The microstructure of low alloy Transformation Induced Plasticity (TRIP) assisted steels has been systematically varied through the combination of computational and experimental methodologies in order to enhance the mechanical performance and to fulfill the requirement of the next generation Advanced High Strength Steels (AHSS). The roles of microstructural parameters, such as phase constitutions, phase stability, and volume fractions on the strength-ductility combination have been revealed. Two model alloy compositions (i.e. Fe-1.5Mn-1.5Si-0.3C, and Fe-3Mn-1Si-0.3C in wt%, nominal composition) were studied. Multiphase microstructures including ferrite, bainite, retained austenite and martensite were obtained through conventional two step heat treatment (i.e. intercritical annealing-IA, and bainitic isothermal transformation-BIT). The effect of phase constitution on the mechanical properties was first characterized experimentally via systematically varying the volume fractions of these phases through computational thermodynamics. It was found that martensite was the main phase to deteriorate ductility, meanwhile the C/VA ratio (i.e. carbon content over the volume fraction of austenite) could be another indicator for the ductility of the multiphase microstructure. Following the microstructural characterization of the multiphase alloys, two microstructural design criteria (i.e. maximizing ferrite and austenite, suppressing athermal martensite) were proposed in order to optimize the corresponding mechanical performance. The volume fraction of ferrite was maximized during the IA with the help of computational thermodyanmics. On the other hand, it turned out theoretically that the martensite suppression could not be avoided on the low Mn contained alloy (i.e. Fe-1.5Mn-1.5Si-0.3C). Nevertheless, the achieved combination of strength (~1300MPa true strength) and ductility (~23% uniform elongation) on the low Mn alloy following the proposed design criteria fulfilled the requirement of the next generation AHSS. To further optimize the microstructure such that the designed criteria can be fully satisfied, further efforts have been made on two aspects: heat treatment and alloy addition. A multi-step BIT treatment was designed and successfully reduced the martensite content on the Fe-1.5Mn-1.5Si-0.3C alloy. Microstructure analysis showed a significant reduction on the volume fraction of martensite after the multi-step BIT as compared to the single BIT step. It was also found that, a slow cooling rate between the two BIT treatments resulted in a better combination of strength and ductility than rapid cooling or conventional one step BIT. Moreover, the athermal martensite formation can be fully suppressed by increasing the Mn content (Fe-3Mn-1Si-0.3C) and through carefully designed heat treatments. The athermal martensite-free alloy provided consistently better ductility than the martensite containing alloy. Finally, a microstructure based semi-empirical constitutive model has been developed to predict the monotonic tensile behavior of the multiphase TRIP assisted steels. The stress rule of mixture and isowork assumption for individual phases was presumed. Mecking-Kocks model was utilized to simulate the flow behavior of ferrite, bainitic ferrite and untransformed retained austenite. The kinetics of strain induced martensitic transformation was modeled following the Olson-Cohen method. The developed model has results in good agreements with the experimental results for both TRIP steels studied with same model parameters.

