Session K05: Phase transitions and microstructural properties of alloys

Multiscale phase transitions of Cu and Fe in an additively manufactured Cu-Fe Alloy under high-Pressure.

A state of the art custom-built direct-metal deposition (DMD) based additive manufacturing (AM) system at the University of Michigan was used to manufacture 50Cu-50Fe alloy with tailored properties for use in high strain/deformation environments. Subsequently, we performed the first high-pressure compression experiments to investigate the structural stability and deformation of this material. Our work shows that the alpha (BCC) phase of Fe is stable up to ~16 GPa before reversibly transforming to HCP, which is at least a few GPa higher than pure bulk Fe material. Furthermore, we observed evidence of the transition of Cu nano-precipitates in Fe from a well-known FCC structure to a metastable BCC phase, which has only been predicted via density functional calculations. Finally, the metastable FCC Fe nano-precipitates within the Cu grains show a modulated nano-twinned structure induced by high-pressure deformation. The results from this work demonstrate the opportunity in AM application for tailored functional materials and extreme stress/deformation applications.   

Strategies for Manipulating Shock Waves using Metallurgical Metamaterials

Recent research has shown the potential for exploiting volume reduction during phase transformations to manipulate shock waves. Volume-changing phase transformation in metals can be described as metastable crushing at the atomic scale, which can reverse transform upon unloading, is stable for finite-amplitude shocks, and occurs on the time scale of 10s of nanoseconds. Modifying the composition alters the transformation pressure. By constructing a sample with a gradient in composition, shock wave behavior can be controlled. However, most equation of state and transformation behavior has focused on pure metals; there is insufficient data to describe the behavior of alloyed metals, even for simple binary systems. In this work we use X-ray diffraction data collected during diamond anvil cell experiments on Fe-xMn alloys to determine the parameters for a phase transformation model as a function of Mn content. Through a finite element framework, we explored how Fe-xMn laminates with spatially varying composition can be used to achieve shock wave manipulation in 1-d shock waves.   

Metallurgical Metamaterials: A strategy for manipulating shock waves using metallurgy

Existing methods to design metamaterials for dynamic loading involve tuning the crushing behavior so that sufficient energy can be absorbed over extremely short time scales. While this approach is effective for manipulating and dispersing low to moderate amplitude loads, when lattice-based metamaterials are subjected to large-amplitude shocks they are destroyed and lose many of their desirable traits. In this work we utilize volume-changing phase transformations to achieve metastable crushing at the atomic scale, which is reversible, stable to finite-amplitude shocks, occurs on the time scale of 10s of nanoseconds, and can be fabricated using conventional techniques. The phase transformation pressure is readily tuned by modifying the local composition and can be described using the standard solution model. As an example, finite element simulations and shock loading experiments are used to show how to design a metallurgical metamaterial from gradient Fe-xMn laminates that can disrupt, disperse, or amplify shock waves whose magnitude ranges from 5-15 GPa. While Fe-based alloys are promising, an assessment is made of the relative potential of most base elements across the periodic table. Development of this new class of metallurgical metamaterials for high-pressure, transient phenomena appears to address several shortcomings of existing metamaterial design approaches for extreme loading scenarios, but is still in its nascent stages of development.   

Thermal Equation of State Measurements on High Entropy Alloys using Large-Volume Press

Eutectic high entropy alloys (EHEAs) have been studied for almost a decade now due to their ultrahigh yield strength and high tensile ductility. There have been limited studies on the behavior of EHEAs under extreme conditions of pressures and temperatures. We have performed high pressure studies on dual-phase nanolamellar structure of EHEA AlCoCrFeNi_2.1 synthesized through laser powder-bed fusion (L-PBF). Comparing x-ray diffraction studies on L-PBF printed cylindrical sample up to 5.5GPa (large-volume Paris-Edinburgh cell) to similar studies on L-PBF printed foil in a diamond anvil cell to pressure of 42 GPa  We show the alternating face-centered cubic (FCC) and  (BCC) body-centered cubic nanolamellae structure transform into single phase FCC completing the transformation at 21 ± 3 GPa. The equation of state measured for the FCC phase between both the large-volume press and DAC experiments are in good agreement.

    

Influence of Fatigue Damage on the Hugoniot Elastic Limit, Phase Transformation, and Spall Behavior of alpha-Iron and 4340 Steel

Strengthening defects such as particle inclusions and dislocation clusters have a significant influence on the Hugoniot elastic limit (HEL) and spall response of materials. The influence of such defects from material processing routes has been thoroughly, though not completely, studied. However, high cycle fatigue loading which can impart extensive and detrimental damage, has received much less attention to date. Here, alpha-Iron (Fe) and 4340 steel samples fatigued to different stages of the anticipated fatigue life are subjected to shock loading from which the HEL and spall strength are extracted. For most experiments conducted in this study, the applied shock stresses remain below the phase transformation stress with the exception of the 4340 steel, where an additional experiment for each fatigue condition probes the influence of fatigue damage when the phase transformation stress is exceeded. Results are discussed with respect to the influence of the degree of fatigue damage on the dynamic mechanical response as well as the influence of alloying on the post-fatigued shock response within Fe-based alloys.   

Effects of impurities and stacking fault energy on shock-induced phase changes in copper alloys

Recent publications suggest a relationship between stacking fault (SF) generation and the face-centered cubic (FCC) to body-centered cubic (BCC) phase transformation in shock-compressed noble metals. Metal alloying is well known to affect many material properties including stacking fault energy (SFE) and phase stability regions. To examine the effect of impurities on stacking fault generation and high-pressure phase boundaries, we performed laser-shock in-situ X-ray diffraction (XRD) experiments on three copper alloys with SFEs between 10 and 90 mJ/m². For the different Cu alloys (≥70% Cu), we found FCC to BCC transition stresses between ~130 and ~280 GPa, highlighting the large effect impurities can have on transition stress. Due to the observed linear relationship between SFE and FCC to BCC transition stress, for small impurity contents, we expect minimal phase boundary changes.