Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys

Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [$ \bar 1 $ \bar 1 11] and [$ \bar 1 $ \bar 1 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.     

Measurement of grain bridging in some Al-Cu and Al-Sn alloys

A quantitative investigation of interdendritic bridging in some Al-Cu and Al-Sn alloys is presented. The experimental measurement methods and bridging fractions at different eutectic liquid fractions are given. The different bridging fractions in Al-Cu and Al-Sn of different compositions have been found, and the effect of grain refinement on the bridging between the grains is analysed. The measurement is based on metallographic investigation of samples that have been quenched just above the eutectic temperature. Electron microprobe analyses across the bridges have been done and the diffusion of Cu in Al has been calculated in order to rule out alternative interpretations of experimental investigations. Finally, the possibilities of incorporating the experimental results into hot tearing models are discussed. IJCMR/501     

Processing,deformation, and failure in superplastic aluminum alloys: Applications of orientation-imaging microscopy

The importance of grain size refinement in enabling superplasticity is reviewed, and the current understanding of grain boundary characteristics is summarized. The application of orientation-imaging microscopy (OIM) methods to the processing response and the deformation and failure modes in superplastic aluminum alloys are illustrated through microtexture analysis and determination of grain boundary characteristics in selected commercial materials. Continuous and discontinuous recrystallization reactions exhibit distinct microtextures and grain boundary characteristics. The application of OIM and microtexture analysis to the evaluation of both deformation and failure mechanisms during superplastic forming is illustrated. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming, sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

Competing deformation mechanisms in nanocrystalline metals and alloys: Coupled motion versus grain boundary sliding

Plastic deformation of nanocrystalline Pd and Cu as well as the demixing systems Cu–Nb and Cu–Fe is studied by means of atomic-scale computer simulations. The microstructures are specifically chosen to facilitate mesoscopic grain boundary sliding. The influence of segregating solutes on the deformation mechanisms is studied and different cases of solute distributions are compared. We find that the competition between mesoscopic grain boundary sliding and coupled grain boundary motion is controlled by the concentration and distribution of segregating solutes. By analyzing the microstructural evolution and dislocation activity we make a connection between the atomistic solute distribution and the mechanisms of deformation, explaining the observed stress–strain behavior. The detailed analysis of the normal grain boundary motion reveals a stick–slip behavior and a coupling factor which is consistent with results from bicrystal simulations.     

机械合金化制备Al-Sn合金及其摩擦性能研究

采用机械合金化和粉末烧结的方法制备了纳米细晶/粗晶Al-Sn合金,研究了粗晶含量对合金微观形貌、摩擦形貌和耐磨性能的影响,探讨了Al-12Sn合金的磨损机制及影响因素。结果表明,机械合金化得到的双相双尺度Al-12Sn合金,在不同载荷下的摩擦系数要高于未添加粗晶的纯纳米晶Al-12Sn合金,但是低于纯粗晶Al-12Sn合金;当粗晶Al-12Sn粉末比例为30%时,烧结Al-12Sn合金具有最佳的抗摩擦磨损性能,其耐磨性能高于纯纳米晶和纯粗晶Al-12Sn合金,提高幅度分别约为1.5倍和2倍;随着粗晶Al-12Sn粉末比例的提高,合金的磨损机制从局部剥落演变为大面积摩擦层的剥落,并且摩擦表面的氧化摩擦层的数量不断降低。     

Producing a gradient-composition nanocrystalline structure on nitrided surfaces of invar-type Fe-Ni alloys using megaplastic deformation

A nanocrystalline Fe-Ni matrix strengthened by dispersed CrN and TiN nitrides has been produced on the ion-plasma-nitrided surfaces of the austenitic Fe-Ni₃₈-Cr₁₅ and Fe-Ni₃₆-Ti₄ alloys using cyclic “nitride dissolution-nitride precipitation” phase transformations induced by megaplastic deformation. The high-pressure torsion of the nitrided alloys has led to the dissolution of the CrN nitrides and Ni₃Ti intermetallic compounds, which appeared in the matrix, in the surface layer and to the mechanical alloying of the nitrided subsurface layer and the unnitriderd bulk of the specimens. Subsequent annealing has resulted in the formation of secondary nitrides, which propagated to a depth substantially exceeding the thickness of the original nitrided layer.     

Electrochemical studies of amorphous,nanocrystalline, and crystalline FeSiB based alloys

Amorphous, nanocrystalline, and crystalline FeSiB based alloys were analyzed by means of the potentiodynamic anodic polarization technique in alkaline and neutral chloride media. The influence on their corrosion behavior of both pH and the addition of Sn, Cu, Nb, and Al was studied. Studies of X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy were performed.     