The unique combination of high specific strength and ductility in third generation advanced high-strength steels (3G-AHSSs) has garnered significant attention from top automotive steel industries. These materials are being considered as potential options for making lighter body components due to their strength and ability to tolerate thinner material cross-sections. However, galvanizing these steels through the continuous hot-dip galvanizing process is challenging, because the main alloying elements such as Mn, Si, Al, and Cr tend to selectively oxidize on the steel surface during the annealing process before being immersed in the galvanizing bath containing Zn(Al, Fe). The presence of these oxides extensively covering the substrate surface can negatively impact reactive wetting, coating adhesion, and overall coating quality. In this study, the selective oxidation kinetics and reactive wetting of a series of Fe-(2-10)Mn-(0.00/0.01/0.03)Sb (at. pct) were determined and a model was proposed for analyzing oxide growth during intercritical annealing prior to galvanizing. Annealing heat treatments were carried out at 676, 725, 775, and 825 ˚C for 60-480s holding time in a N2-5vol pct H2 process with a dew point of –10 ˚C. MnO was formed on all samples after annealing. It was determined that the annealing conditions (temperature and isothermal holding time) affected the external oxide thickness and depth of the oxidation zone, which in turn influenced the MnO growth rate. With increasing the bulk Mn content of the alloy, the Mn elemental flux to the external surface increased, resulting in an increase in the oxidation parabolic rate constant. The average activation energy of internal oxidation for the Fe-2Mn, Fe-6Mn and Fe-10Mn alloys were determined to be 216±15 kJ/mol, 178 ± 18 kJ/mol and 152 ±10 kJ/mol, respectively, which are consistent with the activation energy of oxygen diffusion through MnO interfaces and the bulk diffusion of oxygen in austenite. Moreover, the average activation energy for external oxide growth was ~113±18 kJ/mol, which was attributed to the diffusion of Mn cations along the grain boundaries of the external Mn oxides. It was determined that micro addition of Sb to the Fe-Mn alloys led to a reduction in the oxidation rate constant, external oxide thickness, and internal oxidation zone, which was attributed to Sb segregation at both the external and internal oxide interface, resulting in the reduction of oxygen permeability. The reduction was more significant in the Fe-10Mn alloys, primarily attributable to the increased Sb segregation at the interfaces. The research showed that when the bulk Mn content increased, more antimony (Sb) segregated at both the internal and external oxide/substrate interface. As a result, the oxygen present at these interfaces decreased. This is attributed to the reduction of Sb solubility in α-Fe with increasing Mn and positive interactions between Sb and Mn. Advanced Atom Probe Tomography (APT) analysis confirmed that as more Sb segregated at the interfaces, the excess oxygen reduced due to site competition between O and Sb. Additionally, Sb surface segregation kinetics for Fe-(0.01/0.03)Sb and Fe-2Mn-(0.01/0.03)Sb at.% were determined based on the modified Darken model and linear heating followed by isothermal annealing. After the annealing, Sb segregation was detected on the surface of both the Fe-xSb and Fe-2Mn-xSb alloys, which increase with increasing temperature and holding time. The segregation rate, as determined from the Darken curves, was higher in Fe-Sb alloys compared to Fe-2Mn-Sb alloys, which can be attributed to variations in the crystal structure and the density of defects within the metal matrix. Additionally, the activation energy for Sb diffusion in both Fe-Sb and Fe-2Mn-xSb alloys were determined to be approximately 193±18 kJ/mol closely aligns with the activation energy of Sb bulk diffusion in α-Fe. Simulated galvanizing treatments were conducted on Fe-(2-10)Mn-(0.00/0.03)Sb at.% alloys. It was found that Sb segregation at the external/oxide interface resulted in a decrease in the size and thickness of the external oxide particles, which can facilitate better contact between the zinc bath and the substrate. Furthermore, it was found that Sb segregation at the interface between the external oxide and substrate led to a decrease in the stability of the interfacial region. This effect was attributed to an increase in the local atomic spacing near the interface, caused by Sb segregation. As a result, a local strain was observed near the interface. This localized strain significantly reduced the energy needed to separate the oxide from the metal matrix, contributing to decreased stability of the interfacial region. The higher bulk manganese (Mn) content led to increased segregation of antimony (Sb), resulting in a greater local strain within the interfacial region. These effects, in turn, enhanced the kinetics of the aluminothermic reduction reaction and assisted oxide lift-off. Furthermore, the closely packed Fe-Al intermetallics at the coating/steel interface increased as a result of adding Sb to the steel. In addition, no Sb segregation was observed at interfacial layer/metal interface. This absence of segregation can be attributed to the dissolution of segregated Sb into the liquid zinc. It was determined that Sb, which segregated at the external oxide/substrate interface during annealing, dissolved into the zinc bath and disrupted its bond with iron. This disruption occurred due to the higher electronegativity of Sb compared to Fe with Zinc, as well as the sufficient solubility of Sb in liquid zinc. / Thesis / Doctor of Science (PhD) / The unique combination of high specific strength and ductility exhibited by third-generation advanced high-strength steels has captured the attention of automotive industries. However, challenges arise when attempting to galvanize these steels through continuous hot-dip galvanizing processes. The selective oxidation of alloying elements during annealing can have detrimental effects on reactive wetting and coating adhesion. The objective of this research was to improve the coating quality of Mn-containing steels by introducing micro-additions of Sb. Sb segregation to the surface and interfaces began to occur during annealing. Segregated Sb resulted in a reduction of the oxidation rate. Sb segregation at oxide interfaces also contributed to decreased oxygen permeability. Upon immersion in the liquid zinc bath, both Sb and Fe dissolved into the zinc, leading to the formation of an interfacial layer on the surface, which indicates successful reactive wetting. The findings of this research provide valuable insights for improving galvanizing processes and enhancing coating quality, specifically in the context of third-generation advanced high-strength steels.

