Role of disorder-order transformation in consolidation of ceramics

The ordering of stacking-disordered silicon carbide prepared from the elements by high energy ball milling was investigated during sintering. A sharp increase in density in the temperature region 1700–1800°C was associated with a decrease in the disorder. Samples which had low disorder density showed a more continuous sintering behavior with temperature. Highly dense (up to 99% relative density) SiC can be obtained at 1900°C under a pressure of 70 MPa with no hold time. Similar results were observed for structurally disordered carbon with 10 at% of boron. The sintering behavior exhibited an abrupt density increase in the narrow temperature region of 1450–1600°C and was associated with disorder-order transformation.     

Hot isostatic consolidation and transformation of sigma phase powders

Abstract

Superferritic stainless steel compositions containing 38%Cr and at least 2% each of Ni and Mo can be converted to sigma phase by heat treatment and then to powder by mechanical attrition. This powdered sigma phase has been experimentally processed by powder metallurgical techniques to produce green compacts which were consolidated in a hot isostatic press. Depending on composition, several of the powdered alloys were converted back to ferrite during hipping. The mechanism of the consolidation process, the nature of the microstructures produced, and the prognosis for its industrial exploitation are examined.     

陶瓷材料动态断裂韧性测试

为了测试陶瓷材料动态断裂韧性,利用Hopkinson压杆实验原理和改装的Hopkinson压杆装置,并将试件加工成单边切口梁进行了三点弯曲动态试验.利用改装的Hopkinson压杆装置可直接测得透射应力波,从而直接得到试件变形过程中作用在试件上的支反力.本文定义了无量纲挠度和挠度变化率,给出了几种陶瓷材料在不同挠度变化...     

Failure mode transition in ceramics under dynamic multiaxial compression

An experimental technique based on the Kolsky pressure bar has been developed to investigate the behavior of ceramics under dynamic multiaxial compression. Experimental results for aluminum nitride (AlN), together with data available in the literature, indicate that a Mohr-Coulomb criterion and the Johnson–Holmquist model fit the experimental data for failure in a brittle manner, whereas the ceramic material exhibited pressure insensitive plastic flow at high pressures. A failure surface is constructed which represents the material failure behavior, including brittle failure, brittle/ductile transition and plastic flow, under various pressures. The effect of various material properties on the failure behavior was investigated. The Poisson's ratio is found to be a measure of brittleness for ceramic materials with low spall strength under shock wave loading conditions. Lower value of Poisson's ratio indicates that the material will fail in a brittle manner through axial splitting even under uniaxial strain loading; whereas materials with higher Poisson's ratio may be expected to deform plastically beyond the Hugoniot Elastic Limit (HEL). The applicability of the proposed failure surface to a range of ceramics is explored and the limitations of the model are outlined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Procedures for estimating the strength of hard alloys with dynamic loading

Experimental procedures are described for determining the dynamic strength properties of high-strength hard alloys. Results are provided for dynamic tests in compression, bending, and spailing. Failure of cylindrical strikers is studied in the case of collision with a target.Translated from Problemy Prochnosti, No. 11, pp. 78–88, November, 1992.     

Hot consolidation and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying

The present study is aimed to investigate the consolidation behaviour and mechanical properties of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after mechanical alloying. The consolidation was achieved by cold pressing with conventional sintering, vacuum hot pressing and hot isostatic pressing techniques. The microstructure and mechanical properties were evaluated. The hardness and compressive strength of nanocrystalline equiatomic AlFeTiCrZnCu high entropy alloy after vacuum hot pressing are 9.50 and 2.19 GPa and those after hot isostatic pressing are 10.04 and 2.83 GPa, respectively. The wear resistance is found to be higher than the commercially used materials such as Ni-hard faced alloy.     

Effect of aluminum reinforcement on the dynamic fragmentation of SiC ceramics

The threat of small-to-medium caliber armor piercing projectiles requires efficient protections that can be achieved by using bilayered configurations. They consist of a front face made up of a hard material and a back face made up of a ductile material. These solutions are among the most interesting in terms of mass efficiency. To design such bilayered concepts, one needs to understand and model fragmentation of the ceramic during the first microseconds after impact. This cracking pattern may significantly reduce the multi-hit capability of the armor. A new material made of porous silicon carbide infiltrated with aluminum is considered. The presence of an aluminum skeleton may improve the residual strength after impact. The present study deals with a comparison of fragmentation properties of this new material with those of porous silicon carbide. Quasi-static experiments and edge-on-impact tests are performed. The role of aluminum is discussed. An anisotropic damage model coupled with a fragmentation study is used to analyze the experimental observations.     