Mechanical behavior of nanocrystalline metals and alloys

Nanocrystalline metals and alloys, with average and range of grain sizes typically smaller than 100 nm, have been the subject of considerable research in recent years. Such interest has been spurred by progress in the processing of materials and by advances in computational materials science. It has also been kindled by the recognition that these materials possess some appealing mechanical properties, such as high strength, increased resistance to tribological and environmentally-assisted damage, increasing strength and/or ductility with increasing strain rate, and potential for enhanced superplastic deformation at lower temperatures and faster strain rates. From a scientific standpoint, advances in nanomechanical probes capable of measuring forces and displacements at resolutions of fractions of a picoNewton and nanometer, respectively, and developments in structural characterization have provided unprecedented opportunities to probe the mechanisms underlying mechanical response. In this paper, we present an overview of the mechanical properties of nanocrystalline metals and alloys with the objective of assessing recent advances in the experimental and computational studies of deformation, damage evolution, fracture and fatigue, and highlighting opportunities for further research.     

Cyclic deformation of nanocrystalline and ultrafine-grained nickel

The cyclic deformation behavior of ultrafine-grained (UFG) Ni samples synthesized by the electrodeposition method was studied. Different from those made by severely plastic deformation, the UFG samples used in this study are characterized by large-angle grain boundaries. Behaviors from nanocrystalline Ni and coarse-grained Ni samples were compared with that of ultrafine-grained Ni. With in situ neutron diffraction, unusual evolutions of residual lattice strains as well as cyclic hardening and softening behavior were demonstrated during the cyclic deformation. The microstructural changes investigated by TEM are discussed with respect to the unusual lattice strain and cyclic hardening/softening.     

Hydrogen generation using the corrosion of Al-Sn and Al-Si alloys in an alkaline solution

We investigated the effects of adding Sn and Si to Al alloys on the corrosion of the alloys and the generation of hydrogen from an alkaline solution using the alloys. With increasing Sn content of up to 20 wt% in the Al-Sn alloy, the volume fraction of the Sn phase as a cathodic site at grain boundaries increased, and consequently, the hydrogen generation rate from an alkaline solution by the alloy also increased. In addition, the quenched Al-Sn alloys had smaller grain sizes compared to the furnace-cooled alloys, and accordingly, exhibited a slightly higher hydrogen generation rate. A galvanic cell was formed between the Al grain and the Sn phase of the grain boundary, and accordingly, intergranular type corrosion was observed on the Al-Sn alloys. Compared with the Al-Sn alloys, a more uniform type corrosion was observed on the Al-Si alloys because the nobler Si was uniformly distributed in the eutectic region formed between the primary Al grains. The hydrogen generation rate increased with an increasing Si content up to 10 wt% and was greater for the furnace-cooled samples than that for the quenched samples due to the more clearly formed eutectic structure.     

The deformation physics of nanocrystalline metals: Experiments, analysis, and computations

This article presents a review of the principal mechanisms responsible for the plastic deformation of nanocrystalline metals. As the concentration of grain boundaries increases, with a decrease in grain size there is a gradual shift in the relative importance of the deformation mechanisms away from the ones operating in the conventional polycrystalline domain. This is predicted by molecular dynamics simulations that indicate a preponderance of dislocation emission/annihilation at grain boundaries and grain-boundary sliding when grain sizes are in the range 20–50 nm. Experiments show, in general, a saturation in work hardening at low strains, which is indicative of a steady-state dislocation density. This saturation is accompanied by an increased tendency toward shear localization, which is supportive of dislocation generation and annihilation at grain boundaries. Dislocation analyses recently proposed corroborate the computational predictions and provide a rational foundation for understanding the mechanical response.     

