Ultrafine-Grained Metals

Ultrafine-grained metallic materials produced by severe plastic deformation methods are at the cutting edge of modern materials science. UFG-metals exhibit outstanding properties which make them very interesting for structural or functional engineering applications. Fifteen articles in this special issue address a broad variety of topics: New developments in severe plastic deformation techniques, advances in modeling and simulation of the severe plastic deformation processes, mechanical properties under monotonic and cyclic loading of homogenous and graded UFG structures, dominating deformation mechanisms in UFG materials, advances and strategies for high conductivity UFG-materials, correlation between severe plastic deformation parameters and resulting materials properties and peculiarities in the corrosion behavior of UFG materials. The book covers latest results on ultrafine-grained titanium, aluminum and copper alloys and on UFG iron and steels and thus provides a deep insight to current research activities in the field of ultrafine-grained metals

Quantifying Co-Deformation Effects in Metallic Laminates by Loading–Unloading–Reloading Tensile Tests

Heterostructured materials such as metallic laminates (LMCs) can be specifically tailored to showcase significantly increased mechanical behavior based on the hetero-deformation-induced (HDI) strengthening effect caused by the co-deformation at the vicinity of interfaces. This study introduces a new approach to quantify these co-deformation effects in metallic laminates by characterizing the behavior of inelastic back strain upon unloading. Experimentally, the inelastic back strain (IBS) is determined by cyclic loading–unloading–reloading (LUR) tensile tests. Compared to a linear rule of mixture (ROM) approximation used as a reference, additional amounts of inelastic back strain were measured for different metallic laminate systems, strongly depending on the dissimilarities of yield strength and elastic moduli of constituents and the interface density in the laminates. Conducting finite element analysis, the distribution of residual plastic strain was investigated for the different metallic laminates used in this study. Based on this, a schematic overview of the spatial distribution of the hetero-deformation zone for metallic laminates with dissimilar yield strength and elastic moduli is derived, summarizing the results of this study. As most mechanical components are subject to cyclic stresses during the application, the method provided in this study to characterize the co-deformation behavior of metallic laminates in the microyielding regime enables valuable insights into mechanisms affecting the cyclic deformation behavior of metallic laminates for future applications

Residual Stresses in Ultrafine‐Grained Laminated Metal Composites Analyzed by X‐ray Diffraction and the Hole‐Drilling Method

Accumulative roll bonding is an advanced manufacturing process, which is capable of simultaneously refining the grain size into the nanometer regime and bonding different metallic sheet materials. Herein, homogenous aluminum/aluminum as well as heterogeneous aluminum/steel laminated metal composite (LMCs) are fabricated. The residual stresses are experimentally determined by X‐ray diffraction and the hole‐drilling method. Generally, a complex residual stress profile is found in all LMCs. The level of residual stress strongly depends on the bonded materials. Compressive residual stresses are induced in all sheets in the near surface area. These stresses range from −5 MPa in aluminum to −240 MPa in steel. In the homogenous aluminum/aluminum LMCs, compressive stresses up to −26 MPa in the softer layers and tensile stresses up to 30 MPa in the stronger layers are built up. This is different to heterogeneous aluminum/steel LMCs, where tensile stresses up to 40 MPa in the softer aluminum layers and compressive stresses up to −72 MPa in the inner harder steel layers are present. Based on the results obtained it is possible to directly design the material combination or stacking architecture of ultrafine‐grained LMCs to tailor the residual stress profile

Rotating Scan Strategy Induced Anisotropic Microstructural and Mechanical Behavior of Selective Laser Melted Materials and Their Reduction by Heat Treatments

Choosing a properly optimized rotating scan strategy during the selective laser melting (SLM) process is essential to reduce residual stresses and thus to obtain homogeneous properties. Surprisingly, anisotropic material properties are found in several materials that are built with the often applied rotating stripes scan strategy of Electro‐Optical Systems (EOS) because the scan strategy avoids possible interactions of the laser beam with process by‐products and therefore excludes a range of scanning directions. Herein, the alloys Hastelloy X, Inconel 718, and stainless steel 316L are investigated. Vertically built specimens with a cylindrical gauge geometry show an oval deformation during tensile testing, indicating a mechanical anisotropy in the horizontal x‐ and y‐direction. Tensile tests along the x‐ and y‐direction reveal a deviation of the yield strength of 7% for Hastelloy X. Analyses of the microstructures show differences in the grain morphology, size, and texture in all three coordinate planes of the three materials. This anisotropic behavior can be explained by a detailed study of the texture and the calculated Schmid factors. Heat treatments can reduce the textural and mechanical anisotropy due to recrystallization of grains but requires annealing at sufficiently high temperatures and long times

Development of Microstructure and Mechanical Properties of TiAl6V4 Processed by Wire and Arc Additive Manufacturing

Wire and arc additive manufacturing (WAAM) has the potential to significantly reduce material waste due to the milling of components made of TiAl6V4 (Ti‐64). To keep up with the market development, this resource‐efficient technology is becoming increasingly important to achieve climate policy goals. Therefore, this study not only focuses on the influence of different process parameters, such as torch and wire feed speed, but also different gas mixtures on the microstructure and related mechanical properties, as well as on the scalability by investigating single‐ to multilayer welded structures. The wire feed speed is found to have a major influence on the geometry and mechanical properties. The use of different process gases, i.e., argon (Ar), helium (He), and a mixture of 70% He and 30% Ar neither significantly affect the microstructure nor the mechanical properties. It is also found that a solution heat treatment followed by an annealing step degrades mechanical properties, while an ordinary stress‐relief heat treatment leads to improved mechanical properties. It is shown that by adapting WAAM process and heat treatment parameters, mechanical properties of additively produced specimens can be achieved, which are fully comparable to milled components

Ultrafine-Grained Metals

Ultrafine-grained (UFG) metallic materials are at the cutting edge of modern materials science as they exhibit outstanding properties which make them very interesting for prospective structural or functional engineering applications. [...

