leading to a step-like crack growth path and concomitantly enhanced fatigue ductility. In addition, different cycling dislocation structures were revealed between the PM and ODS-Mo alloys. Cycle fatigue mechanisms responsible for the fatigue behaviors and microstructure evolution during LCF testing were discussed. The best LCF resistance combined with superior uniaxial tensile mechanical properties found in the SL-Mo alloy were rationalized.

Strength degradation during the electron transmission process is always a hidden danger to the overhead transmission lines. In this study, the microstructure evolutions and the strength degradation behaviors of a cold-drawn commercially pure Al conductor (CPAC) were investigated systematically by a series of annealing experiments. The results show that the texture evolution, dislocation recovery and subgrain growth during the recrystallization should be responsible for the strength degradation of CPAC. Besides, the microstructure evolution depends on the annealing temperature. For instance, some of the texture was changed into the one in the CPACs annealed at a temperature of 90 °C; while, there is an obvious increase in the subgrain width when the CPACs were annealed in the high temperature range from 150 °C to 300 °C. Finally, the strength degradation due to the texture evolution and the subgrain coarsening was quantitatively calculated based on the crystallographic analysis.

It is confirmed that the fatigue strength of high-strength metals would not continuously increase as the tensile strength enhances. In order to find out the optimal fatigue strength of maraging steel, the tensile and cyclic pull-push (R = − 1) loading tests of 18Ni maraging steel (250 grade) aged under different conditions were carried out in the present study. Meanwhile, the fracture surface and microstructural differences were also investigated. It is detected that decreasing the ageing temperature can bring about the decrement in interparticle spacing and austenite fraction, which would further improve the tensile strength and change the tensile fracture mode. Increasing tensile strength can effectively improve the fatigue strength in the low-strength region. However, as the strength further increases, the intergranular fracture would happen, which means intense deformation inhomogeneity at grain boundaries, contrarily causing the decrement in fatigue strength. Finally, it is believed that the optimal fatigue strength can be obtained in the high-strength region where there is a transition in the fracture mode. Moreover, it is found that the specimen aged at 550 °C for 5 h with an ultimate tensile strength of 1838 MPa exhibiting the transgranular fracture possesses the highest fatigue strength of 685 MPa.

A high efficient magnetic levitation melting technique was reported for fabricating bulk equiatomic CoCrFeMnNi high-entropy alloy (HEA) ingot with a diameter of 110 mm. The bulk ingot can be either forged or rolled. In particular, fully recrystallized ultrafine-grained (UFG) HEA with a minimum grain size of 503 ± 181 nm was successfully obtained through a simple cold rolling and annealing process. The tensile properties of the HEA were studied by changing the grain size from the UFG regime to the coarse-grained regime. The UFG HEA exhibited a good balance of strength and ductility due to the low stacking fault energy. The linear Hall-Petch relationship was well fitted when the grain sizes range from 503 nm to 88.9 μm. Keywords: High-entropy alloy (HEA), Ultrafine grain, Hall-Petch relationship, Strength, Ductility

We find the existing empirical relations based on monotonic tensile properties and/or hardness cannot satisfactorily predict the low-cycle fatigue (LCF) performance of materials, especially for twinning-induced plasticity (TWIP) steels. Given this, we first identified the different deformation mechanisms under monotonic and cyclic deformation after a comprehensive study of stress–strain behaviors and microstructure evolutions for Fe-Mn-C alloys during tension and LCF, respectively. It is found that the good tensile properties of TWIP steel mainly originate from the large activation of multiple twinning systems, which may be attributed to the grain rotation during tensile deformation; while its LCF performance depends more on the dislocation slip mode, in addition to its strength and plasticity. Based on this, we further investigate the essential relations between microscopic damage mechanism (dislocation–dislocation interaction) and cyclic stress response, and propose a hysteresis loop model based on dislocation annihilation theory, trying to quickly assess the LCF resistance of Fe-Mn-C steels as well as other engineering materials. It is suggested that the hysteresis loop and its evolution can provide significant information on cyclic deformation behavior, e.g., (point) defect multiplication and vacancy aggregation, which may help estimate the LCF properties.

An unusual necking behavior was found in twinning-induced plasticity (TWIP) steels during tensile tests, which is quite different from that observed on most ductile metals. A sharp drop of the strain-hardening rate (Θ) arises before necking initiation, rather than after it, leading to the premature necking of TWIP steels. Through carefully examining the evolution of macroscopic defects at various tensile strains using three-dimensional X-ray tomography (3D-XRT), this premature necking behavior was attributed to the multiplication of macroscopic voids during plastic deformation. Combining with the previous theories and present characterizations on the evolution of macroscopic voids, the mechanism of the unusual necking behavior in TWIP steels was quantificationally revealed.

