卵形弹体侵彻混凝土开坑区侵彻阻力计算

为了研究弹体开坑过程中,弹头表面应力的表达形式,采用高速摄影仪记录弹体在开坑区的位移和时间关系,得到卵形弹体在开坑区的速度和侵深关系。采用最小二乘法对开坑区结束时消耗的弹体动能和侵深关系进行分析,提出开坑过程中弹头表面的应力形式。计算结果表明,该应力形式能较好地描述高速条件下开坑区的侵深和速度关系。     

Extrusions and intrusions in fatigued metals. Part 2. AFM and EBSD study of the early growth of extrusions and intrusions in 316L steel fatigued at room temperature

Early stages of surface relief evolution of persistent slip markings (PSMs), formed in areas where persistent slip bands (PSBs) intersect the free surface, in polycrystalline 316L stainless steel cycled with constant plastic strain amplitude were studied using atomic force microscopy (AFM) and electron backscattering diffraction (EBSD). Focused ion beam (FIB) technique was employed to obtain additional, more detailed information on the shape of PSMs. To reveal true qualitative and quantitative information on the simultaneous growth of intrusions and extrusions within individual PSMs, identical areas both on the specimen surface and on its inverse copy obtained via plastic replica were studied using AFM. Intrusions are preceded by extrusions regardless of orientation of individual grains of the polycrystal. The first intrusions appear largely around 1% of fatigue life at the moment when ‘static’ extrusions are developed. They appear predominantly but not exclusively at the side of extrusions where the emerging active slip plane is inclined to the surface at an acute angle. They grow faster than the stage of stable extrusion growth. Typical morphology of mature PSMs developed at 15% of fatigue life consists of ribbon-like extrusions accompanied by two thin parallel intrusions along both PSB–matrix interfaces. Experimental data on the morphology and growth of extrusions and intrusions are discussed in relation to the theoretical models and computer simulations of surface relief evolution leading to fatigue crack initiation.     

Evolution and characterization of dynamically recrystallized microstructure in a titanium-modified austenitic stainless steel using ultrasonic and EBSD techniques

A C-scan ultrasonic imaging system was used to investigate the microstructural evolution during dynamic recrystallization (DRX) of a 15Cr–15Ni–2.2Mo–Ti modified austenitic stainless steel (alloy D9). Four specimens were forged at 1273 K to different strains in the range 0.1–0.5. Specimens with true strains of 0.2 or lower did not show any variation in the amplitude of the first back-wall echo. However, a visible variation in the C-scan image was observed at and above the 0.3 strain level. This variation was attributed to the evolution of fine grains. The formation of fine grains was related to DRX, as indicated by electron backscattered diffraction. This study also revealed the characteristics of the DRX or ‘necklace grains’, as opposed to the so-called parent grains or rest of the microstructure.     

Selective appearance of ϵ-martensitic transformation and dynamic strain aging in Fe–Mn–C austenitic steels

The influence of stress-induced ε-martensitic transformation on the serrated flow behavior associated with dynamic strain aging was investigated. The ε-martensitic transformation was controlled by changing the deformation temperature and adding Si to Fe–17Mn–xSi–0.3?C alloys. The addition of Si promoted the ε-martensitic transformation, and suppressed the slip deformation due to solution hardening. The initiation of serrations around room temperature was delayed by the promotion of ε-martensitic transformation which initiated plastic deformation. The critical stress for the occurrence of serrations and the critical stress for the occurrence of slip deformation were found to have a linear relationship.     

Accurate determination of the number density of G-phase precipitates in thermally aged duplex stainless steel

G-phase precipitation in the ferrite phase in thermally aged duplex stainless steel (DSS) was investigated. A needle-shaped sample of DSS aged at 673?K for 5000?h was observed by transmission electron microscopy (TEM), and subsequently by atom probe tomography (APT). The precipitates of the G-phase observed by TEM corresponded well to clustering atoms observed by APT. On the other hand, regarding the precipitates of the G-phase that formed in an earlier stage of aging, the present study suggests that not all the precipitates can be detected by TEM. A large area of DSS aged at 673?K for 5000?h containing both the ferrite and austenite phases was observed. The number density of precipitates of the G-phase in the ferrite phase was small in the vicinity of the phase boundary and increased with the distance between the phase boundary and each field of view. The number density reached an almost constant value at a distance of approximately 4?µm from the phase boundary. The suppression of G-phase precipitation in the vicinity of the phase boundary is discussed in terms of the depletion of alloying elements that comprise the G-phase.     