固化球磨纳米晶粉末制备块体超细晶Mg-3Al-Zn合金(英文)

采用粉末冶金技术制备块体超细晶Mg-3Al-Zn合金。首先采用球磨Mg、Al、Zn混合粉末来制备纳米晶粉末,所得的粉末的平均晶粒尺寸为45nm。随后将球磨好的粉末封入铝包套内,分别在室温和633K温度下,在真空烧结炉内进行真空热压。然后将烧结后的样品在423K下挤压以进行进一步的致密化处理。结果表明:致密后的冷压样品的晶粒尺寸为180nm,而热压坯的晶粒尺寸为600nm,冷压样品的屈服强度达464MPa;超细晶镁合金的强化机制主要是细晶强化,这主要是由于HCP结构的材料晶粒尺寸对材料的影响更为明显。固化后冷压样品的最终密度为(1.777±0.006)g/cm3,而热压样品的最终密度为(1.800±0.006)g/cm3。     

Accommodation processes during deformation of nanocrystalline palladium

Atomistic simulations of uniaxial tensile and compressive straining of three-dimensional nanocrystalline palladium were performed at room temperature and different strain rates. Detailed analysis revealed that initial plastic deformation is due to grain boundary sliding accommodated by localized bending inside the grains and the formation of dislocation embryos. Intergranular cracking in the absence of dislocation activity was found at later stages of tensile straining. During compressive straining the sample shows a plastic response which is brought about mainly by intergranular accommodation processes. The contribution of extended partial dislocations emitted from the grain boundaries as well as full dislocations and twinning at later stages of deformation to the total strain was found to be insignificant.     

Grain growth in nanocrystalline iron and Fe-Al alloys

The effects of the annealing temperature and time, cryomilling in liquid nitrogen, and the addition of aluminum powder on the thermal stability and grain growth behavior of nanocrystalline iron were modeled using the Artificial Neural Network (ANN) technique. The developed model can be used as a guide for the quantification of the grain growth by considering the effects of annealing temperature and time. The model also quantified the effect of Al on the thermal stability of cryomilled nanocrystalline Fe. The model results showed that the cryomilling of Fe has a tangible effect on the stabilization of the nanostructure.     

Tailoring and patterning the grain size of nanocrystalline alloys

Nanocrystalline alloys that exhibit grain boundary segregation can access thermodynamically stable or metastable states with the average grain size dictated by the alloying addition. Here we consider nanocrystalline Ni–W alloys and demonstrate that the W content controls the grain size over a very broad range: ∼2–140 nm as compared with ∼2–20 nm in previous work on strongly segregating systems. This trend is attributed to a relatively weak tendency for W segregation to the grain boundaries. Based upon this observation, we introduce a new synthesis technique allowing for precise composition control during the electrodeposition of Ni–W alloys, which, in turn, leads to precise control of the nanocrystalline grain size. This technique offers new possibilities for understanding the structure–property relationships of nanocrystalline solids, such as the breakdown of Hall–Petch strength scaling, and also opens the door to a new class of customizable materials incorporating patterned nanostructures.     

Room-temperature and high-temperature magnetic permeability of Co-doped nanocrystalline alloys

Influence of composition and annealing temperature on structure and magnetic properties of amorphous and nanocrystalline Fe_(78.4—x)Co_xSi_9 B_9 Nb_(2.6)Cu_1(x=27.4,40.0,51.0,78.4) alloys was investigated by X-ray diffraction(XRD) and the temperature dependence of permeability. According to the initial crystallization temperature(T_(x1)) from differential scanning calorimetry(DSC) curves of as-quenched amorphous alloys,490-700 ℃ isothermal annealing was carried out to obtain the characteristic nanocrystalline structure. Furthermore,the soft magnetic properties were measured by temperature evolution of magnetic permeability to obtain the correlation between Co content, annealing temperature and magnetic permeability. The results show that, on the one hand,the annealing temperature exerts a significant effect on phase structure and initial permeability(μ_i). The highertemperature(from 550 to 610 ℃) annealed Co content nanocrystalline samples can remain higher μ_i at elevated temperature. On the other hand, partial substitution Fe by Co can improve the high-temperature magnetic stability;however, the room-temperature permeability of higher Co content alloys decreases obviously at the same time. This phenomenon was analyzed from the viewpoint of the saturation magnetic induction(B_s), magnetic anisotropy(K) and magnetostriction(λ_s).     

Fe基非晶纳米晶的等温退火工艺

通过重力式单辊喷带机制备成分为Fe73.5Si13.5B9Nb3Cu1非晶带材,并对其进行等温退火处理。等温DSC测试结果表明,重力非晶条带的等温晶化过程是受三维扩散控制的、形核率不断减小的晶化过程,以晶化相的稳态长大过程为主。随着退火温度增高,纳米晶α-Fe相的晶格常数不断减小,表明固溶在α-Fe晶格中的Si含量不断增加。与此同时,α-Fe的晶粒尺寸仅仅略有增加。真空退火样品磁性能测试结果表明,该成分样品的最合适的热处理工艺为退火温度552 ℃,保温时间为80 min。