Surface strengthening is an effective method to improve the service life of some important engineering components. In this study, four structural steels with different yield strength were treated by surface spinning strengthening (3S) technology, which created a hardened layer in the surface layer by applying the shear stress and compressive stress. As a result, the grains in the surface layers were refined to different dimensions with the increase of the depth to the surface, and the grains in the topmost surface layer were refined to nano-scale. The microhardness in the hardened layer increases dramatically due to the grain refinement and strain strengthening. Furthermore, an exponential-type microhardness model for the hardened layer was proposed to characterize the surface strengthening behaviors of the four structural steels with different yield strength. Especially, in the model, some parameters related to the nature of the hardened layer, including Hv M /Hv m (maximum microhardness increment ratio), λ (thickness of the hardened layer), R (surface strengthening exponent) and E (surface strengthening energy) were analyzed on the basis of the microstructure and microhardness evolution in the hardened layer. Results show that with the increase of the yield strength, Hv M /Hv m , λ and E/Hv m are decreasing, while R is increasing. The metal with a low yield strength is easy to obtain a thicker hardened layer and a better surface strengthening effect, i.e., higher surface strengthening energy and lower surface strengthening exponent.

Tensile tests were carried out to investigate the differences in fracture mechanisms between Fe-22Mn-0.6C and Fe-30Mn-3Si-3Al (wt pct) twinning-induced plasticity steels. Although both steels possess a strong twinning capability during tensile deformation, they display different tensile fracture modes of shear and necking. The Portevin–le Chatelier band is proposed as the key factor influencing the different fracture mechanisms.

Compared to stress controlled high-cycle fatigue, enhancing strain controlled low-cycle fatigue (LCF) properties of material is much more difficult and less reported. In this study, we introduced two strategies and technologies to improve the LCF performance of Fe-Mn-C twinning induced plasticity (TWIP) steel. One is grain refining without introducing residual stress by traditional cold rolling and following recrystallization annealing (FG sample); the other is to introduce a linear gradient in grain size into TWIP steel by the original processing technology (GS sample). It is found that GS samples exhibit a higher cyclic hardening ability and cyclic saturation stress than that of as-received coarse grain (CG) samples and FG samples, which invalids the rule of mixture. Based on the dependence of the fatigue life ( N f ) on the total strain amplitude (Δe/2), GS shows the longest life at high strain amplitudes, while FG shows the longest life at low strain amplitudes. Judging from the aspect of stress (Δσ/2- N f curve), i.e. the Basquin curve, GS exhibits a better LCF performance than both FG and CG. The excellent fatigue properties of GS are believed to originate from the large generation of geometrically necessary dislocations (GNDs) and the formation of hard core and soft shell structure during cyclic loading. The significant influences of grain size and distribution on fatigue damage mechanisms may provide new and important implications for the optimized microstructural design of the high fatigue performance material.

It is a great challenge to improve the strength of disc superalloys without great loss of plasticity together since the microstructures benefiting the strength always do not avail the plasticity. Interestingly, this study shows that the trade-off relationship between strength and plasticity can be broken through decreasing stacking fault energy (SFE) in newly developed Ni-Co based disc superalloys. Axial tensile tests in the temperature range of 25 to 725 °C were carried out in these alloys with Co content ranging from 5% to 23% (wt.%). It is found that the ultimate tensile strength (UTS) and uniform elongation (UE) are improved synchronously when microtwinning is activated by decreasing the SFE at 650 and 725 °C. In contrast, only UTS is improved when stacking fault (SF) dominates the plastic deformation at 25 and 400 °C. These results may be helpful for designing advanced disc superalloys with relatively excellent strength and plasticity simultaneously.

Fatigue damage localization is a critical problem which greatly impacts on the fatigue strength improvement. As a main cause of fatigue damage concentration, the microstructure inhomogeneity of cold-rolled and annealed Cu-5at%Al alloy has been paid much attention in this study. A notable plateau with constant fatigue strength is discovered in the region of partially recrystallized microstructures which share similar maximum grain size. The existence of such “fatigue strength plateau” suggests that in some specific cases, fatigue strength would vary with neither the tensile strength nor the average grain size, since the fatigue resistance of the damage localized regions remains unchanged.

