The present study reveals the microstructural evolution and corresponding mechanisms occurring during different stages of quenching and partitioning (Q&P) conducted on 0.6C-1.5Si steel using in-situ High Energy X-Ray Diffraction (HEXRD) and high-resolution dilatometry methods. The results support that the symmetry of ferrite is not cubic when first formed since it is fully supersaturated with carbon at the early stages of partitioning. Moreover, by increasing partitioning temperature, the dominant carbon source for austenite enrichment changes from ongoing bainitic ferrite transformation during the partitioning stage to initial martensite formed in the quenching stage. At low partitioning temperatures, a bimodal distribution of low- and high-carbon austenite, 0.6 and 1.9 wt.% carbon, is detected. At higher temperatures, a better distribution of carbon occurs, approaching full homogenization. An initial martensite content of around 11.5 wt.% after partitioning at 280 °C via bainitic ferrite transformation results in higher carbon enrichment of austenite and increased retained austenite amount by approximately 4% in comparison with partitioning at 500 °C. In comparison with austempering heat treatment with no prior martensite, the presence of initial martensite in the Q&P microstructure accelerates the subsequent low-temperature bainitic transformation.

The alloy 21-6-9 is a nitrogen-strengthened austenitic stainless steel often used in aerospace applications due to its high strength, good fabrication properties, and toughness at cryogenic temperatures. However, minimal research has been conducted on alloy 21-6-9 using the additive manufacturing process laser powder-bed fusion (L-PBF). The L-PBF technique has been seen as a key to reducing production time and avoiding costly machining. Therefore, there is an interest in investigating L-PBF-processed 21-6-9 to determine the effects of L-PBF on properties at elevated and cryogenic temperatures. In this study, prior to tensile testing the alloy 21-6-9 underwent heat treatments that simulated aerospace applications and the alloy was analyzed and characterized to evaluate phase stability. The effects of elevated and cryogenic temperatures (77K) on the tensile behavior and microstructure were investigated using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The tensile tests showed that the yield strength and ultimate tensile strength improved, while ductility varied depending on the conditions and test environment. The ultimate tensile strength was approximately 80% higher at 77K than at room temperature, although the elongation decreased by around 90%, possibly due to the formation of strain-induced martensite.

The effect of ultrasonic treatment (UST) and thermal annealing (THA) post-processes on the mechanical properties and the related microstructural mechanisms of the tensile pre-strained 316 stainless steel was investigated. It was shown that both processes reduce the microhardness and the yield point as well as increasing the elongation of the pre-deformed alloy. A 10% reduction of the yield point and 28% increase in the elongation was observed after the higher power UST (500 W), while an enhanced ductility of 56% and 41% reduction of the yield point was measured for the high-temperature THA (800°C) treated steel. The increased ductility was related to de-twinning and dislocation annihilation mechanisms, which increase the mean free path distance of dislocations. The de-twinning mechanism was proposed as the boundary migration mechanism and reverse gliding of the partial dislocations by cyclic shear stress for the THA and UST processes, respectively. Unlike the UST process, the high-temperature thermal annealing was associated with the formation of M₂₃C₆ precipitates, which causes depletion of alloying elements from the vicinity of grain boundaries and makes the alloy more prone to intergranular corrosion. Compared with THA, the advantages of the UST process are as follows: a rapid and straightforward process, low energy consumption, enhanced ductility without significant reduction in strength, and inhibition of grain boundary precipitation.

Carbon partitioning from martensite to austenite is essential for austenite stabilization during quenching and partitioning (Q&P), while a few competitive phenomena, such as bainitic transformation and carbide precipitation, alter the microstructural evolution. So, there is a need of using in-situ in combination with ex-situ characterisation techniques to understand the C partitioning at high temperature in relation to simultaneous competitive phenomena that might occur during the partitioning stage.

