纳米压痕条件下 MgB2 超导块材的压入力学行为研究

本文采用纳米压痕技术对固相烧结法制备的 MgB2 超导块材进行压入力学实验, 对不同压深的载荷-位移曲线和纳米压痕数据的再现性进行了分析, 实验数据使用 Oliver-Pharr 方法计算得出 MgB2 的硬度值, 借助经验方程拟合纳米压痕蠕变曲线求得蠕变速率敏感指数(m ) . 结果表明, 微观结构不均匀性会对材料在压头压入过程中抵抗外力作用时产生影响, 使压痕数据再现性变差; MgB2 的硬度表现出尺寸效应, 即随着压入深度的增加硬度逐渐下降;m 值随压入深度增加而增加是位错滑移速度加快的结果.     

铁基块体非晶合金在纳米压痕过程中的蠕变行为研究

利用纳米压痕技术研究了{［（Fe_0.6Co_0.4）_0.75B_0.2Si_0.05］_0.96Nb_0.04}₉₆Cr₄铁基块体非晶合金的室温蠕变行为及不同的加载速率对该块体非晶合金蠕变变形的影响.{［（Fe_0.6Co_0.4）_0.75B_{0.2< 关键词： 块体非晶合金 蠕变 EVEV模型 蠕变速率敏感指数}     

原位自生20vol.％TiCP/LD7Al基复合材料蠕变的应力指数和门槛应力

在523 K,573 K和623 K恒应力压缩条件下研究了原位自生20vol%TiC_p/LD7Al基复合材料和LD7Al合金的高温蠕变行为.对蠕变速率与外加应力在双对数坐标中进行拟合,获得了复合材料和基体铝合金的应力指数;通过在幂率方程中引入有效应力（σ-σ₀）,对实验数据进行线性回归外推至零蠕变速率得到相应的门槛应力.实验结果显示,复合材料的应力指数和门槛应力均高于LD7Al合金.TiC颗粒的存在,明显改善了LD7Al合金的高温蠕变 关键词： p/LD7Al基复合材料')" href="#">TiC_p/LD7Al基复合材料 蠕变 应力指数 门槛应力     

Li2O-Al2O3-SiO2微晶玻璃表面粗糙度对纳米硬度测试的影响

 介绍了纳米压痕测试技术的基础理论及纳米压痕法常用的Oliver -Pharr方法的计算原理。采用纳米压痕试验测得不同表面粗糙度的Li2O-Al2O3-SiO2微晶玻璃样品的纳米硬度、弹性模量和载荷-位移曲线。结果表明样品表面粗糙度会降低纳米压痕测试结果的稳定性、准确性和可靠性：样品表面粗糙度越小,测得的纳米硬度和弹性模量值波动越小,载荷-位移曲线重合性越高。随着最大载荷的增大,测得的弹性模量逐渐减小,其原因是压痕边缘材料发生了塑形变形。在超光滑表面样品（Ra=0.9 nm）上测得较为准确的Li2O-Al2O3-SiO2微晶玻璃纳米硬度和弹性模量值分别为8.8 GPa和7.79 GPa。纳米压痕测试结果的重合度对于评价超光滑表面完整性研究具有指导意义。     

Li2O-Al2O3-SiO2微晶玻璃加工脆延转变临界条件实验研究

 介绍了Li2O-Al2O3-SiO2微晶玻璃的加工特点。基于纳米划痕技术对Li2O-Al2O3-SiO2微晶玻璃进行了纳米划痕实验,测得微晶玻璃材料脆延转变临界切削深度和临界载荷的平均值分别为125.6 nm和29.78 mN。将实验所得临界切削深度值与基于压痕断裂力学模型建立的脆延转变临界切削深度计算值进行了对比,结果表明,T. G. Bifano基于显微压痕法给出的临界切削深度计算值与实验结果差别较大,结合实验结果对其公式进行了修正;基于压痕断裂力学模型建立的延性域磨削临界切削深度计算值与实验结果相差较小,并分析了产生差异的原因。     