Cu bicrystals of different sizes with a sole twin boundary (TB) inclined at 45° with respect to the loading direction were deformed under unidirectional and cyclic loading, respectively. It is found that the slip bands (SBs) parallel to the TB can be activated near the TB at all scales without obeying the Schmid’s law. It is concerned with the local stress enhancement in the macroscale while it is more closely related to the scarce dislocation sources in the microscale. Moreover, a wedge-shaped zone formed near the TB in the microscale ascribed to the limited specimen size.

In this study, the performance of hot compressive deformation was investigated in two newly developed Ni-Co based superalloys having different stacking fault energy (SFE). It is interesting to discover that nanograins (NGs) can be produced in superalloy deformed at high temperature and relatively low strain rate. With the decrease of SFE and strain rate, the formation of NGs will be promoted obviously, leading to the negative strain rate sensitivity of compressive stress in superalloys. The NGs, generated through the fragmentation of subgrains by microtwins, may become a candidate choice for dynamically strengthening superalloys at high temperature.

In this study, it is found that the high-manganese austenitic FeMnC twinning-induced plasticity (TWIP) steel exhibits negative strain rate sensitivity (SRS), which is contrary to other face-centered cubic (fcc) metals or alloys. The negative SRS phenomenon of Fe-22Mn-0.6C (wt%) TWIP steel is considered to be suppressed or even convert to be positive through adding Al element. Meanwhile, the relation between tensile strength and uniform elongation of the FeMnC(Al) TWIP steels displays a transition from trade-off to synchronously improvement with decreasing the strain rate and Al content. It is suggested that the negative SRS phenomenon of FeMnC TWIP steel can be mainly attributed to the twinning mechanism assisted by dynamic strain aging (DSA). This mechanism can also explain well the negative to positive transition of SRS in FeMnC(Al) TWIP steel with the addition of Al element.

The tensile and high-cycle fatigue tests of Fe-30Mn-0.9C twinning-induced plasticity (TWIP) steel after 30%, 60% and 70% pre-straining were performed. Meanwhile, the surface damage morphologies of post-fatigue specimens and microstructure evolutions of pre-strained and post-fatigue pre-strained specimens were also investigated. It is found that the fatigue properties of the TWIP steel can be effectively improved through pre-straining, because the pre-straining can change the fatigue strength coefficient and exponent, respectively. The improvement of fatigue strength coefficient may be attributed to the strengthening mechanisms induced by both twin boundaries and dislocations; while the variation of fatigue strength exponent should be resulted from the combined effects of deformation homogeneity and slip reversibility, as well as the internal damages. Furthermore, the detailed mechanisms associated with the variations of fatigue strength coefficient and exponent were discussed. This study may enrich the fundamental knowledge about how to improve the high-cycle fatigue properties of TWIP steels.

In this study, a synchronous improvement of the strength and plasticity (SISP) of Ni and Ni-Si alloys was found through introducing supersaturated vacancies into the microstructure. Detailed observations show that these vacancies tended to aggregate to form Frank loops, which impede dislocation annihilation and increase the strain-hardening capability of Ni and Ni alloys. This finding put forward a new approach for simultaneously improving the strength and plasticity of metallic materials. Finally, the influence of the quenching temperature and Si content on this SISP effect was discussed.

Low-cycle fatigue tests were carried out on ultra-fine grained (UFG) Cu and Cu-Zn alloys to reveal the mechanisms of cyclic softening and the effects of dislocation slip mode. Based on careful examinations of the grain coarsening (GC), shear band (SB) evolutions and surface hardness change during cyclic deformation, the microscopic mechanisms of the cyclic softening process and the correlations between GC and SBs were deeply revealed. Besides, a general and coincident relationship was found between the softening velocities and the fatigue lives for UFG Cu and Cu alloys. Finally, it is approved that through alloying to increase the slip planarity, the cyclic softening caused by GC and SBs can be largely restrained such that the fatigue life may be improved effectively.

Dislocations 〈1 1 0〉/2 are usually dissociated into two 〈1 1 2〉/6 partials with a stacking fault in face centered cubic metals. Their behavior depends strongly on the stacking fault width (SFW) in plastic deformation. However, there is no quantitative study to correlate the SFW with the dislocation configuration when these dislocations are grouped together. In this work, the SFW for different dislocation arrays is analyzed within the framework of the elasticity theory of dislocations and then verified by atomistic simulations. The results demonstrate that the spacing of dislocation arrays has to be taken into account for the SFW variation besides the dislocation character. In addition, the SFW variation with the dislocation spacing seems to be independent to temperature. Our approach can also provide a basis for the accurate estimate of the influence of stacking faults on cross-slip, the competition between slip and twinning during plastic deformations in face centered cubic metals.