No description available.

Os Aços Avançados de Alta Resistência ou AHSS (do inglês Advanced High Strength Steels) apresentam muitas vantagens mecânicas em relação aos aços convencionais. Seu uso crescente na indústria automotiva deve-se principalmente à sua capacidade de possibilitar a redução de peso e, ao mesmo tempo, o aumento da segurança aos ocupantes do veículo em caso de colisões. No entanto, apresentam maiores dificuldades no que se refere à conformabilidade (maiores níveis de solicitação e desgaste das ferramentas, menor deformabilidade plástica, etc). Assim, alguns testes para avaliar a conformabilidade destes materiais ganham maior importância. É o caso do Teste de Expansão de Furos, cuja propriedade medida é a Razão de Expansão de Furos (REF). Neste trabalho investiga-se o processo de expansão de furos para o aço avançado de alta resistência (AHSS) martensítico DOCOL 190M, sob as seguintes condições de processo: duas formas de obtenção do furo (jato d’água e usinagem); duas geometrias distintas de punções (cônico de 60º e elíptico); diversos diâmetros do furo inicial; com e sem o uso de lubrificante; com acabamento diferenciado da borda do furo; e expansão com deslocamento do punção em etapas. Os experimentos demonstram que a expansão de furos possui uma estreita relação com a geometria do punção utilizado para a expansão, bem como com o diâmetro do furo inicial, acabamento da borda e condições de lubrificação. A partir dos resultados experimentais de expansão de furos foi possível realizar a calibração de um software de simulação computacional em relação ao dano crítico do material no momento da fatura na borda do furo. / Advanced High Strength Steels (AHSS) offer many mechanical advantages over conventional steels. Its increasing use in the automotive industry is mainly due to its ability to reduce weight and, at the same time, increase occupant safety in the event of collisions. However, they present greater difficulties with respect to the formability (higher levels of solicitation and wear of the tools, lower plastic formability, etc). Thus, some tests to evaluate the formability of these materials come to have greater importance. This is the case of the Hole Expansion Test, whose measured property is the Hole Expansion Ratio (REF). This work investigates the hole expansion process for a martensitic advanced high-strength steel (AHSS), DOCOL 190M, under the following process conditions: two ways of obtaining the hole (water jet and machining); two different geometries of punctures (conical of 60º and elliptical); various diameters of the initial hole; with and without the use of lubricant; with differentiated finishing of the hole edge; and expansion with punch displacement in steps. The experiments demonstrate that the hole expansion has a close relationship with the geometry of the punch used for the expansion, as well as the initial hole diameter, edge finish and lubrication conditions. From the experimental hole expansion results it was possible to carry out the calibration of a computational simulation software in relation to the critical damage of the material at the moment of hole edge rupture.

This thesis is devoted to the numerical investigation of mechanical behavior of Dual phase (DP) steels. Such grade of advanced high strength steels (AHSS) is favorable to the automotive industry due the unique properties such as high strength and ductility with low finished cost. Many experimental and numerical studies have been done to achieve the optimized behavior of DP steels by controlling their microstructure. Experiments are costly and time consuming so in recent years numerical tools are utilized to help the metallurgist before doing experiments. Most of the numerical studies are based on classical (local) constitutive models where no material length scale parameters are incorporated in the model. Although these models are proved to be very effective in modeling the material behavior in the large scales but they fail to address some critical phenomena which are important for our goals. First, they fail to address the size effect phenomena which materials show at microstructural scale. This means that materials show stronger behavior at small scales compared to large scales. Another issue with classical models is the mesh size dependency in modeling the softening behavior of materials. This means that in the finite element context (FEM) the results will be mesh size dependent and no converged solution exist upon mesh refinement. Thereby by applying the classical (local) models one my loose the accuracy on measuring the strength and ductility of DP steels. Among the non-classical (nonlocal) models, gradient-enhanced plasticity models which consider the effect of neighboring point on the behavior of one specific point are proved to be numerically effective and versatile tools to accomplish the two concerns mentioned above. So in this thesis a gradient-enhanced plasticity model which incorporates both the energetic and dissipative material length scales is derived based on the laws of thermodynamics. This model also has a consistent yield-like function for the interface which is an essential part of the higher-order gradient theories. The main issue with utilizing these theories is the implementation which limits the application of these theories for modeling the real problems. Here a straightforward implementation method based on the classical FEM and Meshless method will be proposed which due to its simplicity it can be applied for many problems. The application of the developed model and implementation will be shown on removing the mesh size dependency and capturing the size effect in microstructure level of dual phase steels.

