U. Martin

Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304

Cylindrical specimens of the austenitic stainless steel AISI 304 were shot peened or deep rolled with different peening intensities, and rolling pressures, respectively. The resulting near surface properties were characterized by cross sectioning transmission electron microscopy (TEM), residual stress and phase analysis as well as interference line half-width and microhardness measurements. Cyclic deformation curves were obtained by hysteresis measurements under stress control with zero mean stress. The microstructural alterations in the fatigued surface regions were again characterized by the above mentioned methods. The investigations revealed that both shot peening and deep rolling lead to a complex near surface microstructure, consisting of nanocrystalline regions, deformation bands and strain induced martensitic twin lamellae with high dislocation densities in the austenitic matrix. These microstructural changes severely influence the cyclic deformation behaviour: Plastic strain amplitudes and cyclic creep were drastically decreased by shot peening and especially by deep rolling. Both surface finishing methods were found to decrease crack initiation and propagation rate. Remarkably, the initial residual stress profile and surface strain hardening were not completely eliminated even by applying high cyclic stress amplitudes. This is due to the fact that during cyclic loading dislocation cell structures were only formed in greater depths whereas the nanocrystalline layer remained stable. In the case of deep rolled surfaces, the martensitic layer was even increased by fatigue-induced martensite formation.

Supersaturated solid solution of niobium in copper by mechanical alloying

Abstract Alloys with both high strength and high conductivity have been produced by mechanical alloying. In the present study, copper was mechanically alloyed with 5, 10 and 20 at.% Nb using a planetary ball mill. The Cu–Nb phase diagram shows a negligibly low mutual solubility in the solid state, but high energy ball milling can largely extend the region of solid state solution. Previously, it was observed that niobium partly dissolves in the copper lattice during milling. The present investigation demonstrates that this limit can be extended to a strongly supersaturated Cu solid solution of up to 10 at.% Nb provided the appropriate mechanical alloying method is applied. The change in the powder microstructure was followed by scanning and transmission electron microscopy (TEM) as well as by X-ray diffraction (XRD) analysis. In the case of Cu–5%Nb and Cu–10%Nb a homogeneous single-phase microstructure was obtained after 30 h of milling. Elemental Nb could no more be detected, indicating the formation of a metastable supersaturated Cu–Nb solid solution.

Stacking fault model of ∊-martensite and its DIFFaX implementation

Plastic deformation of highly alloyed austenitic transformation-induced plasticity (TRIP) steels with low stacking fault energy leads typically to the formation of ∊-martensite within the original austenite. The ∊-martensite is often described as a phase having a hexagonal close-packed crystal structure. In this contribution, an alternative structure model is presented that describes ∊-martensite embedded in the austenitic matrix via clustering of stacking faults in austenite. The applicability of the model was tested on experimental X-ray diffraction data measured on a CrMnNi TRIP steel after 15% compression. The model of clustered stacking faults was implemented in the DIFFaX routine; the faulted austenite and ∊-martensite were represented by different stacking fault arrangements. The probabilities of the respective stacking fault arrangements were obtained from fitting the simulated X-ray diffraction patterns to the experimental data. The reliability of the model was proven by scanning and transmission electron microscopy. For visualization of the clusters of stacking faults, the scanning electron microscopy employed electron channelling contrast imaging and electron backscatter diffraction.

Cyclic deformation and near surface microstructures of normalized shot peened steel SAE 1045

Cylindrical specimens of the normalized plain carbon steel SAE 1045 were shot peened and cyclically deformed under stress control. A special cross-sectioning technique was applied to prepare near surface regions for transmission electron microscopy. The resulting properties of these regions were characterized by X-ray diffraction and microhardness measurements. The cyclic deformation curves are affected characteristically by dislocation arrangements induced by shot peening. The stability of these dislocation arrangements and the depth profile obtained by residual stress and interference line half-width value measurements depend strongly on the stress amplitude. High stress amplitudes lead to the formation of dislocation cell structures in the shot peened layers, accompanied by the total disappearance of compressive stresses and the decrease of interference line half-width values down to the initial level. Thus, a clear correlation between stress relaxation, cyclic deformation behaviour and microstructural alterations in shot peened regions is revealed. Experiments carried out on shot peened hollow specimens indicate that the cyclic deformation behaviour is not merely a function of dislocation arrangement and density but also of the thickness of strain hardened near surface material in relation to the diameter of the specimen.

Pitfalls of local and quantitative phase analysis in partially stabilized zirconia

The addition of selected elements into the host structure of ZrO2 stabilizes the tetragonal and cubic phases of zirconia, which are, in their undoped binary form, only stable at high temperatures. From the crystallographic point of view, the increasing amount of the stabilizer causes a continuous transition of the tetragonal zirconia to its cubic modification. In partially stabilized zirconia, local concentration gradients of the stabilizer are frequently present as a consequence of the production process, which results in a coexistence of zirconia domains having different degrees of tetragonality. The presence of the local concentration gradients in such samples and the continuous nature of the phase transformation are features important for many technological applications, but their analysis is not straightforward. Furthermore, these features complicate the quantitative phase analysis in partially stabilized zirconia. For the example of zirconia partially stabilized by magnesium, this contribution illustrates the capabilities and limitations of X-ray and electron backscatter diffraction. In particular, the ability of these experimental methods to reveal the gradual lattice distortion that is associated with the cubic to tetragonal phase transformation in zirconia and the reliability of the quantitative phase analysis are discussed. In this context, it is shown to what extent the choice of the microstructure model influences the result of the phase analysis.

