To increase the safety of steels in high performance cases like crash energy absorption, even better properties of the materials are necessary. To advance this research, a TWIP and a TRIP steel were combined in a laminated composite via roll bonding at 450 °C with the goal of using accumulative roll bonding (ARB) in later research to further enhance the properties reaching an ultra-fine-grained material. Two different TWIP layer thicknesses (2 mm and 3 mm) were experimentally roll bonded with a 3 mm thick TRIP layer each using a 4-high rolling mill. A modular Python-based simulation incorporating coupled solving of ordinary differential equations of the temperatures and the horizontal stress changes of the layers were implemented to predict deformation and bonding behavior. Simulated results matched well with experimental data in terms of final geometry and temperature, while roll force deviations indicated the need for the refining of the used model. Furthermore, experimentally asymmetric layer relationships at the beginning and the addition of a thin (10 µm) Ni interlayer were found to enhance bond strength in high-strength steel laminates.

AbstractThis chapter presents results on the analysis of nonmetallic as well as intermetallic inclusions within a metal matrix. In both, steel and aluminum matrix these impurities cause detrimental effects during production as well as in service, e.g. under mechanical load. In steel, nonmetallic inclusions originate from the steelmaking process and range in the magnitude of ppm. In recycled aluminum alloys, iron-rich intermetallic phases exhibit a volume fraction in the range of percent caused by insufficient scrap separation. Both types of detrimental inclusions/precipitates were investigated within different materials such as case hardening steel, quenched and tempered steel as well as Al-Si cast alloy. In order to reduce the amount of impurities, the effects of appropriate crucible materials, reactive and active melt filtration and chemical composition of the used materials were studied. Therefore, extensive metallographic investigations on sections were conducted with optical microscopy, manual and automated scanning electron microscopy, focused ion beam preparation and transmission electron microscopy aiming to determine the compositions of inclusions and intermetallic phases. Focusing on the morphology of inclusions and intermetallic phases, experiments with electrolytic and chemical extraction as well as X-ray micro tomography were performed. The gained knowledge can be utilized to improve filtration and reduce volume fraction and size of nonmetallic inclusions and intermetallic phases. This enables the design of long-lasting and safe materials.

Ultrasonic fatigue testing (USFT) is a time-efficient method for evaluation of the fatigue limit of metallic alloys in a high and very high cycle range. Propagation of a final crack at the very end of the fatigue life can be clearly detected by USFT parameters and monitoring techniques. In contrast, initiation of fatigue damage during USFT remains unclear. Despite its excellent sensitivity, the implementation of acoustic emission (AE) for damage monitoring is hindered by severe noisiness of acoustic signals from USFT. In addition to resonance-related quasi-stationary noises, USFT in a pulse-pause mode is accompanied by non-stationary ones, which makes recognition of material-related AE signals even more difficult. A special AE processing algorithm was developed to overcome this issue and get useful insights from USFT monitoring. Consistent cropping, Fourier transformation, adaptive filtration and thresholding allowed to calculate noise-free AE activity during USFT of 42CrMo4 steel Remarkably, most of this activity was located in the very beginning of fatigue loading. In comparison to non-failed runout samples, AE activity of samples with fatigue cracks was significantly higher, indicating its relation to fatigue damage. The proposed AE processing principle can be helpful for monitoring of USFT samples and other parts, operating in resonance conditions.

AbstractThis chapter concerns the influence of internal defects (i.e. nonmetallic inclusions, secondary phases and cast defects) on the fatigue lifetime of steel and aluminum alloys in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. The detrimental effect of internal defects depends on multiple factors such as size, morphology, chemical composition, test temperature or position in the material. Specimens were tested after active and/or reactive melt filtration processes of the materials which served to influence the amount and size distribution of internal defects. Fatigue experiments up to 109 cycles were carried out using ultrasonic fatigue testing equipment. In addition, in situ methods, as e.g. full surface view thermography and acoustic emission (AE), were applied to study the processes of crack initiation and propagation, which finally lead to fatigue failure. Furthermore, the cyclically strained samples were subjected to fractographic analysis and the S–N-curves were discussed according to the characteristics of the crack-initiating defects. Based on these investigations, an enhanced knowledge about the correlation of internal defects on the materials’ fatigue strength enables a specific melt filtration strategy adjusted to the materials’ service conditions.

