The inverse Hall-Petch effect in nanocrystalline ZrN coatings

For the purpose of studying the inverse Hall-Petch effect in nanocrystalline hard coatings, nanocrystalline ZrN coatings have been fabricated using magnetron sputtering with grain sizes ranging from 45 nm to 10 nm by varying negative biases from 0 V to 150 V. The transition from the classical Hall-Petch effect to an inverse Hall-Petch effect in nanocrystalline ZrN coatings is observed at a grain size between 19.0 nm and 14.2 nm. The reality of the inverse Hall-Petch effect in the present study is validated by exclusion of other possible effects on hardness of nanocrystalline ZrN coatings, such as porosity, multiphase, chemical composition, texture, and residual stress. Furthermore, a concise model based on lattice dislocations piling up mechanism is proposed to illustrate the breakdown of the Hall-Petch effect and calculate the critical grain size. The predictions of the model fit well with experimental data in some nitride and carbide nanocrystalline coatings. Both experimental and theoretical results indicate that the inverse Hall-Petch effect is an essential property of nanocrystalline hard coatings as similar to nanocrystalline metals and alloys.     

Effect of grain size on creep properties of type 316LN stainless steel

The effect of grain size on creep properties of type 316LN stainless steel has been investigated at 600°C under different stresses. The initial strain at the beginning of creep tests decreased with the decrease of grain size. This was confirmed by the Hall-Petch relationship. The steady state creep rate decreased to a minimum value at the intermediate grain size (dm=80–130 μm) and then increased with the further increase of grain size. This result agreed with Garofalo's model stating that grain boundaries act simultaneously as both dislocation sources and barriers to dislocation movement. The rupture elongation at the intermediate grain size was minimal due to the cavity formed easily by carbide precipitates in the grain boundaries.     

Influence of Temperature on the Inverse Hall-Petch Effect in Nanocrystalline Materials:Phase Field Crystal Simulation

The influence of temperature on the inverse Hall-Petch effect in nanocrystalline(NC) materials is investigated using phase field crystal simulation method.Simulated results indicate that the inverse Hall-Petch effect in NC materials becomes weakened at low temperature.The results also show that the change in microscopic deformation mechanism with temperature variation is the main reason for the weakening of the inverse Hall-Petch effect.At elevated temperature,grain rotation and grain boundary(GB) migration seriously reduce the yield stress so that the NC materials exhibit the inverse Hall-Petch effect.However,at low temperature,both grain rotation and GB migration occur with great difficulty,instead,the dislocations nucleated from the cusp of serrated GBs become active.The lack of grain rotation and GB migration during deformation is mainly responsible for the weakening of the inverse Hall-Petch effect.Furthermore,it is found that since small grain size is favorable for GB migration,the degree of weakening decreases with decreasing average grain size at low temperature.     

晶粒尺寸和孪晶界间距对纳米多晶铝合金塑性变形影响的分子动力学模拟研究

采用分子动力学模拟方法,分别研究了晶粒尺寸和孪晶密度对纳米多晶铝合金塑性变形的影响。模拟结果表明,弛豫后的位错密度对纳米多晶Al的微观结构演变和逆Hall-Petch关系产生了重要影响。变形受晶粒大小限制,在细晶中可形成层错四面体和复杂层错结构,从而激活了晶界的辅助变形。当孪晶界间距（TBS）较大时,Shockley分位错在晶界处形核并增殖。然而,随着TBS的减小,孪晶界成为Shockley分位错的来源。孪晶界上大量的分位错形核会导致孪晶界迁移甚至消失。在塑性变形过程中还观察到形变纳米孪晶。研究结果为开发具有可调节力学性能的先进纳米多晶Al提供了理论基础。     

多晶体材料的晶界强化模型研究

近几十年来,材料的细晶强化研究大量开展。在一般晶粒尺寸范围内,材料的强度随晶粒尺寸的变化是符合Hall-Petch关系的,但在纳米晶体材料中出现了偏离甚至反Hall-Petch关系的现象,因此Hall-Petch关系的使用具有一定的局限性。文章从塑性变形理论出发,建立了晶界强化数学模型,通过该模型,研究了晶粒大小、晶界厚度及晶界体积百分数、晶界能、弹性模量与多晶体材料强度的关系,发现除了晶粒大小及晶界厚度影响材料强度之外,弹性模量、晶界能越大,多晶体材料的强度越高。     