Subsurface damage and microstructure development in precision microground hard ceramics using magnetorheological finishing spots

We demonstrate the use of spots taken with magnetorheological finishing (MRF) for estimating subsurface damage (SSD) depth from deterministic microgrinding for three hard ceramics: aluminum oxynitride (Al(23)O(27)N(5)/ALON), polycrystalline alumina (Al(2)O(3)/PCA), and chemical vapor deposited (CVD) silicon carbide (Si(4)C/SiC). Using various microscopy techniques to characterize the surfaces, we find that the evolution of surface microroughness with the amount of material removed shows two stages. In the first, the damaged layer and SSD induced by microgrinding are removed, and the surface microroughness reaches a low value. Peak-to-valley (p-v) surface microroughness induced from grinding gives a measure of the SSD depth in the first stage. With the removal of additional material, a second stage develops, wherein the interaction of MRF and the material's microstructure is revealed. We study the development of this texture for these hard ceramics with the use of power spectral density to characterize surface features.     

Numerical manifold method for dynamic consolidation of saturated porous media with three-field formulation

In terms of the three-field formulation of Biot's dynamic consolidation theory, the numerical manifold method (NMM) is developed, where the same approximation to skeleton displacement ( u ) and fluid velocity ( w ) is employed and able to reflect incompressible as well as compressible deformation, but the approximation to pore pressure (p ) takes two different types, respectively. The first type of approximation to p is continuous piecewise linear interpolation and the second type assumes that p is a constant within each element. It is verified that using the second type of approximation to p naturally satisfies the inf-sup condition even in the limits of rigid skeleton and very low permeability, avoiding the locking problem accordingly. Energy components done by various forces are calculated to verify the accuracy and stability of the time integration scheme. A mass lumping technique in the NMM framework is employed to effectively reduce the unphysical oscillations and increase computational efficiency, which is another unique advantage of NMM over other numerical methods. A number of numerical tests are conducted to demonstrate the robustness and versatility of the proposed mixed NMM models.     

Multiresolution atomistic simulations of dynamic fracture in nanostructured ceramics and glasses

Multimillion atom molecular-dynamics (MD) simulations are performed to investigate dynamic fracture in glasses and nanostructured ceramics. Using multiresolution algorithms, simulations are carried out for up to 70 ps on massively parallel computers. MD results in amorphous silica (a-SiO₂) reveal the formation of nanoscale cavities ahead of the crack tip. With an increase in applied strain, these cavities grow and coalesce and their coalescence with the advancing crack causes fracture in the system. Recent AFM studies of glasses confirm this behavior. The MD value for the critical stress intensity factor of a-SiO₂ is in good agreement with experiments. Molecular dynamics simulations are also performed for nanostructured silicon nitride (n-Si₃N₄). Structural correlations in n-Si₃N₄ reveal that interfacial regions between nanoparticles are amorphous. Under an external strain, nanoscale cavities nucleate and grow in interfacial regions while the crack meanders through these regions. The fracture toughness of n-Si₃N₄ is found to be six times larger than that of crystalline -Si₃N₄. We also investigate the morphology of fracture surfaces. MD results reveal that fracture surfaces of n-Si₃N₄ are characterized by roughness exponents 0.58 below and 0.84 above a certain crossover length, which is of the order of the size of Si₃N₄ nanoparticles. Experiments on a variety of materials reveal this behavior. The final set of simulations deals with the interaction of water with a crack in strained silicon. These simulations couple MD with a quantum-mechanical (QM) method based on the density functional theory (DFT) so that chemical processes are included. For stress intensity factor K=0.4 MPa m^1/2, we find that a decomposed water molecule becomes attached to dangling bonds at the crack or forms a Si-O-Si structure. At K=0.5 MPa m^1/2, water molecules decompose to oxidize Si or break Si-Si bonds.     

Hot Isostatic Pressing (HIP) technology and its applications to metals and ceramics

This review examines some of the components of this increasingly exploited technology as well as the application of which will surely increase as a result of constant development in equipment design and extensive research in the field of ceramic and metal materials in general for the production of fully dense and reliable parts. Newly developed high temperature HIP equipment can offer potential improvements to material properties relative to more conventional techniques as a possible solution to the manufacture of ceramic and metal components for airframe and structural components where critical and highly stressed applications are required. By the use the near net shape techniques, exotic materials can be used more cost effectively than machining from solid. Designers and manufacturers alike can make better products by introducing HIP to their production route.     

