Toward a quantitative understanding of mechanical behavior of nanocrystalline metals

Focusing on nanocrystalline (nc) pure face-centered cubic metals, where systematic experimental data are available, this paper presents a brief overview of the recent progress made in improving mechanical properties of nc materials, and in quantitatively and mechanistically understanding the underlying mechanisms. The mechanical properties reviewed include strength, ductility, strain rate and temperature dependence, fatigue and tribological properties. The highlighted examples include recent experimental studies in obtaining both high strength and considerable ductility, the compromise between enhanced fatigue limit and reduced crack growth resistance, the stress-assisted dynamic grain growth during deformation, and the relation between rate sensitivity and possible deformation mechanisms. The recent advances in obtaining quantitative and mechanics-based models, developed in line with the related transmission electron microscopy and relevant molecular dynamics observations, are discussed with particular attention to mechanistic models of partial/perfect-dislocation or deformation-twin-mediated deformation processes interacting with grain boundaries, constitutive modeling and simulations of grain size distribution and dynamic grain growth, and physically motivated crystal plasticity modeling of pure Cu with nanoscale growth twins. Sustained research efforts have established a group of nanocrystalline and nanostructured metals that exhibit a combination of high strength and considerable ductility in tension. Accompanying the gradually deepening understanding of the deformation mechanisms and their relative importance, quantitative and mechanisms-based constitutive models that can realistically capture experimentally measured and grain-size-dependent stress–strain behavior, strain-rate sensitivity and even ductility limit are becoming available. Some outstanding issues and future opportunities are listed and discussed.     

Microstructure,Texture and Mechanical Properties of Titanium Grade 2 Processed by ECAP (Route C)

In the present work the properties of titanium grade 2 after ECAP processing with original route and regimes (route C, channel angle \(\varPhi\)?=?120°, deformation temperature 300 °C, number of passes up to 8) were examined. Texture development and microstructure parameters after ECAP processing and after recrystallization were determined using electron back scatter diffraction and analysed. A significant increase of the mechanical strength accompanied by some increase of ductility was observed in the deformed samples. The kernel average misorientation and average grain orientation spread were strongly increased after deformation, which confirms the material refinement and fragmentation. The proportion of low angle boundaries increased after four ECAP passes, but after four consecutive passes high angle grain boundaries became predominant. No deformation twins were observed after four and eight ECAP passes. The material recrystallized after deformation retained a fine grain microstructure. The textures of deformed and recrystallized samples were determined. It was found that texture after 8 passes is more homogeneous that that after 4 passes, which partly explains higher ductility of this first sample.     

On the fatigue crack growth behavior of WC–Co cemented carbides: kinetics description,microstructural effects and fatigue sensitivity

The influence of microstructure and load ratio (R) on the fatigue crack growth (FCG) characteristics of WC–Co cemented carbides are studied. In doing so, five hardmetal grades with different combinations of binder content and carbide grain size are investigated. Attempting to rationalize microstructural effects, key two-phase parameters, i.e. binder thickness and carbide contiguity, are used. On the other hand, the effect of load ratio is evaluated from the FCG behavior measured under R values of 0.1, 0.4 and 0.7. Experimental results indicate that: (1) WC–Co cemented carbides are markedly sensitive to fatigue; and (2) their FCG rates exhibit an extremely large dependence on K_max. Furthermore, both fatigue sensitivity and relative prevalence of K_max over ΔK, as the controlling fatigue mechanics parameter, are found to be significantly dependent upon microstructure. As mean binder free path increases, predominance of static over cyclic failure modes diminishes and a transition from a ceramic-like FCG behavior to a metallic-like one occurs (conversely in relation to contiguity). Consequently, the trade-off between fracture toughness and FCG resistance becomes more pronounced with increasing binder content and carbide grain size. The observed behavior is attributed to the effective low ductility of the constrained binder and its compromising role as the toughening and fatigue-susceptible agent in hardmetals, the latter on the basis that cyclic loading degrades or inhibits toughening mechanisms operative under monotonic loading, i.e. crack bridging and constrained plastic stretching.     

