Andrea Madeira Kliauga

Al2O3/Ti interlayer/AISI 304 diffusion bonded joint : Microstructural characterization of the two interfaces

Abstract Sintered alumina and AISI 304 stainless-steel discs were joined at 900°C by solid state diffusion bonding, making use of a Ti foil acting as thermal stress relief interlayer. The microstructure of the two interfaces thus formed, that is, Al2O3/Ti and Ti/AISI 304 was investigated by a variety of characterization techniques such as scanning and transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) microanalysis, X-ray diffraction and Vickers microhardness, with the following results: (i) precipitation of Ti3Al particles plus Al and oxygen diffusion were detected within the Ti and close to the ceramic; (ii) within the Ti foil, coexistence of α-Ti and β-Ti was observed and ascribed to Fe and Cr diffusion and to β-Ti decomposition during slow cooling from the bonding temperature; and (iii) adjacent to the Ti/steel interface a 5-μm-thick layer was found to contain TiFe and Fe2Ti, as well as sigma phase, M23C6 precipitates and TiC. An interpretation of these results in terms of element partition during the bonding process was attempted.

Interface compounds formed during the diffusion bonding of Al2O3 to Ti

The interfacial reaction products of Ti/Al2O3 joints obtained in the context of real diffusion bonding technology were investigated by means of X-ray diffraction analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. Some Ti reacted with Al2O3 giving titanium oxides, but the main mass transport occurred into the bulk Ti due to Al2O3 dissolution. The formation of a Ti[Al, O] solid solution followed by a order/disorder reaction yielded Ti3Al. Further Al enrichment at the interface could lead to the formation of TiAl, which was not observed in the present work due to either the short residence time at the bonding temperatures or to its lower oxygen solubility. For joints obtained at 800°C and shear test fractured it was ascertained that the crack always propagated within the Ti3Al layer.

The effect of Sn additions on the semi-solid microstructure of an Al-7Si-0.3Mg alloy

Abstract The influence of Sn additions on both the binary Al–Si equilibrium phase diagram and the microstructural evolution of an Al7Si0.3Mg alloy (A356) having a solid/liquid ratio close to 0.5 were studied. Application of the THERMOCALC method showed that in the range 0.2–10.0 wt.%, Sn decreases all the L→S transformation temperatures. The microstructural evolution was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and by quantitative optical microscopy. Results showed that Sn additions decreased the rate of attainment of the equilibrium liquid fraction besides reducing the kinetics of particle spheroidisation and growth. This behaviour was attributed to a decrease of solid–liquid surface energy brought about by the presence of Sn as small globules within the Si or Al phases and at the Al–Si interface. This interpretation was supported by experimental measurements of particle contiguity and by dihedral angle calculations. The less accentuated effect observed on sample 2.0Sn, was explained by a different spatial distribution of the Sn globules.

Deformation analysis on F138 austenitic stainless steel: ECAE and rolling

Twinning is an alternative mechanism to achieve ultra-fine grain structures through severe plastic deformation. The properties induced in a plastically deformed material are highly dependent on the degree of deformation, accumulated deformation energy and details on grain sizes and microstructure, which are on the scale of some tens of nanometers; therefore it is very important to understand misorientation distributions and dislocation arrays developed in the samples. In this work an F138 austenitic stainless steel was solution heat treated, deformed by Equal Channel Angular Extrusion (ECAE) at room temperature up to four passes, and rolled up to 70% thickness reduction at room temperature. The microstructure evolution was analyzed by x-ray diffraction and domain sizes calculated by Convolutional Multiple Whole Profile (CMWP) model, the misorientation boundaries were measured by electron backscattered diffraction (EBSD), and transmission electron microscopy. Mechanical behavior was tested by tensile tests.

Assessing the Power of Electron Back Scattering Diffraction Characterization of Deformed F-138 Steel from the View Point of Crystal Diffraction

Abstract Microstructural characterization by combined EBSD and X-ray diffraction analysis is shown for an FCC material deformed by rolling and further heat treatment on air. The wide variety of x-rays sources combined with EBSD allows some correlations between microstructural parameters to be withdrawn by carefully analyzing x-ray peak broadening. The combination of both techniques provides a deeper understanding of anisotropic accumulation of dislocation arrays on a deformed F 138 stainless steel. The anisotropy results in a smaller peak breadth for crystal orientations with [110] planes contained on the rolling plane, further explained as resulting from a lower storage of dislocations. After heat treatment the effect is reverted and the formerly less misoriented orientations become more uniformly oriented revealing a faster dislocation migration and annihilation and/or trapping.

