High-strain-rate deformation of armco iron induced by spherical and quasi-spherical converging shock waves and the mechanism of the α-ɛ transformation

The structures of two balls made of plain commercial-purity iron and subjected to loading by spherical and quasi-spherical converging shock waves are studied by optical microscopy and transmission electron microscopy at various depths. The high-strain-rate plastic deformation of armco iron under the loading conditions is found to occur via slip, and, in medium and deep layers, it also occurs via the formation of localized deformation bands. An atomic-crystallographic mechanism for the lattice restructuring during the α-? transformation is proposed.     

Deformation behavior of copper upon loading by spherically converging shock waves: Low-intensity loading conditions

X-ray diffraction, optical metallography, transmission electron microscopy, and microhardness measurements were used to perform a layer-by-layer study of the structure of a copper sphere 70 mm in diameter after loading by spherically converging shock waves. It has been found that the high-rate plastic deformation of copper proceeds mostly by slipping, rather than twinning under the used loading conditions. Shear bands are observed at the macrolevel, while a homogeneous dislocation structure, microbands, and microtwins are observed at the microlevel.     

Studying elastic deformation behaviour of cast irons by acoustic emission

Abstract

The deformation of metallic materials includes both an elastic and a plastic deformation. In the case of cast irons, the elastic region becomes less pronounced as the graphite changes from spheroidal to flake shaped, as observed in nodular and grey cast iron, respectively. The present study aims to correlate the shape of the graphite phase with the deformation behaviour, where the plastic deformation and other strain accommodating events are quantified by measurements of the acoustic emission events occurring in the interior of the material at loading. It also aims to explain how the appearance of cast iron stress–strain curves depends on the graphite morphology where, for instance, spheroidal graphite cast irons exhibit a seemingly linear elastic behaviour in contrast to flake graphite cast irons. The present study includes a series of pearlitic cast iron material grades with differences in nodularity and carbon equivalent, respectively. It is shown that as the roundness of the graphite phase increases, the ability to absorb energy increases. The measured acoustic emission indicates that plastic deformation occurs in the seemingly linear elastic region regardless of the cast iron grade, i.e. no cast iron grade exhibits perfect linear elasticity. The plastic deformation rate in the elastic region increases as the roundness of the graphite decreases and as the carbon equivalent increases. It is shown that the plastic deformation governs the resulting modulus of elasticity in all kind of cast irons, i.e. the modulus of elasticity decreases as the yielding of the material increases. The present study improves the understanding of the deformation behaviour in the elastic region of different cast irons. The survey shows that acoustic emission testing is a useful method when studying the deformation behaviour of cast irons.     

Effect of elongated graphite on mechanical properties of hot-rolled ductile iron

The mechanical properties of hot-rolled and then annealed ductile iron were evaluated. The deformation of this two-phase material and the effects of the elongated graphite spheres on the mechanical properties and the development of anisotropy of mechanical properties were studied. An attempt was made to describe the anisotropy of tensile strength in terms of the deformation of graphite spheres and of the root curvature, area fraction, and the interbridges that result in disproportional changes of stress concentration, loading capacity, and the tendency to break the interbridges and link the neighboring deformed graphite spheres.     

Some Aspects of Slip in Germanium

Germanium single crystals strained in tension at 600°C slip on the {111} plane and, macroscopically at least, in the <110> direction. Deformation is inhomogeneous: various localized rotations are observed, as is a banding consistent with secondary slip banding. The structure after deformation is polygonized with a domain size of about 2×10^?3 cm. In relaxation tests, an incubation period prior to flow is observed, of duration inversely related to temperature and applied stress. Under continuous loading, there is a sharp first yield point. A critical resolved shear stress therefore must be cited with respect both to temperature and to rate of loading. At 600°C, when loading proceeds at 2900 psi per min, it is 1310 psi. The yield point phenomenon is suggestive of Cottrell’s solute atom atmosphere theory and five points of qualitative agreement with this theory are found.     

Characteristics of surface roughening during plane strain compression of polycrystalline iron

Roughening behavior of the free surface of polycrystalline iron during plane strain compression is investigated experimentally. The changes in the shape of the free surface, which is roughened during plastic deformation, are observed in the three-dimensions. It is found that the mountains and the valleys of the roughened shape tend to elongate in the constrained direction for the specimen with isotropic grain shape. The shapes of the roughness curves in the loading direction and in the constrained direction are compared. The normalized height distribution of the roughness curve in the constrained direction is symmetric, while that of the roughness curve in the loading direction is asymmetric and positively skew during plane strain compression. Based on a simple simulation of the roughness curves by a random midpoint displacement method, this difference is supposed to be caused by the constraint of the material flow under plane strain condition.     