Effect of microstructure and load ratio on fatigue crack growth behavior of advanced Al-Cu-Li-Mg-Ag Alloys

Fatigue crack growth (FCG) behavior of recently developed three Al-Cu-Li-Mg-Ag alloys, Weldalite 049, X 2095 and MD 345, was examined in air at load ratios of 0.1 and 0.75. It was found that all three alloys showed better resistance to fatigue crack growth than conventional high strength Al alloys. The morphologies of crack growth paths were generally linear, but some showed deflection and branching. And the alloys revealed rough and transgranular fracture surfaces. Among the factors contributing to the excellent resistance of Al-Cu-Li alloys to fatigue crack growth are enhanced slip reversibility and high surface roughness causing a high crack closure level, thus reducing ΔK_eff for crack extension. The fatigue threshold decreased and fatigue crack growth rates increased significantly with increasing the load ratio. This is caused by the decrease in crack closure level at high load ratio. But the fracture mode did not show a significant change with increasing the load ratio.     

The influence of microstructure on the tensile and fatigue behavior of SAE 6150 steel

The tensile and reverse-bending fatigue behaviors of the SAE 6150 steel in the dual-phase (DP), fully martensitic, and tempered states, respectively, have been investigated using mechanical tests, scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) microscopy, and optical microscopy. Residual stresses, inherent microcracks, and retained austenite films in the martensitic steel, quenched from 900 °C, lead to the development of inferior tensile and fatigue strength. Tempering at 700°C relieves the residual stresses associated with martensite, causes the precipitation of microalloy carbides (MACs), and thus results in superior strength, increased fatigue resistance, and moderate ductility. The DP microstructure, consisting of martensite islets in a ferrite matrix, gives rise to a combination of good strength, excellent ductility, and commendable fatigue characteristics. MAC in the tempered steel and martensite islands in the DP variant enhance fatigue performance by causing crack tip deflection and concomitant crack path tortuosity. Strain incompatibility between martensite and ferrite in the DP steel, and cementite films and ferrite in the tempered variant are identified as fatigue crack initiation sites.     

Effect of Carbon Migration on Interface Fatigue Crack Growth Behavior in 9Cr/CrMoV Dissimilar Welded Joint

Fatigue crack growth (FCG) behavior of 9Cr/CrMoV dissimilar welded joint at elevated temperature and different stress ratios was investigated. Attention was paid to the region near the fusion line of 9Cr where carbon-enriched zone (CEZ) and carbon-depleted zone (CDZ) formed due to carbon migration during the welding process. Hard and brittle tempered martensite dominated the stress ratio-insensitive FCG behavior in the coarse grain zone (CGZ) of 9Cr-HAZ. For crack near the CGZ-CEZ interface, crack deflection through the CEZ and into the CDZ was observed, accompanied by an accelerating FCG rate. Compared with the severe plastic deformation near the secondary crack in 9Cr-CGZ, the electron back-scattered diffraction analysis showed less deformation and lower resistance in the direction toward the brittle CEZ, which resulted in the transverse deflection. In spite of the plastic feature in CDZ revealed by fracture morphology, the less carbides due to carbon migration led to lower strength and weaker FCG resistance property in this region. In conclusion, the plasticity deterioration in CEZ and strength loss in CDZ accounted for the FCG path deflection and FCG rate acceleration, respectively, which aggravated the worst FCG resistance property of 9Cr-HAZ in the dissimilar welded joint.     

Behavior of dopant-modified interfaces in metallic nanocrystalline materials

This article provides a concise discussion of thermodynamic and kinetic contributions to microstructure stabilization by dopant atoms located at interfaces in metallic nanocrystalline materials and an overview of recent molecular dynamics simulations used to study the behavior of dopant-modified interfaces in nanocrystalline copper. Molecular dynamics simulations show that antimony dopant atoms randomly positioned at copper grain boundaries can retard grain growth, in agreement with proposed thermodynamic and kinetic predictions. Simulations of uniaxial tensile deformation show a maximum in the flow strength at a grain size of 15 nm and that antimony dopants at the grain boundaries do not appreciably impact the nucleation of partial or full dislocations during uniaxial tensile deformation in nanocrystalline copper.     