In this study, microstructural evolutions of a medium carbon steel ( 0.6C–1.6Si–1.25Mn–1.75Cr wt%) during Q&P treatment were investigated by using an in-situ High-Temperature X-Ray Diffraction (HTXRD) equipment at three partitioning temperatures. Results confirmed that carbon enrichment of austenite at 280 and 400 ℃ originates from partial carbon depletion from martensite and bainitic transformation, while partitioning at 500 ℃ results in the complete depletion of carbon from initial martensite and ferrite formation. Short diffusion distance (~0.13 µm) of carbon at 280 ℃ caused a poor carbon homogenization of austenite and formation of 8 vol% fresh martensite after final quenching. High Si content of the steel stabilized transitional carbides and, concurrently, suppressed Fe₃C formation during Q&P. The outcome of this study could contribute to the design of suitable chemistry and process parameters for producing quenched and partitioned steels.

The microstructure and mechanical properties of 2205 dual-phase pre-strained stainless steel following the high-power ultrasonic treatment (UST) were investigated. Mitigation of the deformation fiber texture, decrease in dislocation density, XRD peak shifting, and hardness reduction corroborated that the ultrasonic treatment can effectively reduce the internal stress of the duplex structure. The EDS and XRD results showed no brittle intermetallic phases were formed after the UST process, unlike conventional thermal treatment methods. Around 18% reduction in microhardness value and a significant ductility improvement of 54% can be achieved using an intermediate amplitude (power) vibration. The microstructure and mechanical properties variation caused by the UST process and the related mechanisms have been discussed in detail. New microstructural mechanisms have been proposed for the interpretation of the observed acoustic stress relief in both α and γ-phases.

Laser powder bed fusion (L-PBF) has attracted great interest in the aerospace and medical sectors because it can produce complex and lightweight parts with high accuracy. Austenitic stainless steel alloy 316 L is widely used in many applications due to its good mechanical properties and high corrosion resistance over a wide temperature range. In this study, L-PBF-processed 316 L was investigated for its suitability in aerospace applications at cryogenic service temperatures and the behavior at cryogenic temperature was compared with room temperature to understand the properties and microstructural changes within this temperature range. Tensile tests were performed at room temperature and at −196 °C to study the mechanical performance and phase changes. The microstructure and fracture surfaces were characterized using scanning electron microscopy, and the phases were analyzed by X-ray diffraction. The results showed a significant increase in the strength of 316 L at −196 °C, while its ductility remained at an acceptable level. The results indicated the formation of ε and α martensite during cryogenic testing, which explained the increase in strength. Nanoindentation revealed different hardness values, indicating the different mechanical properties of austenite (γ), strained austenite, body-centered cubic martensite (α), and hexagonal close-packed martensite (ε) formed during the tensile tests due to mechanical deformation.

The influence of ultrasonic treatment (UST) on the microstructure of AISI‐304 and AISI‐316 stainless steels as two common commercial grades with similar properties but different levels of stacking fault energy (SFE) are compared in this study. The softening effect of the ultrasonic wave on the pre‐deformed structure is demonstrated by microhardness measurements, while the relaxation of tensile residual stresses is affirmed through the X‐ray diffraction (XRD) peak shifting. Electron and optical microscopy revealed a significant impact of ultrasound on the reduction of deformation twins’ fraction. A new mechanism for de‐twinning under the action of ultrasonic vibration is proposed using electron backscatter diffraction (EBSD) analysis. The reduction of 25% and 34% are detected in dislocation density of 10% pre‐deformed tensile sample after 300 W UST for 304SS and 316SS alloys, respectively. The effect of SFE is discussed, and it turned out that cross‐slip is the main mechanism of dislocation annihilation as a result of UST. Observation of the low‐deformation regions close to the grain boundaries indicated the occurrence of the recrystallization phenomenon during UST. Dislocation annihilation, de‐twinning, dislocation absorption to grain boundaries, and recrystallization are regarded as the softening and relaxation mechanisms of UST for austenitic stainless steels.