Microstructural constraints for creep in SiC-whisker-reinforced Al2O3

New and published creep data obtained on a SiC-whisker-reinforced Al₂O₃ composite have been analyzed in terms of an effective grain size and a threshold/critical stress. These concepts allow the formation of a consistent picture of the high-temperature deformation of these composites. For low volume fractions of whiskers, before the formation of a point-contact percolative limit is reached, deformation proceeds via grain-boundary sliding after the applied stress exceeds a temperature-dependent threshold stress. In this regime, the nominal grain size is the most important microstructural feature. For larger volume fractions of whiskers, up to the critical volume fraction for formation of facet-to-facet contact, whiskers inhibit grain-boundary sliding and deformation proceeds by means of pure diffusion. In this regime, the most important microstructural feature is an effective grain size, i.e. the spacing between the whiskers. Deformation proceeds until the stress reaches a temperature-dependent critical stress. At this point, damage occurs by unaccommodated grain-boundary sliding and creep is no longer in a steady state.     

加载方式对铸铁断口形貌的影响

铸铁断口形貌主要受其内部结晶组织和加载方式的影响，铸铁基体和石墨出现的不同形貌是被不同的加载方式所控制的，石墨在不同方式应力影响下，随着基体的拉伸，扭转或压缩而变形，挤出或破碎，呈现出不同的断裂形貌，这种形貌特征有助于判别加载方式的类别，明确铸铁断裂的机理，是失效分析的重要手段。     

Microprocesses of metal dusting on iron‐nickel alloys and their dependence on the alloy composition

Metal dusting of iron proceeds via the formation and disintegration of the metastable carbide Fe₃C, and the resulting fine Fe particles in the coke further catalyse carbon deposition. By contrast, nickel disintegrates directly, and larger grains are released. As revealed by TEM and AEM techniques, in both cases the disintegration proceeds by inward growth of thin graphite filaments, the atomic basal planes of which being oriented perpendicular to the surface thus effecting a high reactivity at the growth front. Consequently, successive alloying of iron with nickel should lead to a change over from one disintegration mechanism to the other, and, in fact, we could evidence that the carbide formation takes place only up to a nickel content of about 5 wt.%. Already at a Ni concentration of 10 wt.% a direct disintegration of the metal proceeds, as it is typical for pure nickel. Furthermore, in all investigated Ni‐Fe alloys a surface‐near enrichment of Ni was observed which indicates a selective corrosion of Fe, decreasing with increasing Ni content of the basic alloy.     

稳态热加载对筒状结构变形和应力影响的研究

焊接热源分别采用双椭球和整条焊缝两种稳态热加载,得到了筒状结构的焊后变形和残余应力分布,并进行了试验验证。结果表明：两种加载方式焊后变形和应力趋势相同。整条焊缝加载峰值出现在两封口板中心位置,峰值为4.931 mm,与试验结果对比误差小。应力峰值相近,峰值近似339 MPa。采用双椭球热源加载,应力分布与试验结果吻合更好。仿真与试验结果误差均能满足工程应用要求,证明了模拟结果的准确性。双椭球热源加载的计算时间是整条焊缝加载的27.2倍,从工程应用层面分析,整条焊缝加载具有重大的使用价值。     

Effect of plastic deformations and heat treatment on the behavior of the coercive force under load

Based on the measurements of the dependences of the coercive force of ferromagnetic steels on elastic tensile stresses, its minimum values and the positions of these minima have been found. Using the method of thermal neutron scattering, it has been experimentally proved that the stresses at which the coercive force is minimum is equal to the average value of the residual body stresses (internal stresses of the first kind). The latter were varied over wide limits with the aid of either a preliminary tensile plastic deformation to different degrees (steel St3) or using different tempering regimes after quenching (steel U8). In the first case, large residual compressive stresses arise in a significant part of grains along the direction of loading, which are caused by the anisotropy of Young’s modulus of iron. As a result of compensating these stresses by external tensile stresses, there appears a minimum of the coercive force, whose value is determined by an increase in the dislocation density at the stage of plastic deformation.     

