Quenching and partitioning (Q&P) &P) processing is a widely accepted heat treatment methodology for creating high strength steels consisting of ferrite, martensite, and austenite, while maintaining relatively low manufacturing costs. Though the research on effects of prior microstructure is limited, an understanding of the heat treatment response of different starting microstructures is critical to processing and creating steels with complex microstructures that contain retained austenite and may afford opportunities to further optimize properties. This study investigates the influence of starting microstructure (ferrite/pearlite versus martensite) and prior levels of cold work (38 verses 58 %) on the microstructural development and mechanical properties of a 0.2 C-2.0 Mn-1.5 Si (wt.%) steel exposed to Q&P &P processing. Samples with a starting martensitic microstructure resulted in higher retained austenite fractions and a more homogeneous microstructure after Q&P &P processing compared to a starting microstructure of ferrite-pearlite. Starting martensitic microstructures also displayed higher work hardening rates and higher uniform elongations. Larger cold reductions saw accelerated dissolution kinetics and austenite formation during intercritical annealing, resulting in more similar final microstructures from the ferrite-pearlite and martensitic starting microstructures. The results presented here indicate that varying prior processing can be a route to manipulate and control austenite stability in a Q&P &P processed steel.

The dynamic spall properties of an additively manufactured (AM), CoCrFeMnNi high-entropy alloy (HEA) were investigated as a function of processing defects. Laser Powder Bed Fusion (LPBF) was used to additively manufacture HEA samples that were subsequently subjected to shock loading through plate impact experiments. Seven different combinations of laser power and scan speeds were explored, ranging from 180 to 280 W and 925-1350 mm/s, respectively. All samples were considered within the bounds of lack-of-fusion and keyholing defects based on initial experiments, but exhibited varying degrees of solidification cracking and microstructural changes. Grain size and grain aspect ratio were found to modestly decrease with faster scan speeds and lower laser powers, while cracking associated with the AM process increased with faster scan speeds and higher laser powers. Spall strength and spall damage did not systematically trend with microstructural characteristics such as grain size, but did exhibit a relationship with pre-existing crack density. Spall properties were found to generally degrade with increasing crack density. Evidence of defect compaction was not observed in soft recovered spall samples, thus pre-existing cracks were proposed to degrade spall properties by acting as stress concentrators and preferred damage nucleation sites during tensile impact loading. Here, the presence of manufacturing-related defects, such as cracks, dominated the dynamic response; however, in the absence of these defects, microstructural changes like grain size and texture would be expected to control dynamic behavior.

dislocation density coupled with a low dislocation nucleation rate. The spall strength of ferrite-pearlite is consistently lower than the spheroidized microstructure, attributed to elongated cementite that is more susceptible to cracking than more spherical cementite precipitates. Despite a high density of boundaries, martensite exhibits the highest spall strength. A large percentage of the boundaries within the martensite microstructure are found to be low energy (i.e. Σ3 or low angle), and are thus less susceptible to spall damage. Overall, the high spall strength of martensite is likely linked to traditional strengthening mechanisms that limit dislocation motion.

The effect of microstructure on the shock response of 1045 steel is investigated via plate impact experiments and postmortem characterization. Three unique microstructures are explored: ferrite-pearlite, martensite, and ferrite with spheroidal cementite (i.e. spheroidized). Two spall recovery experiments, at approximate peak pressures of 3.2 and 3.5 GPa, are conducted to assess the Hugoniot elastic limit (HEL), spall strength, and damage morphology of the various microstructures. The ferrite-pearlite and martensite microstructures exhibit contin-uous yielding at both quasi-static and dynamic rates, while the spheroidized condition displays discontinuous yielding. Discontinuous yielding of the spheroidized microstructure is attributed to a combined low initial dislocation density coupled with a low dislocation nucleation rate. The spall strength of ferrite-pearlite is consistently lower than the spheroidized microstructure, attributed to elongated cementite that is more sus-ceptible to cracking than more spherical cementite precipitates. Despite a high density of boundaries, martensite exhibits the highest spall strength. A large percentage of the boundaries within the martensite microstructure are found to be low energy (i.e. sigma 3 or low angle), and are thus less susceptible to spall damage. Overall, the high spall strength of martensite is likely linked to traditional strengthening mechanisms that limit dislocation motion.