Deformation behaviour and microscopic investigations of cyclically loaded railway wheels and tyres

Abstract Railway wheel and tyre steels were investigated by specimens from defined volume sections representing the local microstructure of original components. As a consequence of the industrial heat treatment and the size of the components, microstructural gradients exist in the wheel rim and tyre cross-section. The ferrite fraction of the ferritic – pearlitic microstructure and the cementite lamellae spacing vary depending on the distance to the running surface (tread). Stress-controlled load increase and single-step push–pull tests were carried out at ambient temperature with servohydraulic testing systems to study fatigue phenomena with special consideration of microstructural details. Mechanical stress–strain hysteresis, thermometrical, and electrical measurements were used for a comprehensive characterization and evaluation of the cyclic deformation behaviour. The measured physical quantities show a strong interrelation with the underlying fatigue process. Light, scanning, and transmission electron...

Cellular Energy Absorbing TRIP-Steel/Mg-PSZ Composite: Honeycomb Structures Fabricated by a New Extrusion Powder Technology

Lightweight linear cellular composite materials on basis of austenite stainless TRIP- (TRansformation Induced Plasticity-) steel as matrix with reinforcements of MgO partially stabilized zirconia (Mg-PSZ) are described. Two-dimensional cellular materials for structural applications are conventionally produced by sheet expansion or corrugation processes. The presented composites are fabricated by a modified ceramic extrusion powder technology. Characterization of the microstructure in as-received and deformed conditions was carried out by optical and scanning electron microscopy. Magnetic balance measurements and electron backscatter diffraction (EBSD) were used to identify the deformation-induced martensite evolution in the cell wall material. The honeycomb composite samples exhibit an increased strain hardening up to a certain engineering compressive strain and an extraordinary high specific energy absorption per unit mass and unit volume, respectively. Based on improved property-to-weight ratio such linear cellular structures will be of interest as crash absorbers or stiffened core materials for aerospace, railway, or automotive applications.

Microstructure of austenitic stainless steels of various phase stabilities after cyclic and tensile deformation

Abstract The microstructures of two metastable high-alloyed CrMnNi cast TRIP steels and a stable AISI 316L austenitic stainless steel were studied in detail after tensile and cyclic deformation. Electron backscattered diffraction was employed to localize the martensitic phase transformation and electron channelling contrast imaging to describe the typical dislocation arrangements. These were complemented by transmission electron microscopy and by scanning transmission electron microscopy performed in a scanning electron microscope. The TRIP steel with the lowest austenite stability shows a more pronounced martensitic phase transformation realized from the austenite via the intermediate formation of ∊-martensite. Martensitic phase transformation also occurred in the stable 316L austenitic stainless steel with a small volume fraction of α′-martensite, but only with cyclic deformation at low temperatures and/or at very high plastic strain amplitudes.

Microstructure Formations in Copper‐Silicon Carbide Composites During Mechanical Alloying in a Planetary Activator

In the present paper the structure formation process of the powder metallurgical produced copper composite materials was studied. The volume part of the reinforcing SiC particles was varied from 5 to 25 wt.-%. It was discovered that while milling in a planetary activator first of all a puff- pastry structure appeared. There are important differences between this structure formation process and other known processes of milling. The homogeneous distribution of SiC particles was obtained after 60-100 minutes of treatment in Gefest 11-3 planetary activator. Phase composition of the powder and composite samples at the interface SiC/Cu (particles/matrix) was analysed after consolidation of the powder mixture and after the high temperature annealing. It was still determined that not only pure copper powder can be as a starting material for Cu-composites production used, but also the wastes of copper mechanical treatment, for instance, copper shaving.

Oxide dispersion-strengthened silver: manufacturing and properties

Novel oxide dispersion-strengthened (ODS) silver alloys were synthesised by mechanical alloying and subsequent consolidation via hot pressing in protective atmosphere. The effect of dispersoid size and volume fraction on the electrical and mechanical properties is exemplified with Cr 2 O 3 , CaO and Y 2 O 3 as model candidates for a systematic study of the principles of particle selection. The results for the development of the microstructure under different processing conditions show that control of the impurity level is of primary importance for achieving high-quality, fully dense material. Hence, a modified cryo-milling technique at liquid-N 2 temperature was applied. Data at ambient temperature are presented, revealing that the mechanical and electrical properties can be tailored within a wide range as a function of the microstructure. These results are explained by microstructural concepts for room temperature yielding and electrical conductivity.