Papers by Keyword: Microtexture

Abstract: The subsurface fatigue crack generation processes in near α type titanium alloy were divided into four steps: (1) development of a saturated dislocation structure by cyclical micro-plastic strain accumulation, (2) generation of localized slip and/or microcracking to relax the stress concentration in the vicinity of a boundary, (3) microcrack growth and transition to main crack, and (4) crack propagation. The experimentals on transgranular facets formation in Ti-Fe-O alloy were reviewed and a subsurface fatigue crack generation model was discussed. The β platelets which were aligned between the recrystallized α grain and the recovered α grain were responsible for the microcrack generation to form (0001) tansgranular facet in the recrystallized α grains. A combination of the shear stress and tensile stress normal to the basal plane may give a trigger of the (0001) microcracking in the recrystallized α grain. The localized shear stress following slip off on the basal plane was activated at the microcrack tip in the recrystallizedαgrain, and the microcrack grew into the recrystallized α grain to form (0001) transgranular facet.

Abstract: Up to now, only a limited amount of metallic materials is investigated for laser additive manufacturing (LAM). However, the demand to widen the application possibilities by enlarging the range of materials for LAM is growing fast. By now, titanium and aluminium alloys are in the focus of research. In contrast, magnesium alloys are rarely used in the field of additive manufacturing, although they possess a low density in combination with a high specific strength. Currently, magnesium structures are mainly produced by casting but during the last years, the use of wrought alloys also increased. A reason for the rare use of magnesium alloys for LAM technologies might be the high flammability of magnesium powders. This difficulty can be avoided by using magnesium wire for laser metal deposition (LMD). In the present study, the microstructural characteristics of a LMD processed AZ31 magnesium alloy are investigated. For this purpose, optical microscopy and scanning electron microscopy were used. With the help of EDX and EBSD analysis, a change of the chemical composition and micro texture with structure height was identified. The relationship of microstructure and local mechanical properties was investigated with the help of Vickers micro hardness testing. Based on the obtained results it can be concluded that the microstructural characteristics of laser additive manufactured magnesium alloys differ from those of titanium and aluminium alloys. Thus, a wider application spectrum of LMD and magnesium alloys can be opened up.

Abstract: In hot rolling, metal oxides formed on steel surface can generally be classified as primary, secondary and tertiary oxide scales, corresponding to the reheating stages, the roughing stages and the finishing passes of continuous mills, respectively. The tertiary oxide scale grows into the final products on the hot-rolled steel strip during the finishing rolling and the subsequent cooling down to ambient temperature. We provide here a systematic overview of the oxidation mechanism, microstructure and microtexture development of the tertiary oxide scale. Mechanism of oxidation and Fe₃O₄ precipitation in tertiary oxide has been given as the fundamental theory. Three main sections has been divided in this review. The first section includes experimental investigations on microstructure evolution from the formation of oxide scale during hot rolling, then through continuous cooling, to Fe₃O₄ precipitation behaviour in storage cooling of hot-coiled strip. By using electron backscatter diffraction (EBSD) to characterise both the steel substrate and the oxide scale concurrently, the second section has further dealed with the texture-based analysis of oxide scale: phase identification, orientation analysis and coincident site lattice (CSL) boundaries. The third section has provided the general type of crystallographic texture and its evolutions in deformed Fe₃O₄ and steel substrate. Finally, the upcoming challenges have been addressed in this intriguing and promising research field.

Abstract: TC4 titanium alloys have been extensively used in the aerospace engineering due to the high specific strength, high temperature resistance and good corrosion resistance. However, unsuitable forging methods will cause unqualified mechanical properties in the height direction of forgings. The microstructure and microtexture of the forgings after two forging processes with different upsetting and drawing times were investigated by optical microscopy (OM) and electron back scattering diffraction (EBSD) technique. The results showed that bimodal microstructure and weak basal {0002} texture can be obtained after forging. With the increase of upsetting and drawing times, lamellar α were curved and coarsen, basal {0002} texture were enhanced, and the special 60°<11-20> preferred orientation between lamellar α phase due to Burgers relationship was avoided. The modification of microtexture and grain boundary distributions can improve the strength of TC4 titanium alloy forging in the height direction.

