Brittle and Ductile Character of Amorphous Solids

Common silicate glasses are among the most brittle of the materials. However, on warming beyond the glass transition temperature $T_g$ glass transforms into one of the most plastic known materials. Bulk metallic glasses exhibit similar phenomenology, indicating that it rests on the disordered structure instead on the nature of the chemical bonds. The micromechanics of a solid with bulk amorphous structure is examined in order to determine the most basic conditions the system must satisfy to be able of plastic flow. The equations for the macroscopic flow, consistent with the constrictions imposed at the atomic scale, prove that a randomly structured bulk material must be either a brittle solid or a liquid, but not a ductile solid. The theory permits to identify a single parameter determining the difference between the brittle solid and the liquid. However, the system is able of perfect ductility if the plastic flow proceeds in two dimensional plane layers that concentrate the strain. Insight is gained on the nature of the glass transition, and the phase occurring between glass transition and melting.     

Microstructure and mechanical properties of large-volume gradient-structure aluminium sheets fabricated by cyclic skin-pass rolling

Materials of a gradient structure have been shown to possess both high strength and high ductility. To date, materials of a gradient structure can only be produced in small quantities. In this paper, we report a novel ‘cyclic skin-pass rolling’ (CSPR) technique capable of producing sheets of gradient structure in large quantities. Both experimental and analytical/numerical investigations are reported. In the experiments on aluminium sheets, the outer layer was subjected to 40 passes of CSPR with a reduction ratio of 1% per pass. After CSPR, the sample surface shows an ultrafine-grained microstructure with a mean grain size of 206?nm, while the annealed microstructure is retained in the core of the sample. Compared with cold-rolled aluminium sheets fabricated with the same total reduction ratio, CSPR-processed aluminium sheets have the same yield stress but improved uniform elongation (2.4 times). The scanning electron microscopy was used to study the fracture surface, and The transmission electron microscopy to examine the microstructure near the fracture end, in order to analyse the improvement in ductility. In addition, the finite element method was used to simulate the roll-sample contact pressure and strain distribution as well as residual stress on the sheet surface during CSPR, and to better understand the mechanism leading to improvement of ductility of the sheets by the CSPR technique.     

Effect of low dose neutron irradiation on the mechanical properties of an AIMgSi alloy

Abstract

An AlMgSi alloy prepared in two different conditions of age-hardening was neutron irradiated to fast fluences up to 2.5 × 10¹⁸n cm^?2 at 50°C. Postirradiation tensile and hardness tests were performed in the range from room temperature up to 350°C. The results show that the alloy in its soft and hard conditions exhibits a pronounced degree of irradiation-induced softening. The degree of softening is found to be dependent upon the ageing treatment given to the alloy before irradiation. The observed softening is suggested to be brought about by irradiation-induced dissolution of the age-hardening precipitates present before irradiation.     

Theoretics-directed effect of copper or aluminum content on the ductility characteristics of Al-based(Al_3Ti,AlTi,AlCu,AlTiCu_2)intermetallic compounds

First-principle simulations have been applied to investigate the effect of copper(Cu) or aluminum(Al) content on the ductility of Al_3Ti,AlTi,AlCu,and AlTiCu_2 alloys.The mechanical stable and elastic properties of Al-based intermetallic compounds are researched by density functional theory with the generalized gradient approximation(DFT-GGA).The calculated lattice constants are in conformity with the previous experimental and theoretical data.The deduced elastic constants show that the investigated Al_3Ti,AlTi,AlCu,and AlTiCu_2 structures are mechanically stable.Shear modulus,Young's modulus,Poisson's ratio,and the ratio B/G have also been figured out by using reckoned elastic constants.A further analysis of Young's modulus and Poisson's ratio reveals that the third added element copper content has significant effects on the Al-Ti-based ICs ductile character.     

不锈钢衬底MgB2厚膜韧性的研究

我们用混和物理化学气相沉积法(HPCVD)在不锈钢衬底上原位制备了以MgB2厚膜(约10微米厚)为过渡层的MgB2超导厚膜(约20微米厚)样品,两层膜总厚约30微米.X光衍射实验表明过渡层为(101)取向的织构膜,表层MgB2超导厚膜接近多晶膜,但有较强(101)取向,两者均含有少量的Mg和MgO杂相.此超导厚膜样品的Tc(onset)=37.8K,Tc(zero)=36.6K,ΔT=1.2K.对此样品的弯曲实验表明,以曲率半径500微米弯曲到180°后,样品仍具有Tc(onset)=37.8K,Tc(zero)=36.4K的超导电性.样品剖面SEM观测表明该膜结构致密,表面厚膜层和过渡层之间连接紧密.样品表面的SEM观测表明虽然样品弯曲导致表面MgB2超导厚膜面出现了裂缝,甚至有小部分膜面的脱落,但过渡层始终紧紧附着在不锈钢衬底上.这表明过渡层MgB2厚膜的存在大大地提高样品整体的柔韧性,展现了不锈钢衬底上的MgB2厚膜超导带(线)巨大的开发潜力和诱人的广阔应用前景.     