Os Aços Avançados de Alta Resistência ou AHSS (do inglês Advanced High Strength Steels) apresentam muitas vantagens mecânicas em relação aos aços convencionais. Seu uso crescente na indústria automotiva deve-se principalmente à sua capacidade de possibilitar a redução de peso e, ao mesmo tempo, o aumento da segurança aos ocupantes do veículo em caso de colisões. No entanto, apresentam maiores dificuldades no que se refere à conformabilidade (maiores níveis de solicitação e desgaste das ferramentas, menor deformabilidade plástica, etc). Assim, alguns testes para avaliar a conformabilidade destes materiais ganham maior importância. É o caso do Teste de Expansão de Furos, cuja propriedade medida é a Razão de Expansão de Furos (REF). Neste trabalho investiga-se o processo de expansão de furos para o aço avançado de alta resistência (AHSS) martensítico DOCOL 190M, sob as seguintes condições de processo: duas formas de obtenção do furo (jato d’água e usinagem); duas geometrias distintas de punções (cônico de 60º e elíptico); diversos diâmetros do furo inicial; com e sem o uso de lubrificante; com acabamento diferenciado da borda do furo; e expansão com deslocamento do punção em etapas. Os experimentos demonstram que a expansão de furos possui uma estreita relação com a geometria do punção utilizado para a expansão, bem como com o diâmetro do furo inicial, acabamento da borda e condições de lubrificação. A partir dos resultados experimentais de expansão de furos foi possível realizar a calibração de um software de simulação computacional em relação ao dano crítico do material no momento da fatura na borda do furo. / Advanced High Strength Steels (AHSS) offer many mechanical advantages over conventional steels. Its increasing use in the automotive industry is mainly due to its ability to reduce weight and, at the same time, increase occupant safety in the event of collisions. However, they present greater difficulties with respect to the formability (higher levels of solicitation and wear of the tools, lower plastic formability, etc). Thus, some tests to evaluate the formability of these materials come to have greater importance. This is the case of the Hole Expansion Test, whose measured property is the Hole Expansion Ratio (REF). This work investigates the hole expansion process for a martensitic advanced high-strength steel (AHSS), DOCOL 190M, under the following process conditions: two ways of obtaining the hole (water jet and machining); two different geometries of punctures (conical of 60º and elliptical); various diameters of the initial hole; with and without the use of lubricant; with differentiated finishing of the hole edge; and expansion with punch displacement in steps. The experiments demonstrate that the hole expansion has a close relationship with the geometry of the punch used for the expansion, as well as the initial hole diameter, edge finish and lubrication conditions. From the experimental hole expansion results it was possible to carry out the calibration of a computational simulation software in relation to the critical damage of the material at the moment of hole edge rupture.

