Improved mechanical properties of V-microalloyed dual phase steel by enhancing martensite deformability

A good combination of ultimate tensile strength(UTS)up to 1365 MPa and total strain to failure(StF)to 15.5％has been achieved due to deformable martensite in the invented vanadium-microalloyed dual-phase(DP)steel,which was manufactured by two-stage annealing of cold rolled steel strip.The employed extensive characterizations revealed that the ductile martensitic phase in this DP steel differ-entiated from ordinarily low-carbon martensitic lath in both morphology and lattice structure.Complex coherent orientation relationships between ferrite,reverse austenite,martensitic phase and vanadium carbide(VC)do exist,leading to a new martensitic transformation mechanism and resultant dual-phase microstructure.Besides,a detailed characterization including essential phase transformation analysis in combination with in situ TEM observation,shows that,all the essential processing including recrystal-lization,reverse austenitic and martensitic transformation,in debt to the particular effects of VC,can be recognized as phase transformations with higher thermodynamic driving force and higher kinetic energy barrier as compared to previously common processing,which actually changes the microstructure and,indirectly leads to higher strength and higher ductility.This synergy of thermodynamics and kinetics can be generalized to improve mechanical properties of present steels.     

增强钢剪切板的周期性能、变形能力及刚度分析

剪切板在提高结构的抗震性能方面起了重要的作用,它们一般出现在薄钢板剪力墙或在偏心支撑框架结构的连接梁腹板处。剪切板的后期压曲能力、变形能力、能量损耗现在已经广泛地被结构工程师接受并在设计中考虑,以获得更经济的设计结果。通过对加强板和未加强板的对比可知,未加强板更易延展,而加强钢板具有很强的屈服区间,故导致了很高的能量损耗。鉴于这两种极端的情况,剪切板的最大延展性和能量消耗响应不可能同时拥有。在数值分析中对增强剪力板的极限强度的效果进行了分析,并研究了加强和未加强剪力板的循环性能。最后,对未加强板滞回曲线中较小的范围,研究了最佳增强效果需要提供的合理能量损耗和延展性。     

Poisson's ratio values for rocks

Compared to other basic mechanical properties of rocks, Poisson's ratio is an elastic constant of which the significance is generally underrated. Yet, in rock mechanics, there is a considerable number of diverse areas which require a prior knowledge or estimation of the value of Poisson's ratio. This paper examines the values and applications of Poisson's ratio in rock mechanics. Following an historical account of the initial controversy, whether it was a material constant or not, the effects of Poisson's ratio in the elastic deformation of materials, intact rocks, and rock masses are briefly reviewed. Also, the reported values of Poisson's ratio for some elements, materials, and minerals are compiled while typical ranges of values are presented for some rocks and granular soils. Finally, Poisson's ratio classifications are recommended for isotropic intact rocks.     

Effect of YSZ addition on the performance of glass-ceramic seals for intermediate temperature solid oxide fuel cell application

The suitability of glass-based seal is evaluated for application in intermediate temperature solid oxide fuel cell (SOFC). Several glass-YSZ composite seals are investigated in temperature range from 650 °C to 800 °C. The leakage rates are obviously reduced with temperature increased. The seal containing 20 wt% YSZ exhibits excellent gas tightness and thermal cycle stability, obtaining the leakage rate of 0.005 sccm cm^?1 under input gas pressure of 6.8 kPa at 750 °C. Stable leakage rates can be maintained after ten thermal cycles, implying that YSZ addition suppresses crack propagation of the seals. It is also explained by both interfacial bondage and chemical compatibility examination. When large-area-cell is operated during five thermal cycling tests, its performance is found to be constant at 750 °C, all of cell tests achieving OCV of 1.2V and power density of 520 mW/cm². The above results demonstrate the possibility of using the H1-20 seals for SOFC application.     

高钢级管线钢应变时效行为分析

应变时效现象会造成管线钢屈服强度和屈强比升高,应力一应变曲线出现明显屈服平台。通过对管线钢及钢管制造和使用过程的分析,指出应变时效行为对钢管生产过程质量控制、现场环焊缝对接以及管线服役性能的稳定性都有不良影响。通过减小钢管管体试样制备温度影响、严格控制防腐层制备工艺加热条件、降低管线钢中的C、N含量以及采用双相显微组织并在终轧时加速冷却后立即进行热处理等方法,可以有效降低应变时效效应。最后,对管线钢的应变时效行为的定量表征提出了建议。     

Design of composite filler with hedgehog-like reinforcement clusters for effective stress relief in ceramic/metal joints