超细晶铁素体钢的力学性能及成形性能研究

采用形变强化铁素体相变超细晶轧制工艺,获得了750MPa级超细晶铁素体钢板,其超细晶铁素体体积比高达93%,晶粒尺寸仅为1μm,并具有优异的成形性能。对超细晶铁素体钢组织的精细分析发现,因铁素体晶界随晶粒超细化而减薄,使得其晶界对材料的力学性能具有双重影响,一方面,屈服强度随着晶界总长度的增加而提高;另一方面,又因晶界减薄而降低。研究表明,超细晶铁素体钢的组织性能关系虽然与霍尔-佩奇关系相吻合,但存在较大偏差,主要表现为斜率显著下降。文章对斜率下降的原因进行了机理分析。     

Using X-ray microbeam diffraction to study the long-range internal stresses in aluminum processed by ECAP

Aluminum alloy 1050 was processed by equal-channel angular pressing (ECAP) using a single pass (equivalent uniaxial strain of about 0.88). Long-range internal stresses (LRISs) were assessed in the grain/subgrain interiors using X-ray microbeam diffraction to measure the spacing of {5 3 1} planes, with normals oriented approximately +27.3°, +4.9° and ?17.5° off the pressing (axial) direction. The results are consistent with mechanical analysis that suggests the maximum tensile plastic-strain after one pass is expected for +22.5°, roughly zero along the pressing axis, and maximum compressive strain for the ?67.5° direction. The magnitude of the measured maximum compressive long-range internal stress is about 0.13σ_a (applied stress) in low-dislocation regions within the grain/subgrain interiors. This work is placed in the context of earlier work where convergent beam electron diffraction was used to analyze LRISs in close proximity to the deformation-induced boundaries. The results are complementary and the measured stresses are consistent with a composite model for long-range internal stresses.     

A closer look at the local responses of twin and grain boundaries in Cu to stress at the nanoscale with possible transition from the P–H to the inverse P–H relation

Nanocrystalline copper is considered to be a candidate for electrical contacts for dynamic systems because of its intrinsic conductivity and enhanced fretting resistance. However, the enhanced electron scattering at high-density grain boundaries significantly deteriorates the overall conductivity of nanocrystalline copper. Recent studies suggest that nanosized twin boundaries in copper might be a solution to such a dilemma. To better understand the general mechanical behavior of nanotwin boundaries, we conducted molecular dynamics simulation studies to investigate responses of both nanotwin and nanograin boundaries in copper to stress at the nanoscale, particularly in the critical range of 5–25 nm where the inverse Petch–Hall relation (P–H) may occur in nanocrystalline copper. The obtained results suggest that the twin boundary blocks dislocation movement more effectively and the degree of emitting dislocations under stress is considerably lower than that of grain boundary, leading to superior mechanical behavior. The inverse P–H relation is not applicable to the nanotwinned system. It is also demonstrated that the inverse P–H relation occurring in nanograined materials does not necessarily result from grain boundary sliding.     

Al-TiB2纳米晶薄膜中的反Hall-Petch效应

多种微结构因素作用的相互交织使纳米晶合金中是否存在与纯金属类似的反Hall-Petch现象难以得到实验证实。选用Al-TiB_2体系,采用二维结构纳米多层膜的方法,实现了对晶粒尺寸因素的孤立和使其独立地改变,研究了晶粒尺寸对薄膜力学性能的作用规律。结果表明:Al-TiB_2过饱和固溶纳米晶薄膜也与纳米晶纯金属Al一样,存在硬度随晶粒尺寸减小从遵从Hall-Petch关系提高转变为偏离Hall-Petch关系,并进一步出现反Hall-Petch效应的3个阶段,实验得到了偏离Hall-Petch关系为32 nm,产生反Hall-Petch现象的临界晶粒尺寸为8 nm,这2个临界晶粒尺寸与分子动力学方法对纳米晶纯金属A1计算的结果相当。     

