Defence Metallurgical Research Laboratory

About: Defence Metallurgical Research Laboratory is a facility organization based out in Hyderabad, India. It is known for research contribution in the topics: Microstructure & Alloy. The organization has 1208 authors who have published 2662 publications receiving 51663 citations.

Effect of Filler Wire Composition on Microstructure and Pitting Corrosion of Nickel Free High Nitrogen Stainless Steel GTA Welds

Abstract: Nitrogen alloyed austenitic stainless steels are recently developed and are finding applications in defence because of improved mechanical properties and corrosion resistance. Welding is the main fabrication technique to join the structural components. During welding, it may result in porosity, solidification cracking in weld zone, liquation cracking in heat affected zone and inferior properties compared to base metal. Selection of filler wire plays a major role to obtain a sound weld and to have a better mechanical and corrosion resistance. In the present work, gas tungsten arc welding of 5 mm thick nickel free high nitrogen stainless steels were carried out. As no suitable matching filler wires have been developed, commercially available high strength fillers of precipitation hardenable (PH) 13-8Mo filler and nickel based (MDN 250) 18Ni filler were used for welding high nitrogen stainless steel. Microstructural studies were carried out using optical microscopy and field emission scanning electron microscopy (FESEM). Electron back scattered diffraction (EBSD) technique was used to determine the grain size, phase analysis and orientation mapping. Hardness values were recorded using Vickers hardness tester. Results of the present investigation established that the weld zone/fusion zone was observed to have delta-ferrite in the austenite matrix for both the welds. Welds made with (MDN 250) 18Ni filler had an unmixed zone adjacent to the weld zone near fusion boundary. Welds made with (PH) 13-8Mo filler had high hardness which might be attributed to relatively finer grains in the weld zone. Improved pitting corrosion resistance for welds made with (PH) 13-8Mo filler might be attributed to the composition of the filler wire. The presence of chromium and molybdenum helped in enhancing the stable passive film when compared to that of welds made with MDN 250 filler.

Microstructural changes induced by ternary additions in a hypo-eutectic titanium-silicon alloy

Abstract: Hypo-eutectic Ti-6.5 wt % Si alloy modified by separate additions of misch metal and low surface tension elements (Na, Sr, Se and Bi) has been examined by microscopic study and thermal analysis. Addition of third element led to modification of microstructure with apparently no significant enhancement of tensile ductility, with the exception of bismuth. Bismuth enhanced the ductility of the alloy by a factor of two and elastic-plastic fracture toughness to 9 MPa m−1/2 from a value of almost zero. The improved ductility of bismuth modified alloy is attributed to the reduced interconnectivity of the eutectic suicide, absence of significant suicide precipitation in the eutectic region and increase in the volume fraction of uniformly distributed dendrites. These changes are accompanied by a decrease in the temperature of eutectic solidification.

Effect of a solid solution on the steady-state creep behavior of an aluminum matrix composite

Abstract: The effect of an alloying element, 4 wt pct Mg, on the steady-state creep behavior of an Al-10 vol pct SiCp composite has been studied. The Al-4 wt pct Mg-10 vol pct SiCp composite has been tested under compression creep in the temperature range 573 to 673 K. The steady-state creep data of the composite show a transition in the creep behavior (regions I and II) depending on the applied stress at 623 and 673 K. The low stress range data (region I) exhibit a stress exponent of about 7 and an activation energy of 76.5 kJ mol-1. These values conform to the dislocation-climb-controlled creep model with pipe diffusion as a rate-controlling mechanism. The intermediate stress range data (region II) exhibit high and variable apparent stress exponents, 18 to 48, and activation energy, 266 kJ mol-1, at a constant stress, σ = 50 MPa, for creep of this composite. This behavior can be rationalized using a substructure-invariant model with a stress exponent of 8 and an activation energy close to the lattice self-diffusion of aluminum together with a threshold stress. The creep data of the Al-Mg-A12O3f composite reported by Dragone and Nix also conform to the substructure-invariant model. The threshold stress and the creep strength of the Al-Mg-SiCp, composite are compared with those of the Al-Mg-Al2O3f and 6061 Al-SiCp.w, composites and discussed in terms of the load-transfer mechanism. Magnesium has been found to be very effective in improving the creep resistance of the Al-SiCp composite.