IMPACT BEHAVIOR OF POLYVINYL ALCOHOL FIBER REINFORCED CEMENTITIOUS COMPOSITES BEAMS

The paper presents results of an investigation conducted to study the impact resistance of polyvinyl alcohol fiber reinforced cementitious composite (PVA-FRCC) beams containing silica fume and fly ash. PVA-FRCC beam specimens of size 100 × 100 × 400 mm were tested under impact loading. The specimen incorporated four different volume fractions i.e. 0, 0.5%, 1.0% and 2.0% of corrugated PVA fibers. Silica fume and fly ash were added for the specimens containing PVA fibers volume fraction of 2.0%. The drop-weight type impact tests were conducted on the test specimens and the number of blows of the hammer required to induce first visible crack and ultimate failure of the specimen were recorded. The results are presented in terms of number of blows required, impact ductility as well as impact energy at first crack and ultimate failure. It has been observed that specimen containing 2.0% PVA fibers gave the best performance under impact loading, and that impact resistance was improved after silica fume and fly ash were added.     

纳米粒子在不锈钢衬底MgB2超导膜中的作用

我们用混合物理化学气相沉积法(hybrid physical-chernical vapor deposition简称为HPCVD)以氩气为背景气体,在不锈钢衬底上于不同条件下制备了一批MgB2超导薄膜样品.用扫描电子显微镜获取了相关的SEM图像,并对膜的成分进行了能谱分析(EDX)的.当把不锈钢衬底MgB2超导薄膜弯曲不同角度之后,膜面上均出现了裂纹.裂纹的数量和宽度随弯曲的角度的增大而增加,但是膜始终紧紧地覆着在衬底上不脱落.因此我们可以说覆着在不锈钢衬底上的MgB2超导薄膜具有了很好的韧性.在膜中我们也发现有大量的数十纳米大小的晶粒.这个尺寸的纳米粒子的作用可以用来平衡MgB2膜内结构和表面晶粒的活性之间的相互作用.MgB2纳米粒子的存在是MgB2超导膜表现出韧性的关键角色.     

Characterization of macroscopic tensile strength of polycrystalline metals with two-scale finite element analysis

The objective of this contribution is to develop an elastic-plastic-damage constitutive model for crystal grain and to incorporate it with two-scale finite element analyses based on mathematical homogenization method, in order to characterize the macroscopic tensile strength of polycrystalline metals. More specifically, the constitutive model for single crystal is obtained by combining hyperelasticity, a rate-independent single crystal plasticity and a continuum damage model. The evolution equations, stress update algorithm and consistent tangent are derived within the framework of standard elastoplasticity at finite strain. By employing two-scale finite element analysis, the ductile behaviour of polycrystalline metals and corresponding tensile strength are evaluated. The importance of finite element formulation is examined by comparing performance of several finite elements and their convergence behaviour is assessed with mesh refinement. Finally, the grain size effect on yield and tensile strength is analysed in order to illustrate the versatility of the proposed two-scale model.     

First-principles study of the effects of selected interstitial atoms on the generalized stacking fault energies,strength, and ductility of Ni

We analyze the influences of interstitial atoms on the generalized stacking fault energy （GSFE）, strength, and ductility of Ni by first-principles calculations. Surface energies and GSFE curves are calculated for the （112） （111） and / 101） （ 1 1 1） systems. Because of the anisotropy of the single crystal, the addition of interstitials tends to promote the strength of Ni by slipping along the （10T） direction while facilitating plastic deformation by slipping along the （115） direction. There is a different impact on the mechanical behavior of Ni when the interstitials are located in the slip plane. The evaluation of the Rice criterion reveals that the addition of the interstitials H and O increases the brittleness in Ni and promotes the probability of cleavage fracture, while the addition of S and N tends to increase the ductility. Besides, P, H, and S have a negligible effect on the deformation tendency in Ni, while the tendency of partial dislocation is more prominent with the addition of N and O. The addition of interstitial atoms tends to increase the high-energy barrier γmax, thereby the second partial resulting from the dislocation tends to reside and move on to the next layer.