30%Cr-Cu复合材料热变形及动态再结晶临界条件

采用放电等离子烧结法(SPS)制备出30%Cr-Cu复合材料,对其致密度、硬度和导电率等相关性能进行测试,并观察分析该复合材料的显微组织。利用Gleeble-1500D型热模拟试验机在变形温度650~950℃、应变速率0.001~10s-1、变形量60%的条件下对30%Cr-Cu复合材料进行热模拟压缩试验。对热压缩试验得到的真应力-应变数据进行拟合、计算和分析,构建该复合材料的本构方程,同时得到材料的加工硬化率θ,利用材料的lnθ-ε曲线出现有拐点和-(lnθ)/ε-ε曲线对应有最小值这一判据,分析该复合材料的动态再结晶临界条件。结果表明:30%Cr-Cu复合材料的真应力-应变曲线主要以动态再结晶软化机制为特征,峰值应力随应变速率的增加和温度的降低而升高;该复合材料的lnθ-ε曲线出现拐点,-(lnθ)/ε-ε曲线对应有最小值,该最小值所对应的应变为临界应变εc,且εc随变形温度的升高和应变速率降低而减小,εc与Zener-Hollomon参数Z的函数关系为εc=2.38×10-3 Z0.1396。     

Numerical simulations of SHPB experiments for the dynamic compressive strength and failure of ceramics

Complementary to a study of the compressive strength of ceramic as a function of strain rate and confinement, numerical simulations of the split-Hopkinson pressure bar (SHPB) experiments have been performed using the two-dimensional wave propagation computer program HEMP. The numerical effort had two main thrusts. Firstly, the interpretation of the experimental data relies on several assumptions. The numerical simulations were used to investigate the validity of these assumptions. The second part of the effort focused on computing the idealized constitutive response of a ceramic within the SHPB experiment. These numerical results were then compared against experimental data. Idealized models examined included a perfectly elastic material, an elastic-perfectly plastic material, and an elastic material with failure. Post-failure material was modeled as having either no strength, or a strength proportional to the mean stress. The effects of confinement were also studied. Conclusions concerning the dynamic behavior of a ceramic up to and after failure are drawn from the numerical study.     

Effect of electrical and mechanical poling history on domain orientation and piezoelectric properties of soft and hard PZT ceramics

Abstract

The superior piezoelectric properties of all polycrystalline ferroelectrics are based on the extent of non-180° domain wall motion under electrical and mechanical poling loads. To distinguish between 180° and non-180° domain wall motion in a soft-doped and a hard-doped lead zirconate titanate (PZT) ceramic, domain texture measurements were performed using x-ray and neutron diffraction after different loading procedures. Comparing the results to measurements of the remanent strain and piezoelectric coefficient allowed the differentiation between different microstructural contributions to the macroscopic parameters. Both types of ceramic showed similar behavior under electric field, but the hard-doped material was more susceptible to mechanical load. A considerable fraction of the piezoelectric coefficient originated from poling by the preferred orientation of 180° domains.     

基于压电动态信息的便携式阻抗测量系统设计

为解决对大型基础设施微小损伤检测所使用的阻抗分析仪昂贵且不便于携带的问题,采用基于压电阻抗的结构健康诊断技术,检测附着于结构表面的压电材料的阻抗,设计开发出一种便携式的小型阻抗测量系统.通过对机械结构中螺栓松动的情况反复实验,证明此便携式阻抗测量系统的精确度与目前商用的多用途阻抗分析仪相差无几,不但可以对机械结构进行实时在线损伤检测,而且对于微小损伤的检测也非常有效.     

Investigation of the dynamic characteristics of functional ceramics with the use of fiber laser vibrometry

The article describes a contactless method of investigating cyclic deformations with the aid of a laser vibrometer based on measuring the Doppler shift of the frequency of laser radiation reflected by the surface of a piezoelement. It explains the results of investigations of the instability of the piezomodule d₃₁ of the piezoceramic TsTBS-3 under the effect of temperature, obtained by the laser vibrometer.Translated from Problemy Prochnosti, No. 5, pp. 70–73, May, 1990.     

Hot pressed ZrB2–SiC–C ultra high temperature ceramics with polycarbosilane as a precursor

ZrB₂–SiC–C ultra high temperature ceramics (UHTCs) have been produced by hot pressing pyrolyzed mixtures of ZrB₂ and polycarbosilane (PCS). Samples with SiC contents of 0%, 5% and 16% in volume derived from PCS were prepared. The phase composition, microstructure and mechanical properties were characterized for composites hot pressed at 2073 K for 60 min under the pressure of 20 MPa in an argon atmosphere. Analysis showed that the addition of PCS improved the relative density from 78% (without PCS addition) to ∼ 100% (with 16% SiC derived from PCS addition). Hardness and fracture toughness of the composite were also improved.     

The spalling of long bars as a reliable method of measuring the dynamic tensile strength of ceramics

An analysis of the spall bar test as a reliable method of determining the dynamic tensile strength of brittle materials is presented. The method is based on the propagation and reflection of elastic waves in bars. Failure occurs when compressive waves are reflected into tensile ones on reaching a free end. The study analyses the hypotheses needed to obtain the true tensile strength with this experimental technique, referring to the requirements of the material and of the experimental procedure. The analysis is complemented with numerical simulations of the testing procedure. The correct way to determine the true dynamic tensile strength of ceramic materials is outlined. Finally, the results of tests of some ceramic materials, three different aluminium oxides, an alumina reinforced with zirconia, silicon carbide and boron carbide are presented.