CVD金刚石膜断裂性能测试装置的研究

文章介绍弛一种利用恒载荷速率加载测试金刚石膜断裂性能的试验方法,并建立了国内第一台金刚石膜断裂性能测试的专用装置,该装置利用弹性压头代替传统的刚性压头,可以成功地解决断裂试验的缓慢加载问题,试验装置的最大载荷为500N,可以在5－500N之间获得准确的载荷,误差不大于1％,最小加载速率为0．5N/s,最大加载速率为25N／s,该装置采用计算机控制,可以直接输出试验结果,该装置采用恒载荷速率（0．5N／s 至2N/s)的加载方式测得的断裂性能比用恒位移速率（0．5mm/min和0．05mm/min)加载方式测得的断裂性能更低,适合高脆性,小尺寸金刚石膜试样断裂性能的测试。     

TiN薄膜在纳米压痕和纳米划痕下的断裂行为

利用磁控溅射方法在Si(111)衬底上制备了具有(111)和(222)择优取向的TiN薄膜. 用纳米压痕和纳米划痕方法研究了该薄膜的变形和断裂行为. 用扫描电子显微镜、纳米压痕原位原子力显微镜及原位光学显微镜并结合加-卸载 曲线及划痕曲线获得了薄膜发生变形和断裂的微观信息. 在压痕试验中, TiN薄膜在压入深度为200 nm时表现为塑性变形及压痕周围的局部断裂, 随着压入深度的增大, 塑性变形和局部断裂变得越显著, 当最大压入深度达到临界值1000 nm时, 薄膜和衬底间发生了界面断裂. 在划痕实验中, 100 mN及200 mN的最大载荷均可以引起界面断裂. 最大为200 mN的载荷使得薄膜发生界面断裂的位置比用100 mN载荷时的位置提前, 但其临界断裂载荷和100 mN时及压痕实验时的临界界面断裂载荷基本相同. 关键词： TiN薄膜 纳米压痕 纳米划痕 界面断裂     

不同晶体取向下纳米压痕的多尺度模拟

采用准连续介质多尺度方法模拟面心立方金属铝单晶薄膜的纳米压痕变形过程.对薄膜分别采用三种不同的晶体取向(分别为x［1 1 1］,y［1 1 0］,z［1 1 2］; x［1 1 2］,y［1 1 1］,z［1 1 0］;x［1 1 0］,y［0 0 1］,x［1 1 0］),得到载荷-位移响应曲线.加载过程中,对晶体内部变形比较剧烈的部分画出原子图,并从微观角度分析产生剧烈变形的原 关键词： 纳米压痕 准连续介质方法 晶体取向 位错成核     

反应等离子喷涂纳米TiN涂层的显微硬度及微观结构研究

阐述了反应等离子喷涂(RPS)方法的基本思想.利用气体隧道等离子喷枪，通过RPS方法在Q23 5钢基底上成功制备了氮化钛涂层.检测了TiN涂层在不同载荷下的显微硬度，结果显示TiN涂层具有明显的硬度压痕尺寸效应，在高载荷下加工硬化效应较弱.XRD,TEM及HRTEM等分析 表明,通过RPS方法制备得到了纳米TiN涂层，涂层由直径约为50—70nm的TiN晶粒及非晶Ti N所组成. 关键词： 反应等离子喷涂 纳米 氮化钛 微观结构     

基于分数阶流变模型的铁基块体非晶合金黏弹性行为研究

采用分数阶黏弹单元替代经典模型中的黏壶, 结合非晶合金在外加载荷作用下的微观结构演化, 建立了以分数阶微积分表示的非晶合金黏弹性本构模型. 并根据Hertz弹性理论及分数阶黏弹性本构模型, 推导了块体非晶合金在纳米压痕球形压头下的位移与载荷及时间关系式. 基于推导的解析式, 对铁基块体非晶合金在表观弹性区的纳米压痕位移与载荷及时间曲线进行了非线性拟合分析. 相较于整数阶模型, 分数阶模型不仅具有较高的拟合精度, 其拟合参数能敏锐地反应加载速率对块体非晶合金黏弹性行为的影响, 且参数的变化规律与载荷作用下非晶合金微观结构演化呈现出较强的相关性.     

Experimental study of room-temperature indentation viscoplastic ‘creep’ in zirconium

In this paper we have studied the mechanisms of so-called ‘indentation creep’ in a zirconium alloy. Nanoindentation was used to obtain strain rate data as the sample was indented at room temperature, at a homologous temperature below that for which creep behaviour would be expected for this material. A high value of strain rate was obtained, consistent with previous work on indentation creep. In order to elucidate the mechanism of time-dependent deformation, a load relaxation experiment was performed by uniaxial loading of a sample of the same alloy. By allowing relaxation of the sample from a peak load in the tensile test machine, a similar stress exponent was obtained to that seen in the nanoindentation creep test. We conclude that for metals, at temperatures below that at which conventional creep will occur, nanoindentation ‘creep’ proceeds through deformation on active slip systems that were initiated by prior loading beyond the plastic limit. It is therefore more appropriate to describe it as a viscoplastic process, and not as creep deformation.     

