Presence Of Subgrains Research Articles

Investigations on the microstructural evolution and mechanical properties of Al-Cu-Al tri-metal strips fabricated through roll bonding and heat-treatment processes

ABSTRACT Aluminum alloys have recently become increasingly popular in the automobile industry. In this work, roll bonding technology was developed to fabricate the composite sandwich structure (tri-metal strip) of AA5083 alloy with a Cu interlayer. The apposite surface treatment, intermittent annealing, and thickness reduction resulted in desirable metallurgical characteristics and mechanical properties. The micrographs revealed crack-free flat joints, indicating excellent bonding between the layers of AA5083 and Cu. Also, the presence of sub-grains and dislocations between boundaries suggested a continuous and discontinuous recrystallization process. The phase composition analysis confirmed the formation of intermetallic compounds at the interface. The fabricated composite exhibited a microhardness of 124.2 HV and a tensile strength of 566.66 MPa.

Hydrogen-doped In2O3 for silicon heterojunction solar cells: Identification of a critical threshold for water content and rf sputtering power

Hydrogen-doped indium oxide (IO:H) layers with very high carrier mobility have been developed by two-step fabrication procedure. In the first step IO:H films were deposited by radio frequency (RF) magnetron sputtering from In2O3 target in Ar/water vapour gas mixtures. Different sputtering powers and partial pressures of H2O (pH2O) have been explored and effects induced by changes of these process parameters on final film structure and on electro-optical properties have been investigated. In the second step a post-deposition annealing under vacuum were performed. During this treatment a complete and complex amorphous-crystalline transition occurred. Growth of large crystalline domains give rise to IO:H films with excellent carrier mobility values, up to 138 cm2/Vs. For each sputtering power used, the pH2O range has been identified which allows to obtain the highest carrier mobility value for IOH post-annealed films. Growth of large crystallites occurred in IO:H annealed films and, at this purpose, a selective chemical etching method has been developed to give better evidence for the polycrystalline grains. IO:H films appeared formed by fairly large crystalline domains with presence of sub-grains. Silicon heterojunction cells fabricated by using IO:H layers as front electrode, compared with those obtained from a conventional ITO showed improved short-circuit current density and the resulting conversion efficiency.

Dark-field X-ray microscopy reveals mosaicity and strain gradients across sub-surface TiC and TiN particles in steel matrix composites

Residual strain gradients and crystallographic mosaicity across embedded ceramic particles within metal-matrix composites have remained experimentally inaccessible. In this work, we use synchrotron dark-field X-ray microscopy to characterize strain and orientation gradients across ~20 µm large TiC and TiN particles embedded in a steel matrix annealed at 480 and 600 °C. The results document gradual crystallographic orientation changes across the particles and the presence of sub-grains, in contrast to electron backscatter diffraction data. These gradients appear, however, not directly correlated with the observed strain gradients, which are attributed mainly to the different strain states within particles’ interiors and envelopes.

Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps

Abstract. Deformation microstructures of albitic plagioclase and K-feldspar were investigated in mylonitic pegmatites from the Austroalpine basement south of the western Tauern Window by polarized light microscopy, electron microscopy and electron backscatter diffraction to evaluate feldspar deformation mechanisms at greenschist facies conditions. The main mylonitic characteristics are alternating almost monophase quartz and albite layers, surrounding porphyroclasts of deformed feldspar and tourmaline. The dominant deformation microstructures of K-feldspar porphyroclasts are intragranular fractures at a high angle to the stretching lineation. The fractures are healed or sealed by polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur along intragranular fractures of tourmaline and strain shadows around other porphyroclasts. These polyphase aggregates indicate dissolution–precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a cuspate interface. This replacement is interpreted to take place by interface-coupled dissolution–precipitation driven by a solubility difference between K-feldspar and albite. Albite porphyroclasts are replaced at boundaries parallel to the foliation by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent and twinned albite porphyroclasts with internal misorientation. An indication of effective dislocation climb with dynamic recovery, for example, by the presence of subgrains, is systematically missing. We interpret the grain size reduction of albite to be the result of coupled dislocation glide and fracturing (low-temperature plasticity). Subsequent growth is by a combination of strain-induced grain boundary migration and formation of growth rims, resulting in an aspect ratio of albite with the long axis within the foliation. This strain-induced replacement by nucleation (associated dislocation glide and microfracturing) and subsequent growth is suggested to result in the observed monophase albite layers, probably together with granular flow. The associated quartz layers show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures: (i) alternating layers of a few hundred micrometres' width, with isometric, fine-grained feldspar (on average 15 µm in diameter) and coarse-grained quartz (a few hundred micrometres in diameter), representing lower strain compared to (ii) alternating thin layers of some tens of micrometres' width composed of fine-grained quartz (<20 µm in diameter) and coarse elongated albite grains (long axis of a few tens of micrometres) defining the foliation, respectively. Our observations indicate that grain size reduction by strain-induced replacement of albite (associated dislocation glide and microfracturing) followed by growth and granular flow simultaneous with dislocation creep of quartz are playing the dominating role in formation of the mylonitic microstructure.