Effect of single and duplex aging on precipitation response,microstructure, and fatigue crack behavior in Al-Li-Cu alloy AF/C-458

The interrelationships between precipitate characteristics and mechanical properties of Al-Li-Cu alloy AF/C-458 were quantified. The microstructure, precipitation response, and fatigue crack growth (FCG) rates in the Al-Li-Cu alloy AF/C-458 were studied following single and duplex aging treatments for varying aging times on specimens that were given a 6% stretch after solution heat treatment. The aging response was studied using hardness and compression yield strength measurements. Quantitative transmission electron microscopy methods were used to characterize average size, volume fraction, number density, and interparticle spacing of strengthening precipitates [i.e., δ’ (Al₃Li) and T₁ (Al₂CuLi)]. Strength and FCG rates for select heat treatments were obtained and were related to the precipitate microstructure and yield strength data.     

Observations of grain boundary impurities in nanocrystalline Al and their influence on microstructural stability and mechanical behaviour

The exceptional properties of nanocrystalline materials lend themselves to a wide range of structural and functional applications. There is recent evidence to suggest that grain boundary impurities may have a dramatic effect on the stability, strength and ductility of nanocrystalline metals and alloys. In this study, transmission electron microscopy and atom probe tomography were used to characterize specimens deposited at different base pressures, thus providing a direct comparison of impurity content with microstructural stability and mechanical behaviour. Atom probe measurements provide clear experimental evidence of grain boundary segregation of oxygen in samples deposited at higher base pressures. It is proposed that these oxygen atoms pin the boundaries, preventing stress-assisted grain growth and resulting in increased strength and loss in ductility. This study provides the first direct experimental evidence that boundary impurities play a critical role in determining the microstructural stability and deformation behaviour of nanocrystalline metals.     

Computer simulation of the effect of ultrasound and annealing on the structure of a two-dimensional severely deformed nanocrystalline material

Qualitative comparison of the effect of ultrasonic treatment and annealing on the structural relaxation of a two-dimensional nanocrystalline material after severe plastic deformation has been performed by the molecular-dynamics method. The treatments of both types lead to a decrease in the dislocation density and to a more equilibrium state of grain boundaries and triple junctions but, unlike the temperature effect, ultrasonic treatment does not result in a considerable grain growth. Thus, the calculations conducted indicate potentialities of the ultrasonic treatment of bulk nanomaterials with the aim of relieving internal stresses without increasing grain size.     

Micromechanisms of fatigue crack growth in a single crystal Inconel 718 nickel-based superalloy

The fatigue crack growth behavior of an experimental, single crystal alloy, of equivalent nominal chemical composition to Inconel 718 is presented. Fracture modes under cyclic loading were determined by scanning electron microscopy. The results of the fractographic analyses are presented on a fracture mechanism map that shows the dependence of the fatigue fracture mechanisms on the maximum stress intensity factor, K_max, and the stress intensity factor range, ΔK. Crack-tip deformation mechanisms associated with fatigue crack growth were studied using transmission electron microscopy. The relative effects of ΔK and K_max on the fatigue crack growth behavior of this material are discussed within the context of a two-parameter crack growth law. The influence of grain boundaries on the fatigue crack growth resistance of materials such as Inconel 718 is also discussed in light of the results of this investigation.     

A New Model for Inverse Hall-Petch Relation of Nanocrystalline Materials

In the present article, a new model for inverse Hall-Petch relation in nanocrystalline materials has been proposed. It is assumed that lattice distortion along grain boundaries can cause internal stresses and high internal stresses along grain boundaries can promote the grain boundary yielding. The designed model was then verified using the nanocrystalline-copper data. The minimum grain size for inverse Hall-Petch relation is determined to be about 11 nm for Cu.     

Effects of laser shock processing,solid solution and aging,and cryogenic treatments on microstructure and thermal fatigue performance of ZCuAl10Fe3Mn2 alloy

The materials used in variable temperature conditions are required to have excellent thermal fatigue performance. The effects of laser shock processing (LSP), solid solution and aging treatment (T6), and cryogenic treatment (CT) on both microstructure and thermal fatigue performance of ZCuAl₁₀Fe₃Mn₂ alloys were studied. Microstructure and crack morphology were then examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The result showed that, after being subjected to the combination treatment of T6+CT+LSP, the optimal mechanical properties and thermal fatigue performance were obtained for the ZCuAl₁₀Fe₃Mn₂ alloy with the tensile strength, hardness, and elongation of 720 MPa, 300.16 HB, and 16%, respectively, and the thermal fatigue life could reach 7,100 cycles when the crack length was 0.1 mm. Moreover, the ZCuAl₁₀Fe₃Mn₂ after combination treatment shows high resistance to oxidation, good adhesion between the matrix and grain boundaries, and dramatically reduced growth rate of crack. During thermal fatigue testing, under the combined action of thermal and alternating stresses, the microstructure around the sample notch oxidized and became loose and porous, which then converted to micro-cracks. Fatigue crack expanded along the grain boundary in the early stage. In the later stage, under the cyclic stress accumulation, the oxidized microstructure separated from the matrix, and the fatigue crack expanded in both intergranular and transgranular ways. The main crack was thick, and the path was meandering.     