The Evolution of Texture and Deformation Anisotropy at an Equal Channel Extruded Aluminum 1050 Alloy

The aim of this work was to analyze the evolution of texture and the deformation anisotropy of a AA1050 after ECAP and further recrystallization. The initial material was processed by two different routes: the first set of samples came from a hot rolled band with a mixture of cube, copper and brass textures and the second from roll casting and presented a strong brass texture. The ECAP deformation followed routes A, Ay (no rotation between passes but keeping the normal to rolling direction parallel to the transverse extrusion axis) and Bc. Texture evaluation was performed by x-ray analysis. Results were analyzed by regular texture and orientation distribution function calculations. The deformation anisotropy was measured by tension tests with specimens oriented 0, 45 and 90o to the extrusion direction.

Microstructural Evolution of a F138 Austenitic Stainless Steel after Deformation by ECAP and HPT

A F138 austenitic stainless steel was solution heat treated, deformed by equal-channel angular pressing (ECAP) at 25, 100, 200, and 300°C. The equivalent strain was ~0.7 per pass and the applied equivalent strain varied from 0.7 to 4.2. The same material was also deformed by high pressure torsion (HPT) at 300 and 480°C, applying 6GPa pressure and 5 turns; the equivalent strain was ~ 4.5 at r/2 and ~5.2 at the vicinity of the disk edge. Microstructure evolution was observed by transmission electron microscopy (TEM) electron back-scattered diffraction (EBSD) and X ray diffraction. The effect of severe plastic deformation was studied at 25 and 300°C: at 25°C further deformation led to the formation of grain subdivision inside deformation bands and the onset of new grains formation after 2 ECAE passes. The deformation at 300 and 400°C up to 6 passes lead to the formation of recrystallized grains of the order of 100 nm size.

Mechanical and Corrosion Properties of a F138 Austenitic Stainless Steel after Deformation by Equal Channel Angular Pressing

A F138 austenitic stainless steel was solution heat treated, deformed by equal-channel angular pressing (ECAP) at 25 and 300°C. The equivalent strain was ~0.7 per pass and the applied equivalent strain varied from 0.7 to 2.8. Microstructure evolution was observed by transmission electron microscopy (TEM) electron back-scattered diffraction (EBSD) and X –ray diffraction. Work hardening behavior was studied by making use of Kocks-Mecking plots and hardness measurements, the influence of deformation on corrosion resistance was evaluated recording anodic polarization curves in 0.9% NaCl solution.

Anisotropic and Heterogeneous Development of Microstructures. Combining Laboratory/Synchrotron X-rays and EBSD on a few SPD Metallic Systems

The onset of Severe Plastic Deformation (SPD) regime is quite instructive on the possible origins of the nano-microstructures developed in metals and alloys. It is known that grain fragmentation and dislocation accumulation, among other defects, proceed at different paces depending fundamentally on grain orientations and active deformation mechanisms. There have been many attempts to characterize nano-microstructure anisotropy, leading all of them to sometimes contradictory conclusions. Moreover, the characterizations rely on different measurements techniques and pos-processing approaches, which can be observing different manifestations of the same phenomena. On the current presentation we show a few experimental and computer pos-processing and simulation approaches, applied to some SPD/alloy systems. Williamson-Hall and Convolutional Multiple Whole Profile (CMWP) techniques will be applied to peak broadening analysis on experimental results stemming from laboratory Cu Ka X-rays, and synchrotron radiation from LNLS (Laboratorio Nacional de Luz Sincrotron, Campinas, Brazil) and Petra III line (HEMS station, at DESY, Hamburg, Germany). Taking advantage of the EBSD capability of giving information on orientational and topological characteristics of grain boundaries, microstructures, grain sizes, etc., we also performed investigations on dislocation density and Geometrically Necessary Dislocation Boundaries (GNDB) and their correlation with texture components. Orientation dependent nano-microstructures and domain sizes are shown on the scheme of generalized pole figures and discussions provide some hints on nano-microstructure anisotropy.

How severe plastic deformation at cryogenic temperature affects strength, fatigue, and impact behaviour of grade 2 titanium

Samples of grade 2 Ti were processed by Equal Channel Angular Pressing (ECAP), either isolated or followed by further deformation by rolling at room temperature and at 170 K. The main interest of the present work was the evaluation of the effect of cryogenic rolling on tensile strength, fatigue limit and Charpy impact absorbed energy. Results show a progressive improvement of strength and endurance limit in the following order: ECAP; ECAP followed by room temperature rolling and ECAP followed by cryogenic rolling. From the examination of the fatigued samples a ductile fracture mode was inferred in all cases; also, the sample processed by cryogenic rolling showed very small and shallow dimples and a small fracture zone, confirming the agency of strength on the fatigue behaviour. The Charpy impact energy followed a similar pattern, with the exception that ECAP produced only a small improvement over the coarse-grained material. Motives for the efficiency of cryogenic deformation by rolling are the reduced grain size and the association of strength and ductility. The production of favourable deformation textures must also be considered.