Effects of cold rolling deformation on microstructure,hardness, and creep behavior of high nitrogen austenitic stainless steel

Effects of cold rolling deformation on the microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel （HNASS） are investigated. Microstructure characterization shows that 70% cold rolling deformation results in significant refinement of the microstructure of this steel, with its average twin thickness reducing from 6.4 μm to 14 nm. Nanoindentation tests at different strain rates demonstrate that the hardness of the steel with nano-scale twins （nt-HNASS） is about 2 times as high as that of steel with micro-scale twins （mt-HNASS）. The hardness of nt-HNASS exhibits a pronounced strain rate dependence with a strain rate sensitivity （m value） of 0.0319, which is far higher than that of mt-HNASS （m = 0.0029）. nt-HNASS shows more significant load plateaus and a higher creep rate than mt-HNASS. Analysis reveals that higher hardness and larger m value of nt-HNASS arise from stronger strain hardening role, which is caused by the higher storage rate of dislocations and the interactions between dislocations and high density twins. The more significant load plateaus and higher creep rates of nt-HNASS are due to the rapid relaxation of the dislocation structures generated during loading.     

Constitutive description of primary and steady-state creep deformation behaviour of tempered martensitic 9Cr–1Mo steel

The model based on the coupled sine hyperbolic creep rate relation with the evolution of internal stress as a function of strain provides better understanding of primary and secondary creep behaviour of tempered martensitic 9Cr–1Mo steel. The predicted evolution of internal stress as an increase in the internal stress value (or, decrease in effective stress) with strain/time appropriately described the observed decrease in creep rate during primary creep in the steel. The applicability of the model has been demonstrated by comparing experimental and predicted creep strain–time and creep rate–strain/time data of 9Cr–1Mo steel at 793 and 873 K for quenched and tempered and simulated post-weld heat treatment conditions. Irrespective of prior heat treatment and test temperature, the optimised parameters associated with the internal stress values exhibited linear variations with applied stress. The influence of prior heat treatment on primary and secondary creep characteristics of the steel is reflected on the rate constant values associated with the model. At all temperatures and heat treatment conditions, good agreement between the experimental and predicted steady-state creep rates demonstrate the further applicability of the model.     

Grain size effects on microstructural stability and creep behaviour of nanotwinned Ni free-standing foils at room temperature

Creep tests were performed on the high stacking fault energy (SFE) nanotwinned (NT) Ni free-standing foils with nearly the same twin thickness at room temperature (RT) to investigate the effects of grain size and loading rate on their microstructural stability and creep behaviour. The grain growth mediated by the twinning/detwinning mechanism at low applied stresses (<800 MPa) and grain refinement via the detwinning mechanism at high applied stresses (>800 MPa) were uncovered in the present NT-Ni foils during RT creep, both of which are attributed to the interactions between dislocations and boundaries. It appears that a higher initial dislocation density leads to a faster primary creep strain rate and a slower steady-state creep strain rate. Unlike the non-twinned metals in which grain growth often enhances the creep strain rate, the twinning/detwinning-mediated grain growth process unexpectedly lowers the steady-state creep strain rate, whereas the detwinning-mediated grain refinement process accelerates the creep strain rate in the studied NT-Ni foils. A modified phase-mixture model combined with Arrhenius laws is put forward to predict the scaling behaviour between the creep strain rate and the applied stress, which also predicts the transition from grain growth-reduced to grain refinement-enhanced steady-state creep strain rate at a critical applied stress. Our findings not only provide deeper insights into the grain size effect on the mechanical behaviour of nanostructured metals with high SFE, but also benefit the microstructure sensitive design of NT metallic materials.     

Dislocation-mediated creep process in nanocrystalline Cu

Nanocrystalline Cu with average grain sizes ranging from ～ 24.4 to 131.3 nm were prepared by the electric brushplating technique.Nanoindentation tests were performed within a wide strain rate range,and the creep process of nanocrystalline Cu during the holding period and its relationship to dislocation and twin structures were examined.It was demonstrated that creep strain and creep strain rate are considerably significant for smaller grain sizes and higher loading strain rates,and are far higher than those predicted by the models of Cobble creep and grain boundary sliding.The analysis based on the calculations and experiments reveals that the significant creep deformation arises from the rapid absorption of high density dislocations stored in the loading regime.Our experiments imply that stored dislocations during loading are highly unstable and dislocation activity can proceed and lead to significant post-loading plasticity.     