Activation energy for vacancy migration in [001] tilt boundaries in nickel

The activation energy for vacancy migration in [001] tilt boundaries in nickel under the action of homogeneous tensile stress in the presence of a grain-boundary dislocation (GBD) was calculated with the help of atomistic computer simulation by the molecular-statics method using embedded-atom-method potentials. The Σ=5 (?=36.9°) special boundary as well as the boundary with a misorientation angle of ?=37.9° whose period contains 20 structural units of the Σ=5 (?=36.9°) boundary and one structural unit of the Σ=5 (?=53.1°) boundary simulating a GBD have been investigated. High applied stresses and long-range GBD-induced stresses are shown to exert no significant effect on the activation energy for vacancy migration. An analysis using calculated data on the energy for vacancy formation [1] shows that, due to the internal stresses in ordered boundaries, the diffusion coefficient along grain boundaries may increase only slightly.     

Hardness of porous nanocrystalline Co-Ni electrodeposits

The Hall-Petch relationship can fail when the grain size is below a critical value of tens of nanometres. This occurs particularly for coatings having porous surfaces. In this study, electrodeposited nanostructured Co-Ni coatings from four different nickel electroplating baths having grain sizes in the range of 11–23 nm have been investigated. The finest grain size, approximately 11 nm, was obtained from a coating developed from the nickel sulphate bath. The Co-Ni coatings have a mixed face centred cubic and hexagonal close-packed structures with varying surface morphologies and different porosities. A cluster-pore mixture model has been proposed by considering no contribution from pores to the hardness. As the porosity effect was taken into consideration, the calculated pore-free hardness is in agreement with the ordinary Hall-Petch relationship even when the grain size is reduced to 11 nm for the Co-Ni coatings with 77±2 at% cobalt. The present model was applied to other porous nanocrystalline coatings, and the Hall-Petch relationship was maintained.     

INTERNAL OXIDATION BEHAVIOR OF Pd-40Ag-1M(M=Zr,Y)ALLOYS

The internal oxidation behavior of Pd-40Ag-1M(M=Zr,Y)alloy wires has been studied inair at 800—1200℃.The relationship between the internal oxidation depth ξ and the reactiontime t can be expressed as ξ= Kt~n,where n=0.5—0.75.The higher the temperature,thelarger the value of n is.The active elements Zr and Y show different internal oxidationcharacters.For the alloys eontaining Zr,the oxidation rate along the grain boundaries isabout twice as high as that in grains,and“lateral oxidation”exists along the grainboundaries.For the alloys containing Y,the oxidation rates in grains and along the grainboundaries are roughly the same,and there is no“lateral oxidation”along the grain bounda-ries.The activation energies of both alloys are in the range of 120—150kJ/mol.Some prop-erties for oxidized alloys were studied.The mechanisms of the internal oxidation were dis-cussed.     

Superstrength of nanostructured metals and alloys produced by severe plastic deformation

Metals and alloys produced by severe plastic deformation (SPD) are characterized by not only an ultrafine grain size, but also other structural features, such as nonequilibrium grain boundaries, nanotwins, grain-boundary segregations, and nanoparticles. The present work deals with the study of the effect of these features on the strength of SPD metals and alloys. In particular, it has been shown that, with segregations on grain boundaries and nonequilibrium boundaries, the yield stress of the material can exceed considerably the values extrapolated to the range of ultrafine grains using the Hall-Petch relationship.     

Relaxation of the residual defect structure in deformed polycrystals under ultrasonic action

Using numerical computer simulation, the behavior of disordered dislocation systems under the action of monochromatic standing sound wave has been investigated in the grain of the model two-dimensional polycrystal containing nonequilibrium grain boundaries. It has been found that the presence of grain boundaries markedly affects the behavior of dislocations. The relaxation process and changes in the level of internal stresses caused by the rearrangement of the dislocation structure due to the ultrasonic action have been studied.     