Os Aços Avançados de Alta Resistência ou AHSS (do inglês Advanced High Strength Steels) apresentam muitas vantagens mecânicas em relação aos aços convencionais. Seu uso crescente na indústria automotiva deve-se principalmente à sua capacidade de possibilitar a redução de peso e, ao mesmo tempo, o aumento da segurança aos ocupantes do veículo em caso de colisões. No entanto, apresentam maiores dificuldades no que se refere à conformabilidade (maiores níveis de solicitação e desgaste das ferramentas, menor deformabilidade plástica, etc). Assim, alguns testes para avaliar a conformabilidade destes materiais ganham maior importância. É o caso do Teste de Expansão de Furos, cuja propriedade medida é a Razão de Expansão de Furos (REF). Neste trabalho investiga-se o processo de expansão de furos para o aço avançado de alta resistência (AHSS) martensítico DOCOL 190M, sob as seguintes condições de processo: duas formas de obtenção do furo (jato d’água e usinagem); duas geometrias distintas de punções (cônico de 60º e elíptico); diversos diâmetros do furo inicial; com e sem o uso de lubrificante; com acabamento diferenciado da borda do furo; e expansão com deslocamento do punção em etapas. Os experimentos demonstram que a expansão de furos possui uma estreita relação com a geometria do punção utilizado para a expansão, bem como com o diâmetro do furo inicial, acabamento da borda e condições de lubrificação. A partir dos resultados experimentais de expansão de furos foi possível realizar a calibração de um software de simulação computacional em relação ao dano crítico do material no momento da fatura na borda do furo. / Advanced High Strength Steels (AHSS) offer many mechanical advantages over conventional steels. Its increasing use in the automotive industry is mainly due to its ability to reduce weight and, at the same time, increase occupant safety in the event of collisions. However, they present greater difficulties with respect to the formability (higher levels of solicitation and wear of the tools, lower plastic formability, etc). Thus, some tests to evaluate the formability of these materials come to have greater importance. This is the case of the Hole Expansion Test, whose measured property is the Hole Expansion Ratio (REF). This work investigates the hole expansion process for a martensitic advanced high-strength steel (AHSS), DOCOL 190M, under the following process conditions: two ways of obtaining the hole (water jet and machining); two different geometries of punctures (conical of 60º and elliptical); various diameters of the initial hole; with and without the use of lubricant; with differentiated finishing of the hole edge; and expansion with punch displacement in steps. The experiments demonstrate that the hole expansion has a close relationship with the geometry of the punch used for the expansion, as well as the initial hole diameter, edge finish and lubrication conditions. From the experimental hole expansion results it was possible to carry out the calibration of a computational simulation software in relation to the critical damage of the material at the moment of hole edge rupture.

No description available.

Third generation advanced high-strength steels (3G-AHSS) have received significant interest from leading auto steel industries and OEMs as candidate materials for reduced mass Body In White (BIW) components due to their unique combination of high specific strength and ductility. However, the continuous hot-dip galvanizing of these steels is challenging due to selective oxidation of the main alloying elements such as Mn, Si, Al and Cr at the steel surface during the annealing step prior to immersion in the galvanizing Zn(Al, Fe) bath, as extensive coverage of the substrate surface by these oxides is detrimental to reactive wetting, good coating adhesion and integrity. Simulated galvanizing treatments were conducted on two prototype Fe-0.1C-6Mn-2Si (wt pct) 3G steels; one as the reference steel and the other with 0.05 wt pct Sn added to the composition. The combined effects of annealing temperature, time, process atmosphere oxygen partial pressure and 0.05 wt pct Sn addition on the selective oxidation of the steel substrates were determined. Subsequently, the reactive wetting of the steels with respect to the pre-immersion surface structures of the samples annealed for 120 s was examined. Annealing heat treatments were carried out at 800˚C and 690˚C in a N2-5 vol pct H2 process atmosphere under three dew points of –50˚C, –30˚C and +5˚C, covering process atmosphere oxygen partial pressures within the range of 1.20  10-27 atm to 1.29  10-20 atm. MnO was present at the outmost layer of the external oxides on all samples after annealing. However, the morphology, distribution, thickness and surface coverage were significantly affected by the experimental variables. Annealing the reference steel under the low dew point process atmospheres, i.e. –50˚C and –30˚C, resulted in the highest Mn surface concentration as well as maximum surface oxide coverage and thickness. The oxides formed under these process atmospheres generally comprised coarse, compact and continuous film forming nodules, whereas the surface morphologies and