Materials Science Forum Vol. 941

Abstract: Classical ring springs are mechanical elements used in industrial applications and in transport for shock absorption and energy dissipation. They are constituted by a stack of internal and external metal rings (typically high strength steel), with tapered surfaces in contact with one another. Under the action of an axial load these surfaces slide, the rings are deformed circumferentially and energy is dissipated due to friction. The main advantages of these springs are the high specific energy stored and the large damping capacity due to sliding friction. Furthermore, the stiffness and damping are independent on the strain rate and the temperature, which limits or avoids the occurrence of any resonance problems. The superelastic materials, characterized by an almost flat stress plateau and large reversible deformation, can be used to replace traditional steels in ring springs giving a significant performance increase. Compared to the traditional version where energy is dissipated only due to friction, in superelastic ring springs there is an increase of the dissipated energy thanks to the internal hysteresis of the material. This paper studies analytically the ring springs in traditional material and in superelastic material, providing equations to dimension these mechanical elements, which enable the designer to customize this useful structural element.

Abstract: The stress-induced martensitic transformation and slip deformation behavior were investigated by the compression test with an in-situ observation in a Ti-6Mo-10Al (mol %) alloy single crystal. Owing to the stress-induced martensitic transformation from the parent β phase to the α′′ martensite phase, the single crystal of α′′ martensite without internal twinnings was successfully obtained at room temperature. By further compression, the slip deformation occurred in the single crystal of α′′ martensite. The operated slip system in the α′′ martensite was analyzed by the two face trace analyses, and the slip direction was determined to be []_o.

Abstract: Sheet metal formability is generally affected by crystallographic texture. In particular, bendability and deep drawability of metals and alloys are closely related to the recrystallization texture of the rolled sheets. It is necessary to quantitatively predict them from a viewpoint of texture control. This study proposes a method for simultaneous prediction of both the bendability and the deep drawability on the basis of the average Taylor factor as a polycrystal calculated by using an orientation distribution function. The normalized Taylor factor (M_n-value) and the r-value are used as measures of bendability and deep-drawability, respectively. The predicted results from ideal orientations demonstrated that {001}<uv0> orientation had excellent bendability and poor deep drawability, whereas {111}<uvw> orientation had poor bendability and excellent deep drawability. In addition, the predicted results for practical FCC and BCC metals indicated that cube texture in FCC metals was unfavorable for deep drawing and the γ-fiber texture of <111>//ND in BCC metals was unfavorable for bending.

Abstract: The effect of the activated slip systems on the temperature dependence of yield stress was investigated in α-Ti by using crystal plasticity finite element method. A model for finite element analysis (FEA) was constructed based on experimental results. The displacement in FEA was applied up to the nominal strain of 4% which is the same strain as the experimental one. Stress-strain curves were obtained, which corresponds to experimental data taken every 50 K between 73 K and 673 K. The used material constants which are temperature dependent were elastic constants, and lattice friction stresses. The lattice friction stresses of basal slip systems were set to be higher than that of pyramidal slip systems at 73 K. Then, the lattice friction stresses were set to be closer as the temperature increases. It was found that the activation of slip systems is strong temperature dependent, and that the yield stress depends on the number of active slip systems.

Abstract: The temperature-dependent mechanical property of a forged Ti-6Al-4V (bimodal structure; α and α+ß phases) was investigated. Tensile test was performed at a wide range of temperatures between 77K to 550K. Fatigue test was also performed with corner notched specimens at the temperature range between room temperature and 550K. The temperature dependency of both the yield stress and activation volume was established from the tensile tests. The strong dependency of yield stress and activation volume was observed with respective to the temperature. At lower temperatures, from 77K to 250K, there was a decrease in yield stress with the increase in temperature, whereas abnormality in the yield stress was observed approximately at 275K, and the yield stress was decreased again with the increase in temperature at 325K. Activation volume was increased with increasing the temperature. The activation volume also shows abnormality at the temperatures where the abnormal trend was observed from yield stress. The reasons for the change in yield stress and activation volume with temperature were studied in detail.

Abstract: Intermetallic TiAl alloys based on the γ-TiAl phase are already used as engineering light-weight high-temperature materials in aircraft and automotive engines. Thereby, they partly substitute the twice as heavy Ni-base superalloys. Present applications are, for example, blades in the low-pressure turbine of advanced aero-engines, turbine wheels for turbocharger systems of car diesel engines as well as engine parts used in racing cars. All these applications require balanced mechanical properties, i.e. certain ductility at room temperature as well as defined creep strength at elevated temperatures. The first part of this paper reviews the alloy design strategy, which was used for the development of a β-solidifying γ-TiAl-based alloy, the so-called “TNM alloy”, which exhibits an excellent hot-deformability. In the meantime, the TNM alloy with the nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in atomic percent, at.%) is introduced in a particular eco-friendly and fuel-saving aero-engine, which is powering a medium-range aircraft since the beginning of 2016. In the second part of this work the microstructural parameters are highlighted, which influence the failure strain at room temperature and creep strength at elevated temperatures. It will be shown how the creep resistance can be improved by tailoring phase fractions as well as the spatial arrangement of the microstructural constituents.

Abstract: Large-strain deformation of aluminum in shear consistently evinces strain softening of roughly 15-20%. Most researchers have suggested that this flow stress decrease is a consequence of a decrease in the average Taylor factor as a consequence of a shear-texture. The authors also consider, now, the possibility that changes in the dislocation climb stress induced by the texture could rationalize the softening. This work reports on an analysis of large strain deformation of aluminum single crystals in the softest orientation {111} <110>. Here softening is not observed. However, this result and other in earlier publications are consistent with dislocation climb being the rate-controlling process that also explains the observed stress versus strain behavior.

Abstract: Bulk ultrafine-grained (UFG) materials usually show superior mechanical and physical properties. The development of micro-mechanical behavior is observed after significant changes in microstructure through high-pressure torsion (HPT) processing. This report summarizes recent results on the evolution of small-scale mechanical response examined by the nanoindentation technique on two UFG materials including a high-entropy alloy and an Al-Mg metal matrix nanocomposite processed by HPT. Special emphasis is placed on demonstrating the interrelationship of essential microstructural changes with increasing torsional strain and applying a post-deformation annealing treatment and the evolution of the micro-mechanical behavior in these UFG materials by estimating the strain rate sensitivity.

Abstract: Recent Al7075 severe friction stir processing (FSP) data gave new insights regarding the relationship among processing, microstructure and high temperature behaviour. Grain boundary sliding, GBS, usually operates with fine, equiaxed and highly misoriented grains although, so far, the variable misorientation is missing from the constitutive equation. A collection of very fine microstructures comprising various grain size and misorientation values is employed to evidence the relative importance of grain size vs misorientation in the superplastic behaviour of the processed alloy. This relationship is included into a new GBS constitutive equation incorporating the average misorientation as a variable.

Abstract: It is well known that magnesium (Mg) shows anisotropic fatigue behavior. However, the fatigue mechanisms have yet to be elucidated. The relationships between crystal orientations and crack initiation behavior in Mg single crystals were investigated by uniaxial tension-compression fatigue tests. Three types of round-bar specimens were prepared. The lording direction of AD, BC and EF specimen were [110], [100] and [0001], respectively. Fatigue tests were carried out with the stress ratio R=-1 and the frequency of 10Hz at room temperature in laboratory air. At stress amplitude (σ_a) over 40 MPa, fatigue lives of BC specimen and EF specimen were the longest and shortest. However, at σ_a =20 MPa, the fatigue life of all specimens were almost the same. It was found that fatigue lives of Mg single crystals strongly depend on crystal orientations and stress.