The morphological and structural development of internal oxides in nickel-aluminum alloys at high temperatures

The formation and development of internal oxides in Ni-Al alloys containing 1–4 wt.% Al in Ni-NiO packs and in 1 atm oxygen at 800 to 1100°C have been studied. The internal oxide particles were relatively fine, closely spaced, and mainly acicular, although more granular near the surface. They were identified as Al₂O₃ at the advancing front, but NiAl₂O₄ at the surface and at a significant distance from that surface. Growth of internal oxide particles resulted in the development of significant compressive stresses in the internal oxide zone when formed in Ni-NiO packs. These stresses led to grainboundary sliding at the higher temperatures and extrusion of weak, internal oxide-denuded zones adjacent to alloy grain boundaries. At the lower temperatures, these stresses also resulted in significant preferential penetration of oxides down grain boundaries and sub-grain boundaries. Stress development and resulting phenomena were much less significant during oxidation in 1 atm oxygen because vacancies injected from the external NiO scale accommodated the volume increase during growth of internal oxide particles.     

Strains and stresses caused by penetrative wetting of grain boundaries by the liquid phase

We demonstrate that the dependence of grain boundary energy on composition leads to generation of normal stresses during grain boundary interdiffusion process. These self-generated stresses facilitate grain boundary embrittlement and rapid penetration of liquid phase along the grain boundaries.     

Superior fatigue crack growth resistance, irreversibility, and fatigue crack growth–microstructure relationship of nanocrystalline alloys

While previous studies have reported that nanocrystalline materials exhibit poor resistance to fatigue crack growth (FCG), the electro-deposited nanocrystalline Ni–Co alloys tested in this paper show superior resistance to FCG. The high damage tolerance of our alloy is attributed to the following: alloying with Co, low internal stresses resulting in stability of the microstructure, and a combination of high strength and ductility. The high density of grain boundaries interact with the dislocations emitted from the crack tip, which impedes FCG, as predicted by the present model and measured experimentally by digital image correlation. Further, the addition of Co increases the strength of the material by refining the grain size, reducing the fraction of low angle grain boundaries, and reducing the stacking fault energy of the material, thereby increasing the prevalence of twinning. The microstructure is stabilized by minimizing the internal stress during a stress relief heat treatment following the electro-deposition process. As a result grain growth does not occur during deformation, leaving dislocation-mediated plasticity as the primary deformation mechanism. The low internal stresses and nanoscale twins preserve the ductility of the material, thereby reaching a balance between strength and ductility, which results in a superior resistance to FCG.     

An elastic analysis of growth stresses during oxidation

A quantitative analysis of oxide growth stresses is carried out for the model advanced by Rhines and Wolf in which new oxide forms along preexisting oxide grain boundaries. The mean oxide stress developed within the oxide is calculated using standard techniques from continuum dislocation theory. This analysis shows that the mean growth stress is compressive and is directed parallel to the oxide/ matrix interface. The growth stress is found to be independent of the oxide scale thickness, provided that the scale is thicker than the oxide grain size. However, in thin scales, the growth stress is very sensitive to oxide scale thickness. The compressive growth stress increases in direct proportion to the width of the new grain boundary oxide layer formed. The oxide scale is expected to either fail by buckling, or the growth mode will change to one in which additional compressive stresses are not generated.     

INFLUENCE OF GRAIN SIZE ON STRENGTH AND HYDROGEN INDUCED CRACKING OF DUPLEX STAINLESS STEELS

The relation between grain size and strength of the duplex stainless steels and influence ofgrain size on properties of hydrogen induced cracking in these steels have been investigated.The Hall-Petch relation between grain size and strength of the steels is also followed.Thesusceptibility to hydrogen induced cracking of the steels increases with increasing grain size.     

Computer simulation of grain-boundary diffusion creep

Under externally applied stress, polycrystalline materials may deform as a result of matter diffusing from grain boundaries in compression to those in relative tension. We model the behaviour of a general structure under several different boundary conditions and show how the stress function along each grain boundary may be obtained by solving a system of linear equations. We further show how knowledge of the stresses enables us to predict the position of the grain boundaries after a small time-step δt and use this to simulate changes in the grain structure over a finite time interval. Our simulations indicate that grain-size distribution is of importance with regard to the onset of buckling. We are also able to track the movement of individual grains during deformation and our results show that, in an irregular grain structure, diffusion creep may cause significant grain rotation.