Inert simulant materials, or “mocks”, are often used as surrogates for plastic-bonded explosives (PBX) in non-detonative tests in order to mitigate hazards. Mocks should reproduce as many non-detonative properties of the explosive as possible, including structural behavior in a variety of thermal and mechanical environments. Recently, the molecular crystal idoxuridine (IDOX) has been identified as an ideal mock for the main component in the explosive polymer-matrix composite PBX 9501, and has performed favorably under quasistatic loading conditions. Here, in order to assess robustness over a range of mechanical environments, plastic-bonded IDOX was compression tested from 0.001/s to 2000/s strain rates and compared to PBX 9501 historical data. Plastic-bonded IDOX showed good agreement to PBX 9501 across these strain rates, justifying continued development and production as a mock.

The goal of this work is to investigate the effect of varying phase fractions on the overall spall strength and damage behavior of a material. Specifically, two plain carbon, ferrite–pearlite steels (1045 and A283) were subjected to spall recovery experiments to investigate the effect of pearlite fraction on spall strength and total damage. The A283 (20% pearlite) alloy exhibited a higher Hugoniot elastic limit and spall strength compared with 1045 (60% pearlite). Discontinuous and continuous yielding behaviors were observed at quasi-static and dynamic rates for A283 and 1045, respectively. The yielding behavior was connected to pearlite fraction and the prevalence of dislocation-emitting, ferrite/cementite interfaces. Postmortem characterization revealed cementite lamellae cracking within pearlite of 1045, suggesting that pearlite reduces spall strength by providing low-energy damage nucleation sites. The rate of damage growth and coalescence was similar between the two alloys; however, 1045 exhibited more continuous cracks than A283, which exhibited a greater prevalence of discrete voids.

The influence of rapid tempering on cementite precipitation in 4340 steel was investigated within the tempered martensite embrittlement (TME) tempering regime. Cementite amount, size, and morphology, and matrix dislocation density were explored for rapid (1 s) and conventional (3600 s) tempering conditions with scanning electron microscopy (SEM) and x-ray diffraction (XRD), respectively, and compared at an equivalent degree of tempering (i.e., hardness). Rapid tempering resulted in an average refinement of cementite diameter by approximately 2–3 nm, and did not significantly alter cementite morphology, as determined by SEM, or phase fraction, as determined by Mössbauer spectroscopy, compared to conventional tempering. Previous studies have shown an improvement in toughness performance associated with the rapid tempering conditions explored here. Given the minimal carbide refinement observed in the present work, carbide size is not thought to be the primary microstructural factor in improving toughness properties of rapidly tempered conditions. Rather, impact toughness is likely influenced by differences in retained austenite content, as proposed in previous studies. The matrix dislocation content was similar between conventional and rapid tempering conditions at a given hardness, suggesting that the cementite refinement associated with rapid tempering was not promoted by the suppression of dislocation recovery.

Interfacial friction is a key aspect to understanding and modelling dynamic processes in which materials interact. However, friction is a complex phenomenon that depends on a multitude of factors, including sliding velocity. Understanding how friction behavior changes as a function of sliding rate is thus crucial for accurately simulating dynamic processes. Recent literature has shown that the split-Hopkinson pressure bar can be adapted for friction measurements associated with high sliding rates. The present work introduces an insert designed to be transferrable between a quasi-static load frame and a compression split-Hopkinson bar, enabling friction measurements across a wide range of sliding velocities (10 -4 – 20 m/s). Here, the split-Hopkinson pressure bar setup is modelled using a multiphysics research code (FLAG), developed at Los Alamos National Laboratory (LANL), to identify and reduce potential issues in the configuration prior to experimental implementation.

Tempered martensite embrittlement (TME) is investigated in two medium carbon, high strength steels, 4340 (low silicon) and 300-M (high silicon), via rapid (1, 10, or 100 s) and conventional (3600 s) tempering. Rapid tempering of 4340 diminishes the depth of the TME toughness trough, where improvements in impact toughness correspond to the suppression of retained austenite decomposition. In 300-M, retained austenite decomposition is suppressed to an even greater extent by rapid tempering. While toughness improves overall after rapid tempering, TME severity remains consistent in 300-M across the tempering conditions examined. Through interrupted tensile tests, it was found that the 300-M conditions that exhibit TME are associated with mechanically unstable retained austenite. Unstable retained austenite is shown to mechanically transform early in the deformation process, presumably resulting in fresh martensite adjacent to interlath cementite that ultimately contributes to TME. The present results emphasize the role of both the thermal decomposition and mechanical transformation of retained austenite in the manifestation of TME.

