Ravi Sankar Kottada

The High Temperature Tensile and Compressive Deformation Characteristics of Magnesia Doped Alumina

The mechanical characteristics of alumina have not yet been characterized completely in tension due in part to strain hardening accompanying grain growth and premature cavitation failure. Tensile tests were conducted on fine grained magnesia doped alumina over a range of strain rates, grain sizes and temperatures to evaluate the stress exponent, inverse grain size exponent and activation energy. Constant stress compression creep tests were also carried out under a similar range of experimental conditions. Extensive microstructural characterization after deformation indicated that there was considerable grain growth during deformation; however, the grains retained their initially equiaxed structure after significant deformation. Although a standard plot of strain rate versus stress indicated a stress exponent of ~2, a complete analysis including the compensation of data for concurrent grain growth revealed that true stress exponent was ~1, consistent with diffusion creep. It is argued that grain rearrangement processes accompanying grain growth will tend to mask the development of an elongated grain structure predicted by diffusion creep processes. In contrast to several ceramics with a significant amount of glassy phase, there is no significant difference between the elevated temperature tensile and compressive behavior of alumina.

Age-Hardening Characteristics of Cu-3Ag-0.5Zr Alloy

Among the copper alloys, the Cu-3Ag-0.5Zr alloy is one of the potential candidates for combustion chamber of liquid rocket engine because of its optimum combination of high strength with thermal conductivity. The present study is a detailed characterization of microstructure, strength, and electrical conductivity during the aging treatment. The aging cycle for Cu-3Ag-0.5Zr alloy after the solution treatment (ST) was optimized to obtain higher hardness without compromising on electrical conductivity. The precipitates responsible for strengthening in aged samples are identified as nanocrystalline Ag precipitates with an average diameter of 9.0±2.0 nm.

Heat-Affected Zone Liquation Cracking Resistance of Friction Stir Processed Aluminum-Copper Alloy AA 2219

In the current work, the effect of friction stir processing on heat-affected zone (HAZ) liquation cracking resistance of aluminum-copper alloy AA 2219 was evaluated. In Gleeble hot-ductility tests and longitudinal Varestraint tests, the FSPed material, despite its very fine dynamically recrystallized equiaxed grain structure, showed considerably higher susceptibility to HAZ liquation cracking when compared to the base material. Detailed microstructural studies showed that the increased cracking susceptibility of the FSPed material is due to (i) increase in the amount of liquating θ phase (equilibrium Al2Cu) and (ii) increase in the population of grain boundary θ particles. An important learning from the current work is that, in certain materials like alloy 2219, the use of FSP as a pretreatment to fusion welding can be counterproductive.

Friction Buttering: A New Technique for Dissimilar Welding

This work offers a fresh perspective on buttering, a technique often considered for fusion welding of dissimilar metals. For the first time, buttering was attempted in solid state using friction deposition. Using this new “friction buttering” technique, fusion welding of two different dissimilar metal pairs (austenitic stainless steel/borated stainless steel and Al-Cu-Mg/Al-Zn-Mg-Cu) was successfully demonstrated. The results show that friction buttering can simplify a tough dissimilar welding problem into a routine fusion welding task.

An innovative spraying setup to obtain uniform salt(s) mixture deposition to investigate hot corrosion

A hot corrosion study via molten salt deposition and its interaction with creep/fatigue play a critical role in predicting the life of gas turbine engine components. To do systematic hot corrosion studies, deposition of molten salts on specimens should be uniform with good adherence. Thus, the present study describes an in-house developed spraying setup that produces uniform and reliable molten salt deposition in a repeatable fashion. The efficacy of the present method was illustrated by depositing 90 wt. % Na2SO4 + 5 wt. % NaCl + 5 wt. % NaVO3 salt mixture on hot corrosion coupons and on creep specimens, and also by comparing with other deposition methods.

High Temperature Deformation Behaviour of a Mg-0.8Al Alloy

There is considerable interest currently in developing magnesium based alloys as replacements for aluminum alloys in automobile applications, due to their high specific strength as compared to aluminum alloys. However, the poor formability of magnesium alloys has restricted their applications; superplasticity can be utilized to form components with complex shapes. In the present study, the compressive deformation characteristics of a Mg-0.8 wt% Al alloy with an initial grain size of 19 +/- 1.0 mum have been studied in the temperature range of 623-673 K and at strain rates ranging from 10(-7) to 10(-3) s(-1). The stress exponent was observed to decrease with a decrease in stress. The results are analyzed in terms of the existing theoretical models for high temperature deformation. Furthermore, the potential for superplasticity in this alloy is explored, based on the mechanical and microstructural characteristics of the alloy.

A simple and versatile machine for creep testing at low loads (6–300 N) and on miniaturized specimens: Application to a Mg-base alloy

High temperature creep testing at a very low load range (<10 N) on miniaturized specimens has always been a challenge due to inherent design limitation (such as significant preload) of the conventional creep testing machines. In the present study, the challenge was overcome by developing a simple and versatile horizontal creep testing machine to conduct creep tests in the loading range of ∼6-300 N in tension and in compression. The competence of the in-house-built horizontal creep machine was validated by conducting creep testing on dog-bone shaped sheet specimens of cast Mg-1Sn-1Ca (TX11) Mg-base alloy over a lower stress range of 1.6-5.9 MPa (equivalent load range of 6.4-18.1 N) at 450 °C and in the high stress range of 20-80 MPa (equivalent load range of 76-310 N) at 175 °C.

Use of Friction Buttering for Overcoming HAZ Liquation Cracking

Multi-layer friction deposits in nickel-based superalloy Inconel 718 were investigated for their heat-affected zone liquation cracking resistance. The friction deposits, owing to their fine-grained microstructure with a large number of subgrain boundaries and fine, uniformly distributed carbide particles, were found to exhibit superior liquation cracking resistance to standard wrought-processed alloy 718. Based on these findings, a new technique involving friction deposition of the base plates prior to fusion welding was proposed for overcoming liquation cracking.

Use of Friction Stir Processing for Improving Heat-Affected Zone Liquation Cracking Resistance of a Cast Magnesium Alloy AZ91D

In this work, a cast magnesium alloy AZ91D was friction stir processed. Detailed microstructural studies and Gleeble hot ductility tests were conducted on the as-cast and the FSPed samples to comparatively assess their heat-affected zone liquation cracking behavior. The results show that the use of FSP as a pretreatment to fusion welding can strikingly improve the heat-affected zone liquation cracking resistance of alloy AZ91D by reducing the amount and size of the low-melting eutectic β (Mg17Al12) as well as by refining the matrix grain size.

Is there Diffusion Creep in Alumina

Although early studies on polycrystalline alumina attributed creep to diffusional processes, a re-analysis of the data and additional experimental results indicated that the creep behavior was non-linear, with a stress exponent of n~2, where ed