Grain boundary engineering: fatigue fracture

Abstract

Grain boundary engineering has revealed significant enhancement of material properties by modifying the populations and connectivity of different types of grain boundaries within the polycrystals. The character and connectivity of grain boundaries in polycrystalline microstructures control the corrosion and mechanical behaviour of materials. A comprehensive review of the previous researches has been carried out to understand this philosophy. Present research thoroughly explores the effect of total strain amplitude on phase transformation, fatigue fracture features, grain size, annealing twinning, different grain connectivity and grain boundary network after strain controlled low cycle fatigue deformation of austenitic stainless steel under ambient temperature. Electron backscatter diffraction technique has been used extensively to investigate the grain boundary characteristics and morphologies. The nominal variation of strain amplitude through cyclic plastic deformation is quantitatively demonstrated completely in connection with the grain boundary microstructure and fractographic features to reveal the mechanism of fatigue fracture of polycrystalline austenite. The extent of boundary modifications has been found to be a function of the number of applied loading cycles and strain amplitudes. It is also investigated that cyclic plasticity induced martensitic transformation strongly influences grain boundary characteristics and modifications of the material’s microstructure/microtexture as a function of strain amplitudes. The experimental results presented here suggest a path to grain boundary engineering during fatigue fracture of austenite polycrystals.     

Proton-induced grain boundary segregation in stainless steel

Abstract

A technique is developed which addresses the problem of irradiation assisted stress corrosion cracking of stainless steels in light water reactors using high energy protons to induce grain boundary segregation. These results represent the first grain boundary segregation measurements in bulk produced by proton irradiation of stainless steel. The technique allows the study of grain boundary composition with negligible sample activation, short irradiation time, rapid sample turnaround and at minimal cost. Scanning Auger electron microscopy is used to obtain grain boundary composition measurements of irradiated and unirradiated samples of ultra high purity (UHP) type 304L stainless steel and UHP type 304L steels with the additions of phosphorus (UHP + P) and sulphur (UHP + S). Results show that irradiation of all three alloys causes significant Ni segregation to the grain boundary and Cr and Fe away from it. Irradiation of the UHP + P alloy also results in segregation of P at the grain boundary from 5.3 to 8.7 at %, over 80 times the bulk value. No radiation-induced grain boundary segregation of S was measured in the UHP + S alloy. Results also indicate that the presence of P or S may enhance radiation-induced segregation of major alloying elements at the boundary. Comparison of irradiated and unirradiated regions of the UHP + P alloy indicate that while a prior thermal treatment segregates P to the grain boundary to 5.3 at %, the major element concentrations at the grain boundary are completely different from those under irradiation.     

钢纤维活性粉末混凝土的动态力学性能

利用?74mmSHPB实验装置对钢纤维活性粉末混凝土(RPC)进行动态压缩实验和动态劈裂拉 伸实验。获得了钢纤维RPC在1~102s-1应变率加载下的动态力学参数。对试件内的动态应力分布进行数 值模拟,验证了动态实验的有效性。结果表明,钢纤维RPC的动态压缩和动态劈裂拉伸的力学性能均表现出 显著的应变率效应。随着应变率的增加,钢纤维RPC冲击压缩破坏应力、冲击压缩破坏应变、弹性模量、动态 劈裂拉伸破坏应力均有一定程度的增加,动态拉压比相对静态拉压比也有显著的提高。     

激光表面织构化对GCr15钢摩擦磨损性能的影响

本文利用Nd:YAG固体脉冲激光对GCr15钢样品表面进行了微坑织构化处理,考察了激光表面织构参数对其摩擦性能的影响,同时通过扫描电镜等对磨斑表面进行了分析.结果表明:与光滑面试样相比,经织构化处理的样品表面在干摩擦条件下虽然摩擦系数较高,但表现出了较好的抗磨性能;在贫油润滑条件下,织构面的摩擦系数与磨损均明显小于光滑面;在低载低速下较小孔径织构面摩擦系数较小,随着速度及载荷的增大,较大孔径织构面表现出更好的摩擦学性能.采用Stribeck曲线探讨了织构化表面与光滑面在贫油润滑下的润滑机理,结果表明织构面在试验条件下均保持油膜流体润滑状态.