Strain rate sensitivity of ultra-fine and nanocrystaline metals and alloys

The mechanical behavior of metals and alloys is strongly related to grain size. In particular, the grain refining leads to the increase in yield strength in the ultra-fine grain (<1 μm) and nanocrystalline (<100 nm) regimes.Instrumented nanoindentation measurements allow a rapid evaluation of mechanical properties of materials, and the possibility to perform tests in a very wide range of loads. The strain rate sensitivity of ultra-fine and nanocrystalline metals can be derived by changing loading rates. The present paper presents the results on the strain rate sensitivity of ultra-fine grain metals produced by equa-channel angular pressing and nanocrystalline materials produced via electrodeposition. The results were obtained by systematic experiments performed at different loading rates (3, 30 and 300 mN/s) showing broad ranges of variations for the investigated metals. Also, the strain rate sensitivity of the studied materials was derived from the load vs. depth curves.     

Creep deformation of single crystals of new Co–Al–W-based alloys with fcc/L12 two-phase microstructures

The creep deformation behaviour of single crystals of Co–Al–W-based alloys with γ?+?γ′ two-phase microstructures has been investigated in tension under a constant stress of 137?MPa in air at 1000°C as a function of the γ′ solvus temperature and the volume fraction of the γ′ phase. When described by the creep strain rate versus time curve, the creep deformation of Co–Al–W-based alloys consists of transition and accelerating regions without a steady-state region, as observed in many modern nickel-based alloys. However, the creep strength of the present Co–Al–W-based alloys is comparable with nickel-based superalloys of the first generation but is much weaker than those of the second and higher generations. Unlike in nickel-based superalloys, the so-called p (parallel)-type raft structure, in which the γ′ phase is elongated along the tensile axis direction, is formed during creep in Co–Al–W-based alloys, being consistent with what is expected from the positive values of lattice misfit between the γ and γ′ phases. As a result, of the alloys investigated, the best creep properties are obtained with the alloy possessing the highest volume fraction (85%) of the γ′ phase, which is far larger than usual for nickel-based superalloys (55–60%).     

Theories of Creep in Ceramics

Mathematical models that have been proposed for creep in ceramics are described. Emphasis is on models involving grain boundary motion (sliding or flow). In Lifshitz models the crystalline grains elongate with strain; the elongation results from diffusion, slip, or solution and precipitation. In Rachinger models the grains do not elongate during creep. The sliding strain can be accommodated by viscous flow of a glassy phase at the grain boundaries, or if there is no boundary glass by diffusion or slip in superplastic models. Sliding of a glass-free boundary can result in cavitation, cracking, or formation of boundary dislocations or triple point folds.

Most models of ceramic creep at high temperatures predict a steady state (stage II) creep rate that depends on the applied stress, grain size, and temperature. A general equation for the creep rate as a function of these factors, as well as the elastic modulus and a diffusion coefficient, is used to compare models. The models give different exponents for the functional dependence of creep rate on grain size and strain and different temperature dependencies. These differences are compared in tables, and the main mechanistic features of the models are described in the text.

The purpose of this review is to describe creep models rather than to compare them with experimental results or to select the most applicable models. There are few critical experimental tests that allow selection of the most accurate models; such experiments are suggested as the next step in choosing between the models for specific experimental results.     

Non-conservation of grain volume during diffusion creep and its effect on denuded zones

It is reasoned in this paper that the traditional assumption of grain volume conservation during diffusion creep is correct only for special grain configurations. An analysis is presented that illustrates this, using a hypothetical arrangement of grains specifically chosen to be stable against both grain boundary sliding and grain rotation, so that the extent of grain volume non-conservation can be illustrated in the absence of these factors. The influence on the development of the ‘denuded’ zones that characterize diffusion creep in particle-containing materials is addressed. The analysis contributes to an explanation for the discrepancies between the creep strain estimated from zone sizes and the overall specimen strain, a discrepancy that has been used in the past as counter evidence for the diffusion creep mechanism. Suggestions are made for the improved modelling of diffusion creep in polycrystalline materials and duplex structures.