冶炼工艺对0Cr15Ni25WMoTi2NbAl合金力学性能的影响

研究了真空感应 电渣和非真空感应 电渣两种工艺冶炼的0Cr1 5Ni2 5WMoTi2NbAl合金的杂质元素含量、组织与性能。因真空感应冶炼过程中晶界偏聚元素Pb得到了有效挥发 ,减轻了晶界的弱化效应 ,因而该合金中高温塑性、持久强度均不同程度提高。随着中温形变速率的降低 (拉伸、大应力持久、小应力持久 ) ,两工艺合金的晶界弱化效应加剧。未发现两者的基本组织以及晶界沉淀相的种类、数量、形貌和分布有明显差异。     

疲劳裂纹扩展过程中焊接残余应力重分布测试

提出了一种新的焊接残余应力在疲劳裂纹扩展过程中重分布规律的测试方法,并给出了相应的计算公式.在裂纹扩展的路径上粘贴若干三向电阻应变片,然后利用动态应变仪实时采集裂纹扩展过程中的应变变化,根据所推导的公式换算成相应的残余应力变化,从而得到裂纹扩展过程中残余应力重分布的规律.利用这种方法,试验测试了高强钢TIG焊接接头在疲劳裂纹扩展过程中的焊接残余应力重分布规律,并与一种解析的方法进行了对比.结果表明,该方法能够更为准确地表征裂纹扩展过程中焊接残余应力重分布的情况.     

Simultaneously Improving Mechanical Properties and Stress Corrosion Cracking Resistance of High-Strength Low-Alloy Steel via Finish Rolling within Non-recrystallization Temperature

The effect of hot rolling process on microstructure evolution,mechanical properties and stress corrosion cracking (SCC) resistance of high-strength low-alloy (HSLA) steels was investigated by varying the finish rolling temperature (FRT) and total rolling reduction.The results revealed granular bainite with large equiaxed grains was obtained by a total rolling reduction of 60％ with the FRT of 950 ℃ (within recrystallization temperature Tr).The larger grain size and much less grain boundaries should account for the relatively lower strength and SCC resistance.A larger rolling reduction of 80％ under the same FRT resulted in the formation of massive martensite-austenite (M/A) constituents and resultant low ductility and SCC resistance.In contrast,a good combination of strength,ductility and SCC resistance was obtained via 80％ rolling reduction with the FRT of 860 ℃ (within non-recrystallization temperature Tnr),probably because of the fine grain size and M/A constituents,as well as a high density of grain boundary network.     

Effect of Microstructure and Environment on Static Crack Growth Resistance in Alloy 706

The relationship between thermo-mechanical processing, resultant microstructure, and mechanical properties has been of interest in the field of metallurgy for centuries. In this work, the effect of heat treatment on microstructure and key mechanical properties important for turbine rotor design has been investigated. Specifically, the tensile yield strength and crack growth resistance for a nickel-iron based superalloy 706 has been examined. Through a systematic study, a correlation was found between the processing parameters and the microstructure. Specifically, differences in grain boundary and grain interior precipitates were identified and correlated with processing conditions. Further, a strong relationship between microstructure and mechanical properties was identified. The type and orientation of grain boundary precipitates affect time-dependent crack propagation resistance, and the size and volume fraction of grain interior precipitates were correlated with tensile yield strength. It was also found that there is a strong environmental effect on time-dependent crack propagation resistance, and the sensitivity to environmental damage is microstructure dependent. Microstructures with η decorated grain boundaries were more resistant to environmental damage through oxygen embrittlement than microstructures with no η phase on the grain boundaries. An effort was made to explore the mechanisms of improving the time-dependent crack propagation resistance through thermo-mechanical processing, and several mechanisms were identified in both the environment-dependent and the environment-independent category. These mechanisms were ranked based on their contributions to crack propagation resistance.