distributions obtained under the +5˚C dew point process atmosphere, which was consistent with the internal oxidation mode, exhibited wider spacing between finer and thinner MnO nodules. The grain boundary internal oxide networks had a multi layer structure with SiO2 and MnSiO3 at the oxide cores and shells, respectively. Significant morphological changes were obtained as a result of Sn addition. The continuous film-like external MnO nodules were modified to a fine and discrete globular morphology, with less surface coverage by the oxides and reduced external oxide thickness. Both the external and internal oxidations followed parabolic growth kinetics, where the depth of the internal oxidation zone decreased with Sn addition and decreasing oxygen partial pressure. Poor reactive wetting was observed for the reference steel substrates that were annealed for 120 s under the –50˚C and –30˚C dew point process atmospheres at 800˚C and under the –50˚C dew point atmosphere at 690˚C, such that no integral metallic coating was formed after the 4 s immersion in the Zn(Al, Fe) bath. By contrast, excellent coating quality was obtained for the Sn-added steels when the –30˚C and +5˚C dew point process atmospheres were employed when annealing at 690˚C. The remainder of the experimental conditions demonstrated good reactive wetting with intermediate coating quality. For the two reference steels annealed at 800˚C under the –50˚C and –30˚C dew point process atmospheres, poor reactive wetting was due to full coverage of the surface by 116 nm and 121 nm thick and continuous MnO films. In the case of the 690˚C  –50˚C reference steel with the external layer thickness of only 35 nm, however, poor wetting was attributed to substantial coverage of the surface by continuous, film-like oxides. In both cases, exposure of the underlying substrate to the bath alloy and an intimate contact between the substrate Fe and the bath dissolved Al could not take place and the formation of the Fe2Al5Znx interfacial layer was hidered. For the processing conditions that satisfactory reactive wetting was obtained despite the pre-immersion selective oxidation of the surfaces, several reactive wetting mechanisms were determined. For the samples with a sufficiently thin external MnO layer, good reactive wetting was attributed to partial reduction of MnO by the bath dissolved Al, as well as bridging of the Mn sub-oxides by the Zn coating or Fe2Al5Znx interfacial intermetallics. Partial or full formation of the Fe2Al5Znx interfacial layer was observed in the successfully galvanized substrates with Fe-Al crystals formed between, underneath and also on top of the reduced oxides. Furthermore, for cases with widely-spaced, fine oxide nodules, it was found that the liquid bath alloy was able to infiltrate the external oxide/substrate interface, resulting in surface oxide lift-off and enhanced coating adhesion. It was globally concluded that the thin, discrete and fine globular morphology of external MnO, resultant of annealing the steel substrates with 0.05 wt pct Sn addition under the process atmosphere oxygen partial pressures consistent with internal oxidation, allowed for an enhanced reactive wetting by the Zn(Al, Fe) galvanizing bath which was manifested by increased amount of Al uptake and population of the Fe2Al5Znx intermetallics at the coating/steel interface. / Thesis / Doctor of Science (PhD)

Lightweighting demands have been achieved by third generation (3G) Advanced High Strength Steels (AHSSs) by a means of increased strength. The challenge faced in doing so, however, is in ensuring that ductility and crashworthiness is efficiently retained. Key methods in which automotive research has been invested to achieve this strength-ductility balance is by microalloying to promote grain refinement, the introduction of precipitates, and the effective use of plasticity enhancing mechanisms. Specifically, the ability to tailor the stability of retained austenite during deformation has been crucial in manipulating the strength-to-ductility ratio of 3G AHSSs using the Transformation Induced Plasticity (TRIP) effect. On the other hand, dual phase (DP) (i.e: non-TRIP-assisted steels) continue to be most significantly manufactured due to their robust thermomechanical processing but are also compromised by their poor damage tolerance. Hence, considerable reports are available regarding the damage tolerance of DP steels, but the ability for the volume expansion associated with the austenite-to-martensite transformation to suppress damage evolution and enhance a steel’s local formability has not yet been thoroughly investigated. Nonetheless, the damage processes that lead to fracture in 3G AHSSs are complex. A full understanding of the underlying phenomena requires a careful assessment of the strain partitioning amongst phases, how the microstructure evolves with strain and how damage, in the form of voids and micro-cracks, nucleates and grows. This can only be accomplished by applying a range of methodologies, including microscopic Digital Image Correlation (µDIC), X-ray Computed Microtomography (µXCT), Electron Backscattered Diffraction (EBSD) and X-ray Diffraction (XRD), all of which can be tracked