Three groups of Cu bicrystals with their component grains sharing one common {111} slip plane were cyclically deformed. By careful design on the loading direction, coplanar slip systems operated in the two component grains. Three kinds of grain boundaries (GBs): a low angle grain boundary (LAGB), a high angle grain boundary (HAGB) and an incoherent twin boundary (ITB), were all impinged by coplanar slip bands (SBs) on both sides. Nonetheless, the three kinds of GBs showed different fatigue cracking behaviors: the fatigue cracks nucleated along the HAGB earlier than along the SBs which produced cracks earlier than both the LAGB and the ITB. It is found that the slip vectors of the two component grains deviate from each other to different degrees for the three kinds of GBs. Lattice dislocations tend to pile up at the HAGB due to the large difference between the slip vectors, while lattice dislocations tend to pass through the LAGB and the ITB ascribed to the small differences between the slip vectors. High ability of dislocation transmission can generate good strain compatibility and low stress concentration near the GBs so as to increase the intergranular fatigue cracking resistance. In addition, lattice dislocation transmission and boundary dislocation motion can induce migration of LAGB and ITB which could improve the intergranular fatigue cracking resistance by consuming part of plastic work done by the cyclic loading. Thus, fatigue cracking resistance of GBs with impingement of coplanar SBs can be improved by increasing the penetrability and mobility of the GBs.

The low-cycle fatigue (including extremely-low-cycle fatigue) behaviors of the Fe Mn C TWIP steels are comprehensively investigated focusing on the effects of the imposed strain amplitude and Mn content on the deformation and damage characteristics. It is found that fatigue performance varies obviously with increasing Mn content, though their tensile properties do not change much. With the increase of strain amplitude or Mn content, the dislocation slip mode changes from planar slip to wavy slip. In order to evaluate the low-cycle fatigue properties, more impartial and reasonable, a hysteresis energy-based fatigue life prediction model is applied and developed. According to the experimental results and model analysis, unlike tensile properties, the low-cycle fatigue performance of the TWIP steel is extremely sensitive to their slip mode. A remarkable improvement of fatigue resistance is found to be related to the increase of Mn content, which may originate from the relatively low damage accumulation rate of planar slip. Specifically, planar slip caused by lowering the stacking fault energy is much more uniformly distributed than which caused by disordering short-range order. Such a variation in fatigue damage micromechanisms is believed to be the major reason for the occurrence of a special bilinear relationship for the Coffin-Manson curve.

The effect of high diffusivity anisotropy in β-Sn grain on electromigration behavior of micro-bumps was clearly demonstrated using Sn-3.0Ag-0.5Cu solder interconnects with only two β-Sn grains. The orientation of β-Sn grain (θ is defined as the angle between the c-axis of β-Sn grain and the electron flow direction) is becoming the most crucial factor to dominate the different electromigration-induced failure modes: 1) the excessive dissolution of the cathode Cu, blocking at the grain boundary and massive precipitation of columnar Cu6Sn5 intermetallic compounds (IMCs) in the small angle θ β-Sn grain occur when electrons flow from a small angle θ β-Sn grain to a large one; 2) void formation and propagation occur at the cathode IMC/solder interface and no Cu6Sn5 IMCs precipitate within the large angle θ β-Sn grain when electrons flow in the opposite direction. The EM-induced failure mechanism of the two β-Sn grain solder interconnects is well explained in viewpoint of atomic diffusion flux in β-Sn.

We explore quantum uncertainty, based on Wigner–Yanase skew information, in various one-dimensional single-electron wave functions. For the power-law function and eigenfunctions in the Aubry–Andre model, the electronic localization properties are well-defined. For them, we find that quantum uncertainty is relatively small and large for delocalized and localized states, respectively. And around the transition points, the first-order derivative of the quantum uncertainty exhibits singular behavior. All these characters can be used as signatures of the transition from a delocalized phase to a localized one. With this criterion, we also study the quantum uncertainty in one-dimensional disorder system with long-range correlated potential. The results show that the first-order derivative of spectrum-averaged quantum uncertainty is minimal at a certain correlation exponent α m for a finite system, and has perfect finite-size scaling behaviors around α m . By extrapolating α m , the threshold value α c ≃ 1.56 ± 0.02 is obtained for the infinite system. Thus we give another perspective and propose a consistent interpretation for the discrepancies about localization property in the long-range correlated potential model. These results suggest that the quantum uncertainty can provide us with a new physical intuition to the localization transition in these models.