Ravi Sankar Kottada

Evaluation of the diffusion coefficient of metal powders was attempted by using the power-law creep model in conjunction with the isothermal densification kinetics during spark plasma sintering (SPS). The diffusion coefficients obtained from the densification data of elemental Fe, Ni and Al powders are found to be higher than those reported in the literature. The higher values of diffusivity can be attributed to electric current effects. Our analysis demonstrates that it is possible to evaluate diffusion coefficients from experimental SPS densification data.

A composite of two different medium entropy alloys (MEAs, i.e., CoCrFeNi and AlCoCrFe) was synthesized using ball milling and spark plasma sintering. The composite microstructure contains a homogenous distribution of fcc and bcc phases with submicron-sized grains and exhibits excellent microstructural and phase stability even after 100 h heat treatment at 800 °C. The composite provides a combination of high compressive strength, adequate plastic strain, and multiple strain-hardening stages at room temperature. This first exploratory study on a MEA composite can be used as a template to other systems and illustrates the feasibility of combining two or more MEAs.

The B2-Aluminide matrix (FeAl and NiAl) with in-situ Al2O3 reinforcement were synthesized using reactive milling. The oxides (Fe2O3 and NiO) were reduced by Al during high energy milling to form Al2O3. The 20 h ball milled powders were consolidated using spark plasma sintering (SPS). To understand the densification mechanisms during SPS, sintering was performed at various temperatures (750–850 °C) and applied pressures (25–100 MPa). The creep parameters are evaluated from the densification data obtained during SPS using the model proposed by Bernard and Granger. In addition, independent constant-stress compression creep studies were conducted on the dense SPS pellets. The creep studies were performed on the composites at 800 °C at different stresses (100–500 MPa). The densification studies and compression creep studies are correlated based on the creep parameters obtained from both these studies and corroborated by TEM studies of the crept samples. This correlation is found to be valid even for the in-situ composites. Thus, the analysis of densification data can be helpful in predicting the creep behavior and useful for designing the new creep resistant alloys or composites.

The hot deformation behaviour of two solid solution alloys Mg-3Al-3Sn (AT33) and Mg-3Al-3Sn-1 Zn (ATZ331) were investigated using processing maps approach, while elucidating the role of Zn. Processing maps were generated using the flow stress data from constant strain rate (10−3 to 10 s−1) compression testing in the temperature range of 300 to 460 °C. Three distinct common domains (Domain I, II and III) were observed in the processing maps of AT33 and ATZ331. Addition of Zn is found to have expanded the domains range in ATZ331. Compared to AT33, an additional domain (Domain IV:T∼340−380∘C,ɛ˙∼10−2−10−1s−1) is observed in ATZ331. The instability map showed instability regions in the temperature and strain rate range in which twinning was predominant. Rate controlling mechanisms were identified by correlating the activation energy (Q) and stress exponent (n) in individual domains. Flow softening mechanisms in individual domains are corroborated by correlating the post-deformed microstructures with the deformation mechanism. The Domain III (T∼400−460∘C,ɛ˙=1−10s−1) is identified as the safe hot working regime for both the alloys AT33 and ATZ331 based on the correlation of microstructural evolution together with the processing maps and kinetic analysis. Dynamic recrystallisation is a dominant flow softening mechanism in Domain I, III and IV, whereas dynamic recovery is predominant in Domain II. On the whole, Zn addition has increased the tendency for cross-slip, and resulted in more recrystallisation in Domain I besides faster recovery in Domain II as compared to AT33.

Joule heating as a primary heating source mechanism was probed during spark plasma sintering (SPS) of pure metal powders (Fe, Ni and Cu). Resistance to electric path was estimated from voltage-current measurements obtained online during these experiments. Resistance was observed to saturate at the same value irrespective of the type of metal powder, after attaining a sintering temperature of ∼0.3Tm. This saturation in resistance is attributed primarily to the Joule heating that occurs at the graphite-foil and punch in an SPS system.

Simultaneous spark plasma sintering (SPS) of metal powders (Fe and Ni) with their respective aluminide powders (FeAl and NiAl) was attempted to produce a cylindrical aluminide coating on a cylindrical metal core. The interdiffusion zone (IDZ) formed between the metal and the aluminide was analyzed to evaluate the diffusion coefficient. The composition variation and the actual temperature estimated at the interface were taken into consideration while evaluating the diffusion coefficient. The diffusion coefficients estimated based on the IDZ are found to be higher than those obtained by conventional methods but are in agreement with those obtained from previous SPS studies.

In the present study, alloying of Ni with Cu was attempted using Ni and Cu elemental powders via high energy ball milling. The morphological and texture evolution during milling was studied. The {001} texture was observed in milled particles when they develop flaky morphology with large (L/D ~ 100) ratio and it changed to random texture when these flakes fragmented into tiny particles on further milling. These observations are rationalized by invoking the deformation mechanisms operating during milling. Extensive twinning was observed in the milled particles. The flaky nature of Ni is found to be stabilized with the addition of Cu with concomitant reduction in strain hardening compared to pure nickel.

A combination of high strength and good ductility was achieved in a precipitation hardenable Cu-3Ag-0.5Zr alloy through cryo-rolling (80% reduction in thickness) and aging in the temperature range (200-500 °C). The high-strength sheets produced by cryo-rolling showed a threefold increase in yield strength compared to that of the solution-treated and aged (220 MPa) sample, while retaining good ductility. An optimum combination of high strength (614 MPa) and ductility (8%) was achieved by 80% cryo-rolling and aging at 320 °C for 1 h. The high strength and good ductility obtained was attributed to various microstructural factors such as deformation twins, ultra-fine grains, high dislocation density and fine nano-sized silver precipitates.

The densification kinetics during spark plasma sintering (SPS) of FeAl and NiAl powders were analyzed using a model proposed by Bernard-Granger and Guizard [10]. Creep parameters obtained through densification data are in good agreement with those obtained from conventional creep experiments. Validity of the model was illustrated for aluminides in the form of deformation mechanism maps. This validation assures plausible confidence to predict creep behavior using densification data obtained during pressure assisted sintering of metallic alloys.