S. K. Varma

Static and Cyclic Oxidation of Ti–44Al and Ti–44Al–xNb Alloys

Binary, Ti–44Al, and ternary, Ti–44Al–xNb (x=4, 8, and 11) alloys were subjected to static and cyclic oxidation in air up to a period of 1 week at 850 and 950°C. The oxidation behavior is characterized by plotting a graph between weight gain as a function of time or number of cycles. The binary alloy undergoes the most severe oxidation while Nb provides resistance to oxidation, which increases with an increase in concentration. Oxides were examined by SEM and EDX was used to perform chemical analysis. Alternate oxide layers rich in alumina and titania were found along with an aluminum-depleted zone underneath the oxide scale. Different microstructures were observed for the alloys subjected to static- and cyclic-heating modes. Defects like twins and stacking faults were generated after a static mode of heating, while transformation products at the grain boundaries dominate the microstructures for samples subjected to cyclic heating.

Structure-sensitive properties during room-temperature wire drawing at various speeds in nickel 200

The effects of drawing speed, cell size and grain size on the yield strength of nickel 200 wires drawn at room temperature up to a true strain of 2.09 have been investigated. The wire drawing speeds in the range from 17 to 140 mm s−1 do not show any effect on the yield strength, cell size and grain size of drawn wires. However, the cell sizes as well as grain sizes decrease with increase in true wire drawing strain when their values are averaged over all the wire drawing speeds at a given strain. Even though the Hall-Petch equation is valid for all the grain diameters observed in this study, the graph suggests that two distinct linear regimes may be more appropriate to properly describe the strengthening mechanisms during wire drawing. The cell diameter has been correlated with the yield strengths of drawn wires by an inverse relationship.

The surface morphology and transient current response of Fe-16 wt% Cr-16 wt% Ni in a scratch test

This letter deals with a study of the electrochemical behaviour of an Fe-16 wt% Cr-16 wt% Ni alloy, immersed in a 0.01 M H 2 SO 4 -0.01 M KCl solution, when scratched with different hardness indentors. The depassivation characteristics have been studied by relating the transient current response to the different loads applied through the indentors to produce scratches

Microstructural development during aging of 2014 aluminum alloy composite

The 2014 aluminum alloy reinforced with 0.1 and 0.15 volume fraction of alumina particles (VFAP) have been solutionized for a range of time from 1.5 to 20 h at 813 K. The effect of solutionizing time (ST) on the age hardening response of the composites has been studied and compared with the characteristics exhibited by the monolith. The results indicate that increasing the ST decreases the time required to get the peak hardness (TPH) values in the monolith but the composites do not show a systematic monotonic behavior. The TPH values first decrease and then increase with an increase in ST at an aging temperature of 473 K for the composite. It has been speculated that he ST influences the concentration of quenched-in vacancies and continued heating may affect the bonding between particles and matrix which can generate additional dislocations throughout the solutionizing process due to curvature effects.

Strain-rate and grain-size effect on substructures and mechanical properties in OFHC copper during tension

The combined effect of grain size (recrystallized grains of 34, 86, 105 and 128 Μm) and strain rate (0.01, 0.05, 0.25, 2.5 and 5 min−1) on the evolution of dislocation substructures and mechanical properties in oxygen-free high conductivity (OFHC) copper during room-temperature tensile testing has been studied. Under identical conditions of deformation, the flow stress values for smaller grain size were higher than those for larger grain sizes with the exception in the case of 86 Μm which has been attributed to the inhomogeneous substructural developments in the microstructures. The cell size decreases monotonically with increase in per cent strain indicating no signs of cell size saturation. The effect of strain rate on the development of dislocation substructures at constant strain is such that the cell size decreases initially but increases with further increase in strain rate for smaller grain sizes of 34 and 86 Μm while a reverse trend has been observed for larger grain sizes of 105 and 128 Μm. A graph of the cell size strengthening coefficient, k, and the strain rate shows three distinct stages in the curves for different grain sizes.

