Materials Science Forum Vols. 706-709

Abstract: There are four prominent orientation relationships (ORs) between directionally grown precipitates and their parent phases in steel. They are ORs between ferrite precipitate and parent austenite (the Kurdjumov and Sachs OR), between orthorhombic cementite precipitate and parent austenite (the Pitsch OR), between cementite precipitate and parent ferrite (the Bagaryatski OR) and between hexagonal molybdenum carbide precipitate and parent ferrite (the Dyson et al. OR). The directed precipitation occurs at low transformation temperatures. The ORs have been explained by the directed growth model. The solid phase transformation of a metastable phase into a stable phase needs the activation energy. The energy is usually supplied in the form of thermal energy. When the nucleation takes place, the strain energy may develop in the stable nucleus and the metastable matrix. The strain energy can result from a difference in density between the nucleus and matrix and the lattice mismatch along the nucleus:matrix interface. The fundamental concept of the model is that the maximum growth rate of precipitate is along the direction that generates the maximum strain energy and the interface energy is minimized. The four ORs are determined, based on the concept, such that the mismatch along the interface between the minimum shear modulus planes of precipitate and its parent phase that are parallel to the maximum Young’s modulus direction of the precipitate is minimized.

Abstract: This paper summarizes the development of new cast and wrought magnesium alloys using computational thermodynamics tools and experimental approach. The Mg-Al-Ca alloys show excellent creep resistance due to the formation of high-temperature (Mg,Al)₂Ca phase. The Mg-Al-Sn alloys are designed for mechanical properties and corrosion resistance through the optimization of Mg₁₇Al₁₂ and Mg₂Sn phases in the microstructure. In the Mg-Zn-Ce system, Zn provides strength through solid solution strengthening while Ce increases the ductility via improved texture. Mg-Nd-Zn is a heat-treatable alloy system based on the precipitation hardening of Mg₁₂Nd phase.

Abstract: A 0.12 wt.% C – 1.26 wt.% Mn steel was studied to evaluate phase transformations that occurred during a specific thermal processing method designed to simulate steel plate surface layer microstructural evolution during processing with intermediate cooling. All process simulations used a Gleeble thermomechanical simulator along with thermal practices developed previously. After intermediate cooling was completed during processing, slight reheating of the plate surface layer region would occur due to heat retained in the plate core. Microstructural evaluation of Gleeble samples quenched at several points along the thermal profile allowed interpretation of microstructural evolution during processing. The microstructure that was present at the point where deformation would be applied consisted of approximately 75% ferrite, 25% austenite and some small, undissolved cementite particles.

Abstract: TMP of Al alloys includes hot working with dynamic substructures and deformation bands for texture components combined with static recovery or recrystallization as well as cold working altered by annealing. The above processes are separately tailored for solute (Al, Mg), dispersoid (Al-0.7Fe) and precipitation hardening alloys; aging combined with deformation can raise strength or improve fatigue or corrosion resistance. Hot and cold rolling with suitable holding intervals are managed to combine deformation and annealing textures for planar anisotropy or for producing less fibrous grains to avoid delamination corrosion; grains may be severely refined by discontinuous or continuous recrystallization for superplastic sheet. In hot-billet and impact extrusion as an addition to substructure and texture strengthening, the intense heating near the die may be employed for precipitate solution with exit quenching for press heat treatment to T5 temper. Similarly, friction stir surface treatment and welding provide intense hot straining with additional softening as metal is swept behind the pin. In combination with some of the above, forging provides grain and dispersoid fibering oriented for crack retardation; semi solid forming competes with this.

Abstract: This paper presents a new fracture assessment method, IST method developed as ISO 27306. The IST method implements an equivalent CTOD ratio, β, for the CTOD toughness correction for constraint loss in structural components. Using β, the standard fracture toughness specimen and structural components are linked at the same level of the Weibull stress. This paper extends the equivalent CTOD concept to weld components. Effects of the weld strength mismatch and residual stress on β are discussed. It is shown on the failure assessment diagram (FAD) that the CTOD toughness correction with β leads to accurate fracture assessments of weld panels, whereas the conventional procedure gives much conservative results.