Understanding the friction behavior between two sliding bodies can inform the design of machines, processing of materials, and simulation of dynamic processes. Kinetic friction is a complex phenomenon that depends on a multitude of factors such as sliding velocity, normal force, contact area, surface roughness, material properties, lubrication conditions, and thermal effects. This literature review covers the major known effects of sliding velocity, normal load, and surface roughness on the measured kinetic friction coefficient in the context of microscopic friction phenomena. Classic macroscale friction models are reviewed to illustrate approaches for simulating friction behavior. Prominent experimental friction setups within the literature are discussed with respect to achievable velocity and pressure regimes. The background information gathered here will be used to inform experimental procedures and modeling strategies of the exploratory research (ER) project titled “Measurement of Dynamic Friction via Kolsky Bar” (20200418ER).

Tempering reactions are critical to microstructure and property control in martensitic steels. Here, retained austenite decomposition and cementite precipitation are monitored using Mössbauer spectroscopy in 4340 and 300-M steel under conventional and rapid tempering conditions. Tempering times are compared at a constant tempered hardness by increasing tempering temperatures associated with short time conditions to achieve equivalent matrix softening to that of longer tempering times. Time-temperature combinations that provide equivalent tempered hardness generated microstructures with similar dislocation densities and cementite precipitation fractions; these mechanisms are controlled by self-diffusion. However, systematic differences in retained austenite content were observed at a given degree of softening, where shorter tempering times exhibited higher levels of retained austenite compared to more conventional conditions. At low temperatures, the differences in retained austenite preservation between explored time-temperature conditions are attributed to corresponding differences in carbon diffusion distance (in austenite), the controlling diffusional process of retained austenite decomposition. At higher temperatures, retained austenite decomposition exhibits C-curve kinetic behavior in 4340. Thus, reduced thermodynamic driving force for cementite and ferrite formation at higher temperature is believed to play a role in restricting retained austenite decomposition within some short-time, high temperature tempering regimes. The addition of silicon pushes cementite precipitation and retained austenite decomposition to higher temperatures, although retained austenite decomposition is suppressed to a greater extent than cementite precipitation. Potential is illustrated for coupling rapid tempering with silicon alloying to produce appreciably tempered martensite (~490 HV) with relatively less retained austenite decomposition compared to conventional tempering conditions.

Rapid tempering involves shorter times and faster heating rates compared to conventional tempering. Utilizing rapid tempering via industrial processes, such as induction heating, not only offers the opportunity for improved mechanical properties, but also for reduced processing times and energy costs. The current study demonstrates an improvement in toughness with rapid tempering by examining impact toughness at a constant tempering parameter for short-time (1 s) and conventional (3600 s) tempering treatments of 4340 steel. Fracture behavior is found to complement the observed toughness behavior, where higher impact energy conditions are associated with a greater percentage of ductile fracture. The role of the Hollomon–Jaffe tempering parameter, as well as the use of hardness as a metric to characterize degree of tempering, is discussed in light of the results comparing rapid and conventional tempering conditions. This study indicates promising mechanical properties associated with short-time tempering, especially within the tempered martensite embrittlement (TME) regime, and exposes challenges associated with predicting performance strictly based on hardness when rapid tempering is utilized.

While tempering reactions under furnace conditions have been extensively studied, short-time tempering at higher temperatures, as might apply to induction tempering, has not been thoroughly explored. The mechanical behavior and phase development of short-time (1, 10, and 100 seconds) and conventionally (3600 seconds) tempered 4340 steel are compared at an equivalent degree of tempering defined by the Hollomon–Jaffe tempering parameter. The tempering parameter accurately predicts hardness values across short-time and conventional tempering conditions, but is less able to describe phase evolution associated with short-time tempering. Room temperature Charpy impact toughness and ductile-to-brittle transition temperature systematically improve with shorter tempering times at an equivalent tempering parameter. In particular, rapid tempering significantly increases toughness within the tempered martensite embrittlement regime. Relative to conventional tempering, shorter tempering times exhibit higher retained austenite content for a given tempering parameter, although no systematic or significant difference in cementite or transition carbide content is observed between time conditions. The retained austenite decomposition behavior indicates that the relationship between the classical tempering stages is altered at short tempering times, where increased overlap and/or “re-ordering” of stage II and III tempering are observed.