G.H. Rubiolo

Strain aging effects on the cyclic behavior of austenitic stainless steels

Abstract The strain cyclic behavior of three austenitic stainless steels (AISI 304H, 316H and 316L) has been investigated at temperatures ranging from room temperature to 1023 K. Cyclic hardening curves obtained by plotting the peak stress per cycle against the number of cycles showed a similar trend for the three types of steels. At low temperatures a rapid hardening is followed by a softening until a saturation peak stress is attained. At temperatures higher than 523 K softening is gradually replaced by a more pronounced cyclic hardening and an increase in saturation peak stress. A maximum in this value is obtained at approximately 823 K for a total strain of 0.01 and a strain rate of 2 × 10 −3 s −1 . When the saturation peak stress is plotted against temperature for three different strain rates (2 × 10 −3 , 2 × 10 −4 , 2 × 10 −5 s −1 ) two peaks are present in the region where strain aging effects are probably to occur. This region is characterized by a negative strain rate dependence and an enhanced hardness of the material. Cell formation and dislocation-impurity atoms interactions are responsible for the hardness observed. Transmission Electron Microscopy observations corroborate the proposed model.

Improving the physical properties of starch using a new kind of water dispersible nano-hybrid reinforcement

Plasticized cassava starch matrix composites reinforced by a multi-wall carbon nanotube (MWCNT)-hercynite (FeAl2O4) nanomaterial were developed. The hybrid nanomaterial consists of FeAl2O4 nanoparticles anchored strongly to the surface of the MWCNT. This nano-hybrid filler shows an irregular geometry, which provides a strong mechanical interlocking with the matrix, and excellent stability in water, ensuring a good dispersion in the starch matrix. The composite containing 0.04wt.% of the nano-hybrid filler displays increments of 370% in the Youngs modulus, 138% in tensile strength and 350% in tensile toughness and a 70% decrease in water vapor permeability relative to the matrix material. All of these significant improvements are explained in terms of the nano-hybrid filler homogenous dispersion and its high affinity with both plasticizers, glycerol and water, which induces crystallization without deterioration of the tensile toughness.

Amplitude dependent damping study in austenitic stainless steels 316H and 304H. Its relation with the microstructure

The behaviour of the micro-yield stress in AISI 316H and 304H will be related to the shape of the dislocation arrangement that results from different thermomechanical treatments. The physical mechanism which produces the peak in the saturation stress against temperature curve at 623 K, in the fatigue test, will be explained considering the interaction between dislocations and carbon atoms in solid solution. Furthermore, the physical mechanism which controls another peak which also appears in the saturation stress against temperature curve at 823 K will be explained on the basis of an interaction of dislocation with fine precipitates, which contain chromium. This last mechanism allows one to explain the difference in intensity of the peak at 823 K between the 316H and 304H steels.

First-principles study of atomic ordering in bcc Cu–Al

The order?disorder transitions and phase stability in the body centered cubic structure of Cu?Al binary alloys are studied by means of theoretical methods. The total energy of different ordered compounds sharing a common bcc Bravais lattice was calculated within the framework of density functional theory. A set of effective cluster interactions was calculated through a cluster expansion (CE) of the total energies. The finite temperature phase diagram of bcc Cu?Al was obtained using the CE formalism coupled with the cluster variation method calculation of the configurational entropy. These results are confronted with a simpler semi-empirical approach based on effective pair interactions obtained from experiment. Both approaches predict a single first-order A2/DO3 transition for compositions close to Cu3Al, in agreement with the most recent experimental results.