Abstract: Ti6Al4V titanium alloy has been characterized for its prospective applications as an implant material. The surface treatments performed have brought about enhanced surface properties of these alloys and have produced corrosion resistant oxide films with increased bioactive properties. Characterization of the alloy surface has revealed the presence of a duplex oxide structure over the surface treated specimens, composed of an inner barrier layer and an outer porous layer. The inner barrier layer has imparted a high corrosion resistance to the alloy while the outer porous layer which is responsible for the increased roughness of the surface treated alloy specimens, has encouraged formation and deposition of apatite into the oxide pores and further resulted in an increase in cell adhesion over the alloy surface. Anodization and heat treatment procedures have proved advantageous to titanium alloys in terms of producing oxide films that can offer these alloys an improved biological performance.

Abstract: Mechanical Spectroscopy provides information on microstructural features of materials not obtained by other techniques. In general, it is used for investigating physical phenomena, however it can be very useful also for solving problems related to industrial processes. This work describes and discusses some applications of metallurgical interest.

Abstract: Recent research on biological materials and bioartificial systems has created one of the most dynamic field at the confluence of physical sciences, molecular engineering, cell biology, materials sciences, biotechnology and (nano) medicine. This field concerns better understanding of living systems, design of bio-inspired materials, synthesis of bioartificial technologies with new properties depending on their multi-scale architectures. Biological and man-made systems show the first level of organization at the nanoscale, where the fundamental properties and functions are settled (e.g., proteome and genome). The nanoscale properties reflect on larger scales: mesoscale, microscale, and continuum. Mechanisms by which phenomena at the different length and time scales are coupled and influence each other is the central issue in linking properties to functionalities, with a dramatic impact in designing and engineering biosystems. To get insights into the progressive trough-scales cascade effects-from molecular to macroscale level and from nanoseconds to life expectancy duration-multiscale/multiphysics models are required, dealing with inorganic, biological and hybrid matter. Thus, bioartificial systems technology depends upon our ability in assembling molecules into objects, hierarchically along several length scales, and in disassembling objects into molecules, in a tailored manner. As a peculiar feature, in bioartificial systems, the definition of the interactions between artificial and biological components needs to incorporate the “time” variable, in order to reproduce the evolution of the overall system, and to simulate complex phenomena as biodegradation and tissue remodeling. Herein, a number of paradigmatic multiscale models that attend the investigation of biological systems and the engineering of bioartificial systems is reviewed and discussed.

Abstract: Due to the high rate of dynamic recovery associated with the large stacking fault energy of the bcc structure, classical "discontinuous" dynamic recrystallization, occurring by nucleation and growth of new grains is not observed in the β phase of titanium alloys. Instead, the following mechanisms take place: at low and moderate strains (ε < 1), the original flattened (compression) or sheared (torsion) grains are still recognizable, although their boundaries are strongly serrated. In this strain range, grain size (thickness) results from both the convection and the migration of grain boundaries. At intermediate strains, "geometric" dynamic recrystallization leading to "pinching off" events of the original grains is observed, whereas at larger strains (ε > 5), grain fragmentation occurs by the generation of new grain boundaries ("continuous" dynamic recrystallization). The associated flow stress often exhibits pronounced softening and the resulting (equiaxed) grain size can be much smaller than the initial one. It is worth to note that a very similar sequence of mechanisms takes place in ferritic steels, as well as in aluminium alloys, in spite of their different crystallographic structure. In this paper, the above mechanisms will be illustrated by a set of data pertaining to titanium alloys.

Abstract: Oxygen enhances the strength of titanium alloys in general; however, excess oxygen can make titanium alloys brittle. On the other hand, oxygen enhances the precipitation of the α phase and suppresses the formation of the ω phase. Thus, using the optimal amount of oxygen is important to improve the mechanical properties of titanium alloys. The role of oxygen in titanium alloys is still not well understood. The effect of oxygen on the mechanical behavior of a β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr (referred to as TNTZ), which is used for biomedical applications, was investigated in this study. Oxygen was found to stabilize the ω phase in TNTZ. This behavior of oxygen is unusual considering the known behavior of oxygen in titanium alloys: oxygen is known to suppress the formation of the ω phase in titanium alloys. A small amount of oxygen increases the tensile strength but decreases the ductility of TNTZ. On the other hand, a large amount of oxygen, of around 0.7 mass%, increases both the tensile strength and the ductility of TNTZ. This phenomenon is unexpected.