Residual microstructure of a shaped-charge jet fragment

A recovered copper shaped-charge jet fragment has been built up by copper electrodeposition for the first time to allow it to be systematically sectioned and polished for detailed observations by optical and electron microscopy. The residual jet fragment microstructure was observed to have a recrystallized grain structure and dislocation substructures similar to those in the undeformed copper shaped charge liner cone. However, the average grain size in the recrystallized jet fragment was 15 μm compared to 45 μm for the liner. More significantly, however, SEM examination near the tips (or ends) of the jet fragment exhibited voids and coalesced void tunnels elongated axially within the fragment geometry, which are believed to have resulted during jet elongation and breakup by diffusion and viscous growth at high strain and strain rate. The observation of additional porosity in the interior of the jet fragment is supported by the lack of any similar observations in the surrounding and supporting (built up) copper electrodeposit.

The effect of stacking fault energy on the microstructural development during room temperature wire drawing in Cu, Al and their dilute alloys

The effect of stacking fault energy (SFE) on the evolution of microstructures during wire drawing at room temperature has been studied in pure aluminium, pure copper and Cu-2.2% Al andCu-4.5% Al alloys which covers a range of SFE values from 4 to 166 mJ m−2. The compositions are expressed in atomic parts per million by weight. The microstructures have been characterized from samples obtained by deforming rods of these materials to true wire drawing strain values of up to 1.47. A decrease in the SFE value changes the deformation mechanisms from the formation of cell structure and their size refinement in a high SFE material to the formation of deformation bands and deformation twins in a low SFE materials. The Cu-2.2% Al alloy deforms by deformation bands at low true strain values while deformation twins within the bands control the deformation mechanisms at higher true strain values. The alloy, Cu-4.5% Al, with the lowest SFE value deforms only by deformation twins even at low true strain values and the presence of overlapping and intersecting deformation twins are the dominating features as the rods are drawn to higher true wire drawing strains.

In situ surface oxidation of Ti-44Al-11Nb alloy at room temperature

The in situ surface oxidation of polycrystalline Ti-44Al-11Nb (compositions are in atomic %) alloy was studied at room temperature in an ultrahigh vacuum (66.6 to 80.0 nPa). The native oxide of Ti-44Al-11Nb was removed by sputtering the surface using 2.8 kV argon ions and then high-purity oxygen was carefully dosed onto the aluminide surface. After each oxygen dosing the surface species, metallic states and oxidized states, were detected by X-ray photoelectron spectroscopy (XPS). After analyzing the binding energy shifts and the concentration of oxidized species, aluminum oxidized first, then titanium followed by niobium. A reaction sequence model, incorporating the rate-determining step of a dissociative adsorption of oxygen, shows to be consistent with the initial oxidation rate of the Ti-44Al-11Nb intermetallic.

Corrosive wear behavior of 7075 aluminum alloy and its composite containing Al2O3 particles

Corrosive wear behavior of 7075 aluminum alloy and a composite containing 0.10 volume fraction of alumina particles (VFAP) has been evaluated. Transient current (TC) generated as a result of impacting a rotating cylindrical electrode immersed in a 0.1M NaCl solution with a Vickers diamond hardness indenter has been used to measure the corrosive wear response. Age hardenable 7075 alloy shows TC values that are sensitive to prior solutionizing time. The effect of alumina particles in a 7075 aluminum alloy matrix has been studied by comparing the TC values of a monolith along with composites under almost identical experimental conditions. The role of microstructural features associated with composites, such as dislocations generated after solutionizing treatment and during the corrosive wear process, has been observed with the help of near surface microstructures through transmission electron microscopy (TEM). Deformation induced dislocations, as well as those that are due to differences in the coefficient of thermal expansion (CTE) values between the particles and the matrix during solutionizing, have been attributed to the experimentally observed TC values. They may also be affected by the aging response of the monolith and composites, depending on solutionizing time.

Corrosive wear behavior of 2014 and 6061 aluminum alloy composites

Alloys of 2014 and 6061 aluminum reinforced with 0.1 volume fraction of alumina particles (VFAP) were subjected to impact scratching during a corrosive wear process. The transient currents generated due to the impact were measured in the two composites as well as in their respective monoliths. The effect of solutionizing time on the transient currents was correlated to the near surface microstructures, scratch morphology, concentration of quenched-in vacancies, and changes in grain sizes. It was observed that the transient current values increase with an increase in solutionizing time, indicating that the corrosive wear behavior is not strongly affected by the grain boundaries. However, a combination of pitting and the galvanic corrosion may account for the typical corrosive wear behavior exhibited by the alloys and the composites of this study.