In ceramic/metal brazing, high residual stress usually causes fracture in the ceramic substrates. Therefore, composite fillers consisting of ductile matrix featuring low yield strength and elastic modulus, as well as reinforcements featuring low coefficient of thermal expansion (CTE), are employed. However, for AgCuTi-base composite fillers, the reinforcements are likely to weaken the deformability of brazing seams (BS), i.e. increase the elastic modulus and yield stress of BS. To address this issue, we developed a new AgCu-base composite filler with “hedgehog-like” reinforcement clusters. Herein, AgCu + NbB₂ composite filler was used to braze ZrB₂–SiC and TC4-TiB_w, wherein ductile Ag(s,s) and Cu(s,s) dominated BS, thereby ensuring high deformability; in addition, a NbB layer along with limited TiB whiskers was formed at the NbB₂/brazes interface with the orientation relationship of (-11-1)_NbB//(0-1-1)_TiB and [-314]_NbB//[12-2]_TiB, resulting in “hedgehog-like” reinforcement clusters. This structure exhibited the following advantages: (i) crystallographic misfit at the interface of reinforcement clusters/brazes and the controlled formation of TiB whiskers minimised the conventional fine grain strengthening and dispersion strengthening effects of BS; (ii) isolated reinforcement clusters decelerated the increase in the elastic property of BS; (iii) “hedgehog-like” structure absorbed fracture energy by deflecting cracks; and (iv) the formation of brittle compounds could be avoided. In other words, the designed reinforcement clusters reduced the deformability expense and compensated for the mechanical-resistance of BS. Therefore, the maximum joint strength was obtained with 2 wt% NbB₂, which increased the shear strength of the joint six times compared to that with 0 wt% NbB₂. For the joints failing inside ceramic substrates, (i) the key factor for limiting joint strength was the residual stress inside ceramics and (ii) the remission of the trade-off between thermal expansion mismatch and deformability expense contributed to relaxing residual stress, thereby confirming the reliability of our design.     

Numerical evaluation of strength and deformability of fractured rocks

Knowledge of the strength and deformability of fractured rocks is important for design, construction and stability evaluation of slopes, foundations and underground excavations in civil and mining engineering. However, laboratory tests of intact rock samples cannot provide information about the strength and deformation behaviors of fractured rock masses that include many fractures of varying sizes, orientations and locations. On the other hand, large-scale in situ tests of fractured rock masses are economically costly and often not practical in reality at present. Therefore, numerical modeling becomes necessary. Numerical predicting using discrete element methods(DEM) is a suitable approach for such modeling because of their advantages of explicit representations of both fractures system geometry and their constitutive behaviors of fractures, besides that of intact rock matrix. In this study, to generically determine the compressive strength of fractured rock masses, a series of numerical experiments were performed on two-dimensional discrete fracture network models based on the realistic geometrical and mechanical data of fracture systems from feld mapping. We used the UDEC code and a numerical servo-controlled program for controlling the progressive compressive loading process to avoid sudden violent failure of the models. The two loading conditions applied are similar to the standard laboratory testing for intact rock samples in order to check possible differences caused by such loading conditions. Numerical results show that the strength of fractured rocks increases with the increasing confning pressure, and that deformation behavior of fractured rocks follows elasto-plastic model with a trend of strain hardening. The stresses and strains obtained from these numerical experiments were used to ft the well-known Mohr-Coulomb(MC) and Hoek-Brown(H-B) failure criteria, represented by equivalent material properties defning these two criteria. The results show that both criteria can provide fair estimates of the co     

Length-scale-dependent deformation and fracture behavior of Cu/X (X = Nb, Zr) multilayers: The constraining effects of the ductile phase on the brittle phase

The plastic deformation and fracture behavior of two different types of Cu/X (X = Nb, Zr) nanostructured multilayered films (NMFs) were systematically investigated over wide ranges of modulation period (λ) and modulation ratio (η, the ratio of X layer thickness to Cu layer thickness). It was found that both the ductility and fracture mode of the NMFs were predominantly related to the constraining effect of ductile Cu layers on microcrack-initiating X layers, which showed a significant length-scale dependence on λ and η. Experimental observations and theoretical analyses also revealed a transition in strengthening mechanism, from single dislocation slip in confined layers to a load-bearing effect, when the Cu layer thickness was reduced to below ∼15 nm by either decreasing λ or increasing η. This is due to the intense suppression of dislocation activities in the thin Cu layers, which causes a remarkable reduction in the deformability of the Cu layers. Concomitantly, the constraining effect of Cu layers on microcrack propagation is weakened, which can be used to explain the experimentally observed λ and η-dependent fracture mode transition from shear mode to an opening mode. Furthermore, the fracture toughness of the NMFs is also found to be sensitive to both λ and η. A fracture mechanism-based micromechanical model is developed to quantitatively assess the length-scale-dependent fracture toughness, and these calculations are in good agreement with experimental findings.     

Thermo-hydro-mechanical behavior of clay rock for deep geological disposal of high-level radioactive waste

In the context of deep geological disposal of radioactive waste in clay formations, the thermo-hydromechanical(THM) behavior of the indurated Callovo-Oxfordian and Opalinus clay rocks has been extensively investigated in our laboratory under repository relevant conditions:(1) rock stress covering the range from the lithostatic state to redistributed levels after excavation;(2) variation of the humidity in the openings due to ventilation as well as hydraulic drained and undrained boundary conditions;(3)gas generation from corrosion of metallic components within repositories; and(4) thermal loading from high-level radioactive waste up to the designed maximum temperature of 90℃ and even beyond to150℃. Various important aspects concerning the long-term barrier functions of the clay host rocks have been studied:(1) fundamental concept for effective stress in the porous clay-water system;(2) stressdriven deformation and damage as well as resulting permeability changes;(3) moisture influences on mechanical properties;(4) self-sealing of fractures under mechanical load and swelling/slaking of clay minerals upon water uptake;(5) gas migration in fractured and resealed claystones; and(6) thermal impact on the hydro-mechanical behavior and properties. Major findings from the investigations are summarized in this paper.