Accumulative roll bonding (ARB) is a repeated cladding process in which two or more sheets of material are joined together by rolling at temperatures below recrystallization. The present review is focused on ARB of high‐alloy steels, which, among other laminated metal composites (LMCs), deliver the highest mechanical properties. After a brief description of high‐strength steels, history, and state of the art of LMCs, the principal roll bonding mechanism is explained. Further, the methodology of ARB of steels and variable parameters (stacking, temperature, etc.) are discussed. Known examples of steel–steel laminates are summarized with respect to their rolling temperature and mechanical properties. Further, the main toughening mechanisms of steel‐based LMCs are listed. The most promising candidates of high‐alloy steel laminates are presented in more detail. The important deformation mechanisms of twinning‐ and transformation‐induced plasticity (TWIP and TRIP) high‐alloy steels are explained. Microstructural changes and layer bonding as well as mechanical properties and damage behavior of two‐ and four‐layered TRIP/TWIP steel laminates are illustrated, including some specific phenomena, such as deformation lenses. Finally, by summarizing the analyzed data on steel laminates, conclusions and outlook are formulated.

The combination of strength of transformation-induced plasticity (TRIP) steel and ductility of twinning-induced plasticity (TWIP) steel can be achieved by manufacturing laminated composites via cold roll bonding (CRB). Work hardening of the surface before CRB produces deformation lenses (DLs), which play significant role in bonding, but are reported rarely in the literature. The present work aimed to study the DLs at the bonding interface of the laminated composite made of high-alloy TRIP and TWIP steels manufactured by CRB. The DLs and interfaces were investigated by means of scanning and transmission electron microscopy, roughness measurement, tensile and peel tests. Laminates showed ultimate tensile strength up to 900 MPa and elongation up to 45% maintaining the layer’s integrity up to failure. The TWIP–TWIP interface has shown higher maximum peel strength (up to 195 N/cm) than that of a TRIP–TWIP interface (up to 130 N/cm), which was found to be in direct proportion to the overall area of DLs. Bonding of the laminate layers was found to occur between DL fragments.

For the first time a four-layered laminate made of X5CrMnNi-16-6-6 (TRIP steel) and X5CrMnNi16-6-9 (TWIP steel) was manufactured by accumulative roll bonding (ARB). An intermediate annealing between rolling passes allowed to achieve good bonding between the laminate layers. The microstructure of the laminate at each step of production was analysed with the main focus on the bonding interface and deformation lenses by means of scanning electron microscopy, including chemical and orientation mapping techniques, and microhardness measurements. Additionally, a special measuring routine was developed to reveal spatial distribution of the grain size along the cross section. Finally, tensile specimens were cut out from the laminate and tested under quasi static loading with subsequent fracture surface analysis. Repeated rolling and annealing of the steel layers resulted in the alteration of the grain size and microhardness, whereas significant texture appeared only after tensile loading. The grain size in the layer of TWIP steel after first roll bonding and annealing was found to drop down near the surface, whereas no such effect was found in TRIP steel. The microstructure of a deformation lens was found to fully change from severely deformed brittle steel matrix with excessive oxygen content after rolling to a ductile mixture of sub-micron globular oxides and austenite grains after annealing. These ductile deformation lenses ensure excellent bonding of layers and microcrack blunting. This allowed to achieve a remarkable combination of mechanical properties of the four-layered TRIP-TWIP laminate with a yield strength up to 800 MPa and an elongation to failure up to 45%, which resulted in the work of deformation up to 41 GPa.%.