High resolution images of microstructure of TIG welded Zn–22Al–2Cu alloy plates

By applying the tungsten inert gas welding process, plates of the Zn–22Al–2Cu (wt-%) alloy were welded. With the aid of a scanning electron microscope, high resolution images of microstructures in the welded zone were obtained. The images showed the presence of subgrains inside the dendrites (in the fusion zone) and inside the equiaxed grains (in the heat affected zone). Inside the subgrains, a pearlitic microstructure (lamellar or granular) was observed. X-ray diffraction analysis revealed that the α, α′S, α′T, η, η′S, η′T and ϵ crystalline phases are present in the microstructure. The elemental analysis by energy dispersive X-ray spectroscopy showed that the lamellar microstructure is composed by lamellas of the α′T and η′T phases. In the interdendritic region, elemental analysis showed the presence of the η′S, η, α and ϵ phases. In the intergranular region the elemental analysis showed the presence of the η′T, α, α′T and ϵ phases.

Microstructural and textural characterization of a hot-rolled IF steel

The present paper reports the experimental results on the microstructural and textural characterization of a hot-rolled IF steel. The IF steel was hot-rolled in multiple passes in the austenitic field (1,070 °C) followed by air-cooling. SEM, TEM, and LOM were used to image the microstructure of the material. The global texture was determined by X-ray diffraction (XRD) technique. The mesotexture of selected regions was investigated by electron backscatter diffraction (EBSD). Results show the presence of a diffuse (nearly random) and weak texture in the hot-band that consists of recrystallized polygonal grains and subgrains. The fraction of boundaries with misorientations comprised in the interval 2° ≤ ψ < 15° was found to be lower than 5%. It can be concluded that these low angle boundaries and the presence of subgrains can be associated to the existence of a few areas softened by recovery during or after hot rolling in austenitic field.

Development of Superplastic AlCuAgMgZr Alloys for Easy Forming

Two aluminium alloys containing up to 6% Cu, 0.8% Ag, 0.7 % Mg and 0.4 % Zr (all in wt %) were continuously cast and hot rolled at 673 K. Thermomechanical treatment involving particle stimulated nucleation (PSN) process was used in order to obtain fine grain size. The microstructure studies after this treatment revealed the presence of subgrains of 0.5-2 μm in size with a misorientation of 3 - 15°. In spite of the presence of zirconium rich and copper rich particles the grain growth up to 10-20 μm was observed after annealing at temperatures above 673K. This was also observed in samples deformed at the temperature of 673K. It affected the superplastic deformation properties of the investigated alloys at higher temperatures in a negative way. The superplastic behaviour was characterised by the m value, which was determined by step strain - rate change tests. The tests were conducted at a slow initial strain rate, which increased stepwise to high constant strain rates. The highest m value obtained approached 0.7 for the lowest temperature (623K) and the strain rates (10 -3 - 10 -4 s -1 ). A value close to 0.5 was observed for temperatures 623-673K and the same strain rates. The lowest m values were found for temperatures T> 623K and higher strain rates. Structure studies of the superplastically deformed samples revealed grain growth and presence of cavities after elongation of a few hundreds percent. Presence of dislocations within grains indicates the participation of the crystallographic slip in the superplastic deformation at 673K.