Study on microstructure and properties of surface mechanical attrition treated gradient nanostructured alloys

Abstract

The current thesis focused on understanding the microstructure evolution and properties of the gradient nanostructured alloys (AISI 304L, AISI 2205, and AZ91D). Surface mechanical attrition treatment, a novel severe surface deformation technique, was explored to develop a gradient-structured layer on the alloys. Microstructure evolution, corrosion, oxidation, and the nanomechanical response of the SMATed layer were investigated in detail. The role of ball size (one of the crucial SMAT parameters) and alloy chemistry on passivation was explored in this work. Moreover, the effect of the SMATed layer on surface alloying (nitriding) was studied in this work. The topmost layer of the SMATed AISI 304 steel comprised nanograins of size 15 ± 10 nm. The SMAT process caused 2±3 times improvement in surface hardness of the AISI 304 stainless steel. Ball size, a vital SMAT parameter, played a significant role in controlling the microstructure and corrosion behaviour of the steel. Larger diameter (6 mm) balls induced a thicker gradient-VWUXcWXUed la\eU (a450 ȝP), a higheU SURSRUWiRQ of deformation-induced martensite (DIM # 50%), a higher density of dislocations and twins, and higher lattice strain. SMATed layer revealed the presence of considerable compressive residual stress. TEM results confirmed the grain size gradient in the near surface region and {111} type primary deformation twins throughout the deformed sub surface layer. Further, TEM investigation revealed two different modes of DIM transformation: (i) single steS, i.e., Ȗ o Į¶, and (ii) two steps, i.e., Ȗ o İ o Į¶, transformation, which simultaneously operated in the SMATed layer. SMAT enhanced the corrosion properties of the steel. However, the extent of overall corrosion improvement diminished due to the SMAT processing of steel using the larger diameter (6 mm) balls. SMAT altered the composition of the passive layer and increased its thickness. Furthermore, the surface behaviour of SMATed and plasma-nitrided AISI 2205 and AISI 304L steels was investigated in this thesis. The intersection of the mechanical twins formed the submicron-size rhombic blocks in the surface region of the SMATed AISI 304L steel. However, such microstructural feature was absent in the SMATed AISI 2205 steel. A passive film formed on the SMATed AISI 304L steel was relatively more unstable than the AISI 2205 steel at an elevated electric potential and in a plasma environment. The plasma-nitriding response was affected due to the different passivation behaviour of the SMATed, and non-SMATed steels. The SMAT process enhanced the high-temperature oxidation resistance of the AISI 304L steel in the temperature range of 600-800 °C. SMATed surface was prone to form denser, thinner, and adherent scale, dominated by nanocrystals of Cr- and Mn rich oxides. XPS results confirmed the dominance of Fe in the top layer of the oxide scale formed on the non-SMATed specimen. Ni-oxide formation tendency was poor during oxidation of the steel. The SMATed specimens revealed the following zones in the oxide scale: (i) Cr/Mn depleted outer layer, (ii) Cr-/Mn-rich inner layer, and (iii) gradually decreasing Cr/Mn region. A gradient-VWUXcWXUed la\eU Rf a500 ȝP WhickQeVV ZiWh iPSURYed (a2.5 WiPeV) surface hardness was successfully obtained on the AZ91D alloy surface via SMAT. TEM results confirmed nanograins of ~125 nm in topmost region and multiple deformation twin-modes, including {1012} <1011> dense twins and {1011} - {1012} double twinning in SMATed layer. Twining of secondary twins was established in the TEM analysis. The combined isothermal calorimetry and pressure measurement technique, a novel and powerful tool, was explored for in situ monitoring of magnesium cRUURViRQ iQ 0.9% NaCl VRlXWiRQ. The SMATed VSeciPeQ¶V lRZeU cRUURViRQ UeViVWaQce was attributed to the combined effect of the high density of defects, rougher surface, aQd VPalleU YRlXPe fUacWiRQ Rf ȕ phase at the surface.