Despite rapid development of laser powder bed fusion (L-PBF) and its monitoring techniques, there is still a lack of in situ crack detection methods, among which acoustic emission (AE) is one of the most sensitive. To elaborate on this topic, in situ AE monitoring was applied to L-PBF manufacturing of a high-strength Al92Mn6Ce2 (at. %) alloy and combined with subsequent X-ray computed tomography. By using a structure borne high-frequency sensor, even a simple threshold-based monitoring was able to detect AE activity associated with cracking, which occurred not only during L-PBF itself, but also after the build job was completed, i.e. in the cooling phase. AE data analysis revealed that crack-related signals can easily be separated from the background noise (e.g. inert gas circulation pump) through their specific shape of a waveform, as well as their energy, skewness and kurtosis. Thus, AE was verified to be a promising method for L-PBF monitoring, enabling to detect formation of cracks regardless of their spatial and temporal occurrence.

The present study investigates the influence of melt conditioning and filtration on iron-rich phases in AlSi9Cu3 alloy. This method avoids the formation of brittle β phase (Al4.5FeSi) and reduces the iron content by sedimentation and subsequent filtration. The comparison of this method to conventional casting (reference) with a higher Fe content is performed by means of scanning electron microscopy, tensile tests, ultrasonic fatigue tests, X-ray diffraction and X-ray microtomography. The reference batch revealed a high proportion of β plates, which are responsible for low strength compared to melt conditioned batch under uniaxial tensile stress. The fatigue properties of melt conditioned batch are significantly improved compared to the reference state. X-ray microtomography scans before and after ultrasonic fatigue tests were evaluated by machine learning algorithms (Trainable Weka Segmentation). The superposition of the segmented fatigue crack with the initial, undeformed state was performed for the first time and showed that the fatigue crack path is strongly influenced by the brittle Fe-rich phases.

In the present work austenitic stainless steel X2CrMnNi16-7-9 was additively manufactured using electron beam melting (EBM) technique. In order to reach the highest work of deformation processing parameters such as scan speed, beam current and volume energy were varied. Different properties of manufactured samples, including chemical composition, density, roughness, yield and tensile strength, ductility, grain morphology, texture, and phase composition were thoroughly examined by means of SEM, EDX, EBSD, tensile testing, Archimedes and other methods. The correlation between the parameters was analysed and optimal processing settings allowed to reach work of deformation up to 39 GPa with remarkable elongation up to 110%. It was also found that tuning of the Mn evaporation during EBM by varying the beam energy resulted in the change of stacking fault energy and martensitic transformation temperatures of the steel. This led to the change of the austenite stability and plasticity mechanisms, such as the occurrence of deformation induced martensite in the initially TWIP steel. Comparison of phase diagrams of the studied X2CrMnNi16-7-9 steel and similar X5CrMnNi16-6-6 steel has proven that the reason for in situ EBM grain refinement in the latter one is the presence of primary bcc phase field.

The ultrasonic fatigue testing (USFT) is an effective method for rapid determination of the fatigue properties of structural materials under high cycle (≥106 cycles) loading. However, the occurrence and accumulation of fatigue damage with this test method remain uncertain due to the limitations of the existing measurement methods. Currently used monitoring methods allow detecting the fatigue cracks, but only in the late stages of failure. Despite the superior sensitivity to localized processes in materials, the use of the acoustic emission (AE) method in ultrasonic testing is extremely difficult due to the presence of resonant noise. This work aimed to suppress resonant noise and extract the signal for early detection of fatigue damage. The authors tested the samples of the AlSi9Cu3 aluminum alloy under the asymmetric cyclic loading (R=0.1) at a resonant frequency of 19.5 kHz with a non-threshold AE registration. The fracture surfaces were analyzed by electron and optical microscopy. The authors processed AE by two different methods: (1) the digital filtering method consisted of detecting resonant noise and removing it from the spectrum; (2) the φ-function method consisted of differentiating the spectrogram by time. The processed spectrograms were integrated by the frequency with further extraction of the AE events using the threshold method. The digital filtering method revealed a correlation between AE signals and fatigue damage, whereas the undamaged control sample showed no signals. The φ-function technique demonstrated ambiguous results, showing high AE activity on the control sample.

