Microstructures and properties of additively manufactured alloys processed by severe plastic deformation

Abstract

For the first time, the microstructure-property relationship of additively manufactured (AM) alloys processed by severe plastic deformation (SPD) was investigated in this PhD project. In this study, experiments were conducted on single-material 316L stainless steel (316L SS) and multi-material 316L SS/nickel 718 (IN 718) superalloy fabricated by selective laser melting (SLM) and then processed by high-pressure torsion (HPT). These include x-ray computed tomography (XCT), Vickers hardness (HV), x-ray diffraction (XRD), nanoindentation, electrochemical and wear tests, as well as optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) observations. This study aims to investigate the influence of ultrafine grains (UFG) and nano-scale grains (NG) obtained by HPT processing on the porosity, hardness, deformation mechanism, and corrosion and wear performances of SLM-fabricated alloys. 316L SS was initially additively manufactured by SLM and then processed by HPT through 1/4 – 10 revolutions at room temperature under 6 GPa of pressure at 1 rpm to produce UFG and NG microstructures. About 89 – 91% reduction of the spherical gas-induced pores is attained even at minimum torsional strain (1/4 HPT revolution) as determined from OM, due to the combined compressive force and extreme HPT-imposed torsional strain. HV measurements indicate significant hardness increase that saturates throughout the disk after 10 revolutions, suggesting the evolution towards microstructural homogeneity with increasing torsional strain. XRD spectra show that the alloy retains its single γ-austenite FCC structure even after extreme HPT straining. XRD line broadening analysis reveal significant decrease in crystallite size, Dc and considerable increase in dislocation density, ρ with larger HPT straining. TEM and XRD analysis suggest three stages of deformation mechanisms that are related to the microstructural features and the corresponding equivalent strain values, εeq. Primary twins, dislocations, and twin-matrix lamellae are dominant in stage 1 (εeq. = ~ 0 – 10). Shear banding of the twin-matrix lamellae and secondary nanotwins are prevalent in stage 2 (εeq. = ~ 10 – 40), while the equiaxed nanosized grains at stage 3 (εeq. > 40) indicate that an equilibrium or saturation stage is achieved. A physicalbased model was then established to evaluate the contribution of grain boundaries, dislocations, twins, and solid solution on the hardness increase attained by this alloy. The calculated strain rate sensitivity (SRS), m and activation volume, 푉푝 ∗ values from the results of nanoindentation measurements at both constant and varied strain rates were correlated with the microstructural changes from micro- to nanoregime to evaluate the evolution in plasticity and plastic deformation mechanism for all processing conditions. The high m and small 푉푝 ∗ values calculated for the as-received and HPT-processed disks suggest that reasonably high plasticity level is maintained, and grain boundary (GB)-mediated activities play a major role in the evolution of plastic deformation mechanism. Electrochemical tests, SEM observations, and energy dispersive x-ray spectroscopy (EDX) analysis indicate improved overall corrosion performance in 3.5% NaCl solution after HPT processing as evidenced by the consistently lower corrosion rates compared to the as-received AM 316L SS. The enhanced corrosion performance is attributed to the substantial porosity elimination, homogeneous distribution of UFG/NG microstructures, and the absence of martensite formation. Dry sliding wear tests demonstrate improved wear performance after HPT processing, as implied by the steady reduction in coefficient of friction (COF), mass loss, mloss and specific wear rate, kW values with increasing HPT torsional strains compared to the as-received AM 316L SS. The improvement in the overall wear resistance could be attributed to the combined grain refinement-induced high hardness and the formation of iron oxides that act as solid lubricant to lower the friction between the contact surfaces. In addition, SEM and EDX analysis suggest that the wear mechanism transitioned from severe abrasive wear for the as-received AM 316L SS to a combination of mild abrasive, adhesive, and tribo-oxidative wear for all HPT processing conditions. For the multi-material 316L SS/IN 718, about 91% of irregular shaped process-induced pores is eliminated after only 1/4 HPT revolution via the similar pore closure mechanisms from HPT-processed 316L SS as before. HV measurements suggest that hardness saturation is only achieved at the peripheral regions of the disks, as demonstrated by the consistently high HV values at the disk edges compared to the disk centre. XRD analysis shows that the interfacial 316L SS/IN 718 region retains its γ-austenite FCC structure and some (Nb,Ti)C phase throughout all processing conditions. Substantial decrease in Dc and remarkable increase in, ρ at the interfacial region are attained with larger HPT straining. TEM, EDX, and XRD analysis, and the physical-based strengthening model suggest that the hardness increase at the periphery of the interfacial region is the result of grain boundaries, dislocations, solid solution, and precipitates, with the additional contribution of nanotwins after 1 and 10 HPT revolutions. In the future, hip replacement bio-implants and small gas turbine blades are envisaged to be developed by exploiting the advantages of this hybrid AM/SPD approach, particularly the design flexibility of AM and the excellent strength, and superior corrosion and wear performance attained via SPD processing. Finally, the results from the present study show that the primary aim of establishing processmicrostructure-property relationships for SPD-processed AM alloys has been achieved