Particle Based Alloying by Accumulative Roll Bonding in the System Al-Cu

The formation of alloys by particle reinforcement during accumulative roll bonding (ARB), and subsequent annealing, is introduced on the basis of the binary alloy system Al-Cu, where strength and electrical conductivity are examined in different microstructural states. An ultimate tensile strength (UTS) of 430 MPa for Al with 1.4 vol.% Cu was reached after three ARB cycles, which almost equals UTS of the commercially available Al-Cu alloy AA2017A with a similar copper content. Regarding electrical conductivity, the UFG structure had no significant influence. Alloying of aluminum with copper leads to a linear decrease in conductivity of 0.78 µΩ∙cm/at.% following the Nordheim rule. On the copper-rich side, alloying with aluminum leads to a slight strengthening, but drastically reduces conductivity. A linear decrease of electrical conductivity of 1.19 µΩ∙cm/at.% was obtained

Influence of backpressure during ECAP on the monotonic and cyclic deformation behavior of AA5754 and Cu99.5

Ultrafine-grained (UFG) metals produced by equal channel angular pressing (ECAP) exhibit outstanding mechanical properties. They show high strength under monotonic loading as well as strongly enhanced fatigue lives in the Wöhler S-N-plot compared to their coarse grained (CG) counterparts. It could be shown that the fatigue lives can be significantly enhanced further by applying backpressure during ECAP. Besides the positive effect of backpressure on the processability of hard to deform materials via ECAP, the hydrostatic stress induced by backpressure also influences the mechanical properties under monotonic and cyclic loading. Therefore the influence of backpressure on ECAPed Cu99.5 and on the ECAPed aluminum alloy AA5754 was investigated. It is shown that backpressure has no effect on the hardness and grain size in Cu99.5 but changes the grain boundary misorientation to higher fractions of low angle grain boundaries. Also the temperature dependency of the yield strength as well as the hardening behavior under monotonic compression is affected. The cyclic deformation behavior of Cu99.5 is not strongly influenced by backpressure, but the mean stress level changes drastically. The fatigue life increases with the application of backpressure at low plastic amplitudes due to a change in the crack initiation and propagation. Aim of this work is the investigation of the influence of backpressure during equal channel angular pressing (ECAP) on the mechanical properties under monotonic and cyclic loading. Therefore we performed hardness measurements, compression, and fatigue tests on ECAPed Cu99.5 and AA5754. The results are discussed in terms of microstructure and relevant deformation and damage mechanisms

Influence of Zn and Sn on the Precipitation Behavior of New Al–Mg–Si Alloys

In this study, we demonstrate how Zn and Sn influence hardening behavior and cluster formation during pre-aging and paint bake treatment in Al–Mg–Si alloys via hardness tests, tensile tests, and atom probe tomography. Compared to the standard alloy, the Sn-modified variant shows reduced cluster size and yield strength in the pre-aged condition. During the paint bake cycle, the clusters start to grow very fast and the alloy exhibits the highest strength increment. This behavior is attributed to the high vacancy binding energy of Sn. Adding Zn increases the formation kinetics and the size of Mg–Si co-clusters, generating higher yield strength values for both the pre-aged and paint baked conditions. Simultaneous addition of Zn and Sn creates a synergistic effect and produces an alloy that exhibits moderate strength (and good formability) in the pre-aged condition and accelerated hardening behavior during the paint bake cycle.ISSN:1996-194

Microstructure and Mechanical Properties of Accumulative Roll-Bonded AA1050A/AA5005 Laminated Metal Composites

Laminated metal composites (LMCs) with alternating layers of commercial pure aluminum AA1050A and aluminum alloy AA5005 were produced by accumulative roll-bonding (ARB). In order to vary the layer thickness and the number of layer interfaces, different numbers of ARB cycles (4, 8, 10, 12, 14 and 16) were performed. The microstructure and mechanical properties were characterized in detail. Up to 8 ARB cycles, the ultrafine-grained (UFG) microstructure of the layers in the LMC evolves almost equally to those in AA1050A and AA5005 mono-material sheets. However, the grain size in the composites tends to have smaller values. Nevertheless, the local mechanical properties of the individual layers in the LMCs are very similar to those of the mono-material sheets, and the macroscopic static mechanical properties of the LMCs can be calculated as the mean value of the mono-material sheets applying a linear rule of mixture. In contrast, for more than 12 ARB cycles, a homogenous microstructure was obtained where the individual layers within the composite cannot be visually separated any longer; thus, the hardness is at one constant and a high level across the whole sheet thickness. This results also in a significant higher strength in tensile testing. It was revealed that, with decreasing layer thickness, the layer interfaces become more and more dominating