as deformation proceeds. This PhD thesis uses a novel post µDIC data processing technique to prove that a reduction in strain gradient, linked to the evolution Geometrically Necessary Dislocations (GNDs), at dissimilar phase interfaces is attainable with vanadium-microalloying and with use of the TRIP effect. A local strain gradient post µDIC data processing technique was developed and first applied on 3G DP steels to show that the microcompatibility between ferrite and martensite directly at the interface is considerably improved with vanadium-microalloying. This in turn microscopically explains this DP steel’s increased local formability/damage tolerance with vanadium micro-additions. Moreover, when applying this novel µDIC technique on two other 3G experimental steels of interest, an ultrahigh strength Quench & Partition (Q&P) steel and a continuous galvanizing line (CGL)-compatible Medium-Mn (med-Mn) steel, an even slower evolution of microstrain gradients at dissimilar phase interfaces was observed. This indicates that, although vanadium-microalloying can improve the damage tolerance of a DP steel, its ability to achieve the ultrahigh strengths is a direct result of the severe inhibition of dislocation motion at dissimilar phase boundaries. Eventually, at high strains, these local strain gradients cannot be maintained and results in premature damage nucleation. By comparison, at such high strains, distinct evidence of damage nucleation was not apparent in the 3G TRIP-assisted steels which is the result of a slow strain gradient evolution delayed by the effective use of TRIP. This finding triggered a further investigation into isolating the impact the rate of TRIP exhaustion has on damage development. By intercritically annealing this prototype med-Mn steel (0.15C-5.8Mn-1.8Al-0.71Si) with a martensitic starting microstructure, within a narrow temperature interval (from 665 to 710°C), it was possible to make significant changes in the steel’s rate of TRIP exhaustion without making considerable changes to its physical microstructure. This steel exhibits the largest true strain at fracture (ɛf = 0.61), meets U.S. Department of Energy (DoE) mechanical targets (28,809 MPa%), and shows sustained monotonic work hardening when intercritically annealed at an intermediate IA temperature of 685°C for 120s. In addition, this IA condition showed optimal damage tolerance properties as an abundance of voids nucleated during its tensile deformation, but their growth was suppressed by prolonging TRIP over a large strain range. There is reason to believe that the heterogeneous distribution of austenite and Mn throughout this 685°C IA condition compared to the other two enabled its suppressed TRIP kinetics and in turn improved damage tolerance. The impact that changes in stress-state, from a stress triaxiality of 0.33-0.89, has on microstrain partitioning, TRIP kinetics and damage evolution was tested on this med-Mn at its 685°C IA condition. With the machining of notches on tensile specimens, it was seen that a high stress triaxiality (0.74-0.89) accelerated the rate of TRIP, whereas the introduction of shear, through a misaligned notched specimen design, delayed TRIP kinetics. The change in mean stress imposed by the notches was deemed to have played an active role in TRIP exhaustion during the material’s tensile deformation. A unique electropolishing micro-speckle patterning technique was applied to show that the amount of strain that can be accommodated by the steel’s the polygonal ferrite-tempered martensitic regions are considerably impacted by external modifications in stress-state. While damages studies using different such notched tensile geometries revealed that once a critical void size is reached in this med-Mn steel, coalescence proceeds at an increasing, exponential rate up to fracture. It continues to remain a challenge to quantify the effects microstrain partitioning, TRIP kinetics and damage evolution separately, opening new avenues for future experimental and modeling investigations. / Thesis / Candidate in Philosophy / A lot of research up to now has been invested in the automotive industry to create steels that are lightweight, strong and show improved crashworthiness. The means by which this has been achieved is with the use of innovative processing routes to manufacture and implement Advanced High Strength Steels (AHSSs) in a vehicle’s body-in-white. Nonetheless, the constant global pressure to reduce greenhouse gas emissions has eventually driven research to a third-generation class of ultrahigh strength, lightweight AHSSs. These steels retain the weight savings of their second-generation counterparts but are more cost-effective to manufacture and can be adapted to current industrial line capabilities. Considerable work has been done to enable the manufacturing of 3G steels, yet the steel characteristics which underpin fracture, thereby affecting the crashworthiness of these steels, continues to be weakly understood. As such, at a microscopic scale, this thesis uses three different promising 3G AHSSs candidates to evaluate the impact their unique steel characteristics has on the ability to resist damage evolution and fracture.