铁镍梯度材料抗低应力多冲碰撞塑性变形性能研究

目的研究梯度材料抗低应力多冲碰撞塑性变形能力。方法对纯铁材料、铁镍突变材料和铁镍梯度材料进行多次冲击碰撞试验,分析3种材料的累积变形量、不同层深处的应变和形变硬化程度、表面微观组织。结果总形变量、不同层深处的应变和形变硬化程度方面,纯铁材料略大于铁镍突变材料,远大于铁镍梯度材料。铁镍突变材料和纯铁材料表层为单相多晶组织,且界面密度较小;铁镍梯度材料表层为多相多晶组织,且界面的密度大,其形变被抑制。结论铁镍梯度材料的抗低应力多冲碰撞塑性变形的能力最强,铁镍突变材料次之,纯铁材料最差。     

低合金耐磨铸铁共晶碳化物形态改善的研究

钎对低合金耐磨铸铁组织中共晶碳化物呈连续网状分布问题,探讨了热处理对变形低合金耐磨铸铁中共晶碳化物形态的影响.结果表明,高温热处理结合变形可使共晶碳化物形态发生显著变化,碳化物由连续的网状变为条块状;当耐磨铸铁经20%热变形后再经970℃保温3h高温处理后,网状共晶碳化物基本上被条块状碳化物所取代.     

亚共晶白口铸铁塑性变形行为的观察和分析

用金相显微镜，扫描电镜和透射电镜对经锻造和轧制等塑性变形的亚共晶白口铸铁组织进行了观察和分析，提示了共晶碳化物和基体的塑性变形行为，特别展示了被认为不能性变形的硬脆相共晶碳化物在变形应力驱使下由位错的运动，晶体滑移，扭转进行塑性变形的机制。为菜氏体钢铁材料塑性变形的研究提供了科学的证据。     

固液混合铸造高铬铸铁的热变形性能研究

采用一种新的材料制备技术-固液混合铸造工艺制备了高铬铸铁合金坯料,对合金进行了高温热压试验,并对热压过程和合金组织进行了观察和分析。研究结果表明,固液混合铸造的高铬铸铁合金相对于普通铸造合金的热变形性能得到明显提高,累计墩粗变形比可达到34%以上,固液混合铸造工艺对高铬铸铁合金组织的改善是提高材料热变形性能的主要原因。     

热变形对多元合金耐磨铸铁组织和性能的影响

研究了热变形量对多元合金耐磨铸铁组织和冲击韧度的影响,结果表明,热变形能改变耐磨铸铁组织中共晶碳化物的形状与分布,促进颗粒状碳化物的析出,提高耐磨铸铁的冲击韧度。当该耐磨铸铁的热变形量为40％时,其冲击韧度最佳。     

石墨蠕化率对蠕墨铸铁受热变形趋势的影响

通过模拟蠕墨铸铁制动受热变形试验,采用薄片试样,模拟列车蠕墨铸铁制动盘使用时的受力情况,设计受热变形试验工装,给予试样两端一个相同的、较小的变形量(弹性变形范围内),模拟列车制动工况温度150℃,加热保温30 h,自然冷却后,在自由状态下测量试样的永久变形量,对试验数据进行统计分析,通过重复做多组试验来研究不同石墨蠕化...     

Role of W on the dislocation evolution in Ni-W alloy during tension followed by compression loading

In this paper, we report deformation behavior during tension followed by compression loading for Nickel and Nickel-Tungsten alloy (15 at% W) single crystals using molecular dynamics simulations to investigate the role of W on the dislocation evolution in Ni-W alloy. The stress-strain responses of single crystals under uniaxial tension followed by compression loading after different pre-strains (i.e. 0.10 and 0.20 true strains for pure Ni; 0.10 and 0.24 true strains for Ni-15 at% W alloy) are simulated at strain rate of 10⁸ s^?1 and at the temperature of 300 K. Dislocation mobility, dislocation-dislocation interaction and dislocation-twin interactions are thoroughly investigated to evaluate their influence on deformation behaviour during reverse loading. Slip dominated deformation mechanism prevails during forward loading but both twin and slip are found to be operative during reverse loading for Ni single crystal. It is observed that the dominant deformation mechanism is twin for both forward and reverse loading in case of Ni-15 at% W alloy single crystal.     

轻量化倾翻保护结构侧向加载与时域变形研究

工程机械倾翻保护结构(ROPS)轻量化是其安全设计的发展趋势,对轻量化改制后某轮式装载机ROPS进行侧向加载,通过贴应变片方法对其时域变形进行研究分析。实验结果表明:ROPS横梁与立柱各减薄5 mm后,改用Q345B材质和加强焊缝连接可满足ISO3471侧向加载要求和能量吸收标准。侧向加载力达到标准值时,能量值为标准值的30%,能量值达到标准值时侧向承载极限为标准值的1.4倍;横梁(W)中心位置靠近加载区域先进入塑性变形区,两侧变形呈近似对称分布;横梁(L)出现塑性铰呈近似S型变形;立柱(Z)靠近加载端位置先进入塑性变形,中心以下部位不会发生塑性变形,刚度远比横梁大。