Showing papers by "Eberhard Kerscher published in 2015"

On the role of microstructure in governing the fatigue behaviour of nanostructured bainitic steels

Abstract: Nanostructured bainite is not a novel laboratory-scale steel anymore and the interest on the commercial production of these microstructures by steelmakers and end-users is now conceivable. These microstructures are achieved through the isothermal transformation of high-carbon high-silicon steels at low temperature, leading to nanoscale plates of ferrite with thickness of 20–40 nm and retained austenite. Nanostructured bainitic steels present the highest strength/toughness combinations ever recorded in bainitic steels (2.2 GPa/40 MPa m 1/2 ) and the potential for engineering components is alluring. However, fatigue properties, responsible of the durability of a component, remain to be examined. In order to understand the role of the microstructure during the fatigue crack propagation, the crack path in three nanoscale bainitic structures has been analysed on the basis of the relationships between grain misorientations and grain boundaries by Electron Backscatter Diffraction. Active slip systems in bainitic ferrite and crack deflection at grain boundaries have been identified, while retained austenite is cast doubt on its role.

Comparing the pitting corrosion behavior of prominent Zr-based bulk metallic glasses

Abstract: Five well-known Zr-based alloys of the systems Zr–Cu–Al–(Ni–Nb, Ni–Ti, Ag) (Cu = 15.4–36 at.%) with the highest glass-forming ability were comparatively analyzed regarding their pitting corrosion resistance and repassivation ability in a chloride-containing solution. Potentiodynamic polarization measurements were conducted in the neutral 0.01 M Na2SO4 + 0.1 M NaCl electrolyte and local corrosion damages were subsequently investigated with high resolution scanning electron microscopy (HR-SEM) coupled with energy dispersive x-ray spectroscopy (EDX). Both pitting and repassivation potential correlate with the Cu concentration, i.e., those potentials decrease with increasing Cu content. Pit morphology is not composition dependent: while initially hemispherical pits then develop an irregular shape and a porous rim. Corrosion products are rich in Cu, O, and often Cl species. A combination of low Cu and high Nb or Ti contents is most beneficial for a high pitting resistance of Zr-based bulk metallic glasses. The bulk glassy Zr57Cu15.4Al10Ni12.6Nb5 (Vit 106) and Zr52.5Cu17.9Al10Ni14.6Ti5 (Vit 105) alloys exhibit the highest pitting resistance.

Fatigue Behavior of Ultrafine-Grained Medium Carbon Steel with Different Carbide Morphologies Processed by High Pressure Torsion

Abstract: The increased attention ultrafine grained (UFG) materials have received over the last decade has been inspired by their high strength in combination with a remarkable ductility, which is a promising combination for good fatigue properties. In this paper, we focus on the effect of different carbide morphologies in the initial microstructure on the fatigue behavior after high pressure torsion (HPT) treatment of SAE 1045 steels. The two initial carbide morphologies are spheroidized as well as tempered states. The HPT processing increased the hardness of the spheroidized and tempered states from 169 HV and 388 HV to a maximum of 511 HV and 758 HV, respectively. The endurance limit increased linearly with hardness up to about 500 HV independent of the carbide morphology. The fracture surfaces revealed mostly flat fatigue fracture surfaces with crack initiation at the surface or, more often, at non-metallic inclusions. Morphology and crack initiation mechanisms were changed by the severe plastic deformation. The residual fracture surface of specimens with spheroidal initial microstructures showed well-defined dimple structures also after HPT at high fatigue limits and high hardness values. In contrast, the specimens with a tempered initial microstructure showed rather brittle and rough residual fracture surfaces after HPT.

Stress corrosion cracking of a Zr-based bulk metallic glass

Abstract: The stress corrosion cracking behaviour of the bulk glassy Zr 52.5 Cu 17.9 Al 10 Ni 14.6 Ti 5 alloy (Vitreloy 105) in 0.01 M Na 2 SO 4 +0.01 M NaCl electrolyte was investigated under static three-point bending and anodic potentiostatic control by means of in situ stress and current measurements and subsequent fractography analysis. Pitting takes place preferentially at edges of flat rectangular samples and those pits act as precursors to cracking. Features corresponding to shear banding, mechanical brittle cracking and anodic dissolution assisted cracking were found on the fracture surface. The presence of striations indicates a discontinuous step-wise crack propagation mode. A mechanism is proposed in which crack tip blunting is attributed to shear banding and re-sharpening is attributed to preferential anodic dissolution along shear bands.

Stress-Corrosion Interactions in Zr-Based Bulk Metallic Glasses

Abstract: Stress-corrosion interactions in materials may lead to early unpredictable catastrophic failure of structural parts, which can have dramatic effects. In Zr-based bulk metallic glasses, such interactions are particularly important as these have very high yield strength, limited ductility, and are relatively susceptible to localized corrosion in halide-containing aqueous environments. Relevant features of the mechanical and corrosion behavior of Zr-based bulk metallic glasses are described, and an account of knowledge regarding corrosion-deformation interactions gathered from ex situ experimental procedures is provided. Subsequently the literature on key phenomena including hydrogen damage, stress corrosion cracking, and corrosion fatigue is reviewed. Critical factors for such phenomena will be highlighted. The review also presents an outlook for the topic.

The Applicability of Nanoindentation for the Examination of Microstructured Areas in CP Titanium Samples

Abstract: Microscale machining processes modifying component surfaces are applied in various fields: from drive technology through plant engineering to medical technology. The impact of the fine structures formed in the material on its behavior and mechanical properties need to be investigated in more detail. Besides the examination of the cyclic and quasi-static behavior of micromachined samples, analysing the near-surface microstructure and the hardness is of great importance. The nanoindentation allows for the determination of local mechanical properties in small analysis volumes. Its applicability to determine the hardness in micromachined near-surface areas including the metallographic preparation for commercially pure titanium is presented in this work.

Fatigue Behavior and Lifetime Prediction of Unidirectionally Wire Reinforced Lightweight Metal Matrix Composites

Abstract: In the field of lightweight construction for transportation means, hybrid structures composed of high-strength and low-density materials exhibit a high potential for application. The composite extrusion process allows an easy embedding of metallic reinforcements into a multitude of light metal alloys. The current work shows that different metallic wire reinforced light alloys with a content of 11.1 vol% lead to a significant increase in lifetime of different aluminum and magnesium alloys under fully reversed stress controlled fatigue loading. Based on the knowledge of the quasi-static behavior of the single components and the fatigue behavior of the matrix material, a new lifetime model is used to predict the lifetime for different unidirectionally reinforced material systems.