Abstract: The influence of High-Pressure Torsion (HPT) on texture and superplasticity in an Al-7075 was studied using X-ray diffraction and tensile testing. The alloy was processed by HPT at room temperature under a pressure of 6.0 GPA up to a maximum of 20 turns. The pole figures were measured at mid-radius of the disks after 1, 5, 10 and 20 turns. The results show the presence of a typical torsion texture during HPT, in particular, the C{001}<110> component was found to develop preferentially. With increasing deformation, the A {111}<110> and the C components are reinforced after 5 turns and the texture tends to be random with the presence of a fibre texture near the center. Moreover, the fraction of C components tends to gradually decrease and a fairly isotropic microtexture is apparent after 20 turns. Tensile testing showed the development of excellent superplastic properties in this alloy with elongations up to ~700% when testing at a temperature of 623 K.

Abstract: Microstructure and microtexture evolution during batch annealing of warm-rolled Ti-IF steel sheets were investigated in this paper. It was founded that α fiber texture and the relatively weak γ fiber texture were formed in warm-rolled and air-cooled Ti-IF steel sheets. In the early stage of recrystallization, the {111} recrystallization texture was formed from the deformed {111} grains in warm-rolled Ti-IF steels. In the later stage of recrystallization, the α fiber texture was consumed and the γ fiber texture in recrystallized grains was further developed. The main recrystallization texture characteristics of warm-rolled Ti-IF steel sheets had been decided in the early stages of recrystallization, and the oriented nucleation mechanism played a leading role in the formation of recrystallization texture in warm-rolled Ti-IF steel sheets.

Abstract: Titanium (Ti) alloys are used in critical, fatigue limited applications in aircraft and aircraft engines. Current design practices are, of necessity, conservative in order to minimize risk of unexpected failures. Among the sources of this conservatism are the inherent variations in the number of loading cycles the materials can withstand prior to fatigue crack initiation, the uncertainty in crack propagation lifetime prediction and the need to set safe minimum component life values. While the stochastic nature of fatigue is well-known, improved characterization methods have begun to provide a better understanding of the crack initiation process. This paper describes recent work designed to provide an improved understanding of the relationship between thermomechanical processing history, microstructure, texture and the fatigue behavior of α+β Ti alloys. Due to length limitations, the paper focuses on two important aspects of fatigue life variation: the effects of microstructural discontinuities on fatigue and the role of facet formation during crack initiation and the early stages of fatigue crack growth.

Abstract: Mullite (3Al₂O₃•2SiO₂) undergoes a phase transition at 30 GPa with forming aligned nanocrystalline fragments in an amorphous phase. The direction of the crystal axes of mullite nanocrystals with the grain sizes less than 10 nm is that preserved from the starting specimen. To clarify the mechanism of the nanofragmentation in mullite, compositional and structural effects are investigated by comparative studies using several mullite-related aluminosilicates. Consequently, we proposed that the oxygen vacancies in the crystal structure in mullite play an important role to formation of the nanofragmentation textures. Also, we performed impact experiments using mullite as a bumper material, simulating a Whipple bumper shield for spacecrafts. Damage of impact could be considerably less with mullite bumper shield than with aluminum alloy bumper shield, suggesting that mullite could be an candidate for a Whipple bumper materials in the next generation.

Abstract: The Nb-1%Zr-0.1%C (wt%) alloy is one of the most promising refractory metal alloys having an excellent combination of high temperature properties. Such a combination of properties makes it suitable for several structural applications in the Compact High Temperature Reactor (CHTR). In order to produce the alloy in different shape and sizes a new thermo-mechanical route has been established. The central idea behind the development of such thermo-mechanical route is to reduce the working temperature and provide suitable intermediate annealing treatments to develop desired microstructures. The present paper reports about the application of orientation imaging microscopy in optimizing annealing parameters like temperature and time as a function of the extent of deformation. Samples were also characterized by optical microscopy and transmission electron microscopy techniques. It has been shown that a heat treatment of 1300°C for 3 hour could produce nearly full-recrystallized microstructure. This paper also discusses about the carbide precipitation, their morphologies, chemical compositions and orientations with the matrix phase.

Abstract: In this study a systematic characterization of the microstructural and textural evolution in each thermo-mechanical processing step of Zircaloy-4 rod fabrication has been carried out. The possible micro-mechanisms leading to the observed microstructural evolutions have been discussed. The thermo-mechanical steps followed resulted in a completely recrystallized microstructure and retention of the hot-extruded texture in the finished product.