Ultrasonic fatigue testing (USFT) is an effective method for the rapid characterisation of the high cycle fatigue properties of structural materials. However, the process of initiation and progression of fatigue damage remains uncertain in this way of testing due to the limitations of existing measuring techniques. The acoustic emission (AE) method was developed in the present work to pave a new way to monitor the fatigue process during USFT. The proposed new methodology revealed the AE activity related to fatigue damage, allowing to distinguish between surface and internal fatigue crack initiation and to follow the development of fatigue damage.

A detailed microscopic analysis of fracture surfaces of 42CrMo4-hardened steel after ultrasonic fatigue testing revealed globular and cylindrical particles located in ridges along the crack propagation direction. Observed particles could be easily taken for non-metallic inclusions; however, chemical analysis showed that they are of the same composition as the steel matrix. The formation of such round-shaped debris was found to be a result of cutting out the matrix fragments by 'en passant' cracks interaction and their subsequent fretting burnishing. Possible correlation of the parameters of ridges and debris with cyclic plastic zone width and martensite structure is discussed.

The influence of Co micro-alloying (1 at. %) on the shear band diffusion and the relaxation processes in a model PdNiP bulk metallic glass is investigated. The shear bands are induced by one-pass cold-rolling. In addition to a fast shear band diffusion branch (Dsb≃10−16m2/s at 473 K), with the diffusivity being similar to that observed for the cold-rolled standard Pd40Ni40P20 composition, an ultrafast diffusion branch (Dsb≃10−14m2/s at the same temperature) is found to exist in the micro-alloyed glass. Combined with previously reported observations of faster relaxation of both the Boson peak height and the fictive temperature, the results indicate that Co micro-alloying affects the excess free volume distribution and thus changes the potential energy landscape of the glass, introducing a higher number of local atomic arrangements prone to the formation of shear transformation zones under plastic deformation.

Recent experimental studies revealed the presence of Volterra dislocation-type long-range elastic strain/stress field around a shear band (SB) terminated in a bulk metallic glass (BMG). The corollary from this finding is that shear bands can interact with these stress fields. In other words, the mutual behaviour of SBs should be affected by their stress fields superimposed with the external stresses. In order to verify this suggestion, the topography of the regions surrounding SBs terminated in the BMGs was carefully analysed. The surfaces of several BMGs, deformed by compression and indentation, were investigated with a high spatial resolution by means of scanning white-light interferometry (SWLI). Along with the evidence for the interaction between SBs, different scenarios of the SB propagation have been observed. Specifically, the SB path deviation, mutual blocking, and deflection of SBs were revealed along with the significant differences between the topologies of the mode II (in-plane) and mode III (out of plane) SBs. While the type II shear manifests a linear propagation path and a monotonically increasing shear offset, the type III shear is associated with a curved, segmented path and a non-monotonically varying shear offset. The systematic application of the “classic” elastic Volterra’s theory of dislocations to the behaviour of SBs in BMGs provides new insight into the widely reported experimental phenomena concerning the SB morphology, which is further detailed in the present work.

Carbon‐bonded ceramic foam filters with different functional coatings are immersed in a 42CrMo4 steel melt within a steel casting simulator. The solidified steel is analyzed with respect to the size distribution and the chemical composition of the remaining nonmetallic inclusions (NMI). Cyclic loading and quasi‐static tests are performed to determine the fatigue limit, the strength, deformability, and toughness of the steel after filter immersion. The immersion of filter with calcium hexaluminate (CA6) coating significantly reduces the population of small (4–20 μm) NMIs. This leads to an increased deformability and, thus, ability for energy dissipation during deformation. However, the maximum size of NMIs is increased from 100 to 150 μm, which results in fatigue limit reduction, despite the decrease in NMIs total density. The majority of inclusions are found to be pure alumina. Large (up to 150 μm) plate‐like alumina inclusions introduce most of the detrimental effects on cyclic strength, whereas significant effect on quasi‐static strength is not found.

Very high cycle fatigue (VHCF) is fatigue caused by the growth of an internal fatigue crack in materials under stresses below the standard fatigue limit and number of cycles beyond 10 7 . The fracture surface of steels and alloys after VHCF can be divided into distinct zones, such as the fine granular area (FGA) and the so-called “fisheye”. Differences in the morphology of the crack surface can be numerically estimated by the roughness parameter. Murakami Y. et al. showed that the magnitude of the linear roughness Ra is proportional to the stress intensity factor, whereas Shiozawa K. et al. measured Ra within the FGA and fisheye. Stanzl-Tschegg S. et al. revealed presence of the smooth area (SA) between the FGA and the fisheye. The aim of this work is the quantitative fractographic analysis of this smooth area, which was not reported in the literature so far. Hardened and nitrided specimens of 42CrMo4 steel were used for ultrasonic fatigue testing under symmetric loading conditions ( R =-1) at a resonant frequency of 19.5 kHz. Fracture surfaces after fatigue failure were examined by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Smooth area was experimentally defined as a fatigue crack zone between the FGA and the fisheye. This zone is characterized by (i) visual smoothness and (ii) minimal values of the surface roughness parameters: Sq =4.15 μm (roughness), S'q =0.60 μm (microroughness) and Rs =1.02 (normalized surface area). It has been established that the SA is the result of the propagation of an internal fatigue crack at the stage preceding the stage of a stable crack growth.

Quenched and nitrided 42CrMo4 specimens, treated by different reactive filters were tested in very high cycle fatigue regime using ultrasonic method. Despite total alumina reduction fatigue limit showed no improvement due to large alumina plates whose parameters were studied. Areal roughness of fatigue crack zones was measured by confocal laser scanning microscopy. Periodic arrest marks were studied and used for crack growth rate calculation. Smooth area was experimentally defined as a zone between fine granular area and fisheye by imaging, roughness minima, fixed Delta K = 6.4 and crack growth rate of similar to 10(-10) m/cycle. Classification of internal crack zones is proposed.

A possible explanation of the square scaling observed in the time dependence of the velocity of shear motions in a metallic glass has been proposed. Numerous experiments demonstrating this scaling in the time variation range by four orders of magnitude and in the shear band velocity range by nine orders of magnitude indicate its universality and the possibility of separating mechanisms of strain localization in metallic glasses.

Recent progress in steel refining shows significant reduction of non-metallic inclusions (NMIs) of which alumina (Al2O3) is one of the most problematic. Among other refining methods, metal melt filtration by ceramic foam filters shows promising results in steel cleaning. In the present work the influence of alumina inclusions on the fatigue behavior is investigated after the reaction of steel melt with filters. Different batches are compared where carbon-bonded alumina foam filters with different coatings were introduced into the steel melt of 42CrMo4 for 10 s (so called “finger test”). Fatigue tests were performed using ultrasonic fatigue testing (USFT) up to 109 cycles. Specimens were nitrided in order to prevent crack initiation from the surface and to study internal failure on NMIs. Surface hardening of quenched steel increased fatigue limit significantly. Metallographic sections were analyzed using optical and scanning electron microscopy (SEM) for the estimation of NMIs distribution properties. NMI size distribution analysis based on maximum Feret diameter (instead of area) is found to be an effective method for detecting plate-shaped inclusions. Fracture surfaces after fatigue tests were investigated by methods of SEM and confocal laser scanning microscopy (CLSM), revealing that plate-like NMIs initiate crack with all their area even being inclined to the crack plane. Properties of crack initiating NMIs – alumina plates and MnS dendrites – are compared and analyzed. Formation of alumina NMI as plate lead to significant enlargement of its stress-concentrating area in comparison to the spherical shape of the same volume. Thus, total NMI content reduction in steel could give no fatigue limit improvement if NMI morphology changes from spherical to plate-like.