Intermixing of Fe and Cu on the atomic scale by high-pressure torsion as revealed by DC- and AC-SQUID susceptometry and atom probe tomography

The capability of high-pressure torsion on the preparation of supersaturated solid solutions, consisting of Cu-14Fe (wt.%), is studied. From microstructural investigations a steady state is obtained with nanocrystalline grains. The as-deformed state is analyzed with atom probe tomography, revealing an enhanced solubility and the presence of Fe-rich particles. The DC-hysteresis loop shows suppressed long range interactions in the as-deformed state and evolves towards a typical bulk hysteresis loop when annealed at 500{\deg}C. AC-susceptometry measurements of the as-deformed state reveal the presence of a superparamagnetic blocking peak, as well as a magnetic frustrated phase, whereas the transition of the latter follows the Almeida-Thouless line, coinciding with the microstructural investigations by atom probe tomography. AC-susceptometry shows that the frustrated state vanishes for annealing at 250{\deg}C

Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels

This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation

Precipitation reactions during the intrinsic heat treatment of laser additive manufacturing

Laser additive manufacturing (LAM) allows to produce complex and highly customized metallic parts from a computer aided design file (CAD) by melting metallic powder with a focused laser beam. The inherent geometrical design freedom this technique offers enables tremendous weight savings and parts with a complexity that would be impossible to achieve by conventional manufacturing techniques. However, many alloys face problems of either poor processability in LAM or insufficient strength. Even those alloys that are well processable, often do not exploit the full potential of LAM processes as they were typically designed and optimized for conventional processing routes. This work aims at designing new alloys custom-tailored to LAM processes making use of some unique features of these processes. For example, the cyclic re-heating occurring during the process, the so called intrinsic heat treatment (IHT), is used to trigger precipitation reactions already during the process avoiding an aging heat treatment for precipitation strengthened materials. Furthermore, the potential of triggering phase transformations in conventional alloys used in LAM is evaluated and LAM-produced and conventionally-produced parts are compared. The complex microstructures of all samples are characterized at different length scales ranging from cm to nm by light optical microscopy (LOM), scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and atom probe tomography (APT). After showing that during the IHT of a directed energy deposition (DED) processing of a conventional 18Ni-300 Maraging steel slight clustering occurs, simple ternary Fe-Ni-Al and Fe-Ni-Ti steels are developed that respond very well to the in-situ strengthening approach using the IHT. Rapid alloy prototyping approaches using compositionally graded samples are used to efficiently screen a large variation in compositions and find the optimal ones that show the desired microstructure and a strong response to the IHT. In an Fe17Ni10Al (at%) steel, exceptionally high number densities of 1025 NiAl precipitates per m3 are achieved in the as-DED-produced state that lead to a significant increase in hardness and strength. In an Fe18Ni6Ti (at%) steel it is demonstrated how the sequence of two phase transformations (martensite transformation and precipitation) is necessary to obtain precipitation hardening by IHT. Dense networks of Ni3Ti precipitates are triggered in the as-produced state. Furthermore, a detailed understanding of the thermal history of DED-produced materials opens pathways to locally control the microstructure. A combined alloy and process design approach for Al-Sc-Zr alloys allows to produce parts in-situ strengthened by a high number density of thermally stable Al3(Sc,Zr) precipitates. Coarsening of Al3(Sc,Zr) precipitates that takes place already during the IHT in a commercial Al-Sc-Zr alloy can be stopped via the control of solidification conditions together with addition of Zr to the alloy. This allows for enough Zr in the Al matrix to form a Zr-rich shell around Al3Sc precipitates upon IHT and to stop coarsening. The approach of in-situ strengthening via the IHT should be applicable to a wide range of precipitation hardening alloys as well as to further LAM processes such as laser powder bed fusion (LPBF). The IHT cannot only be used for in-situ strengthening precipitation hardening alloys but also be used to trigger phase transformations in general. In a commercial Ti-6Al-4V alloy, the influence of the IHT on the decomposition of the brittle martensitic microstructure is investigated. This thesis shows the tremendous potential of alloy design targeted at laser additive manufacturing (LAM) and the effectiveness of the intrinsic heat treatment (IHT) to trigger phase transformations in-situ already during the LAM process

Massive nanoprecipitation in an Fe-19Ni- x Al maraging steel triggered by the intrinsic heat treatment during laser metal deposition

Due to the layer-by-layer build-up of additively manufactured parts, the deposited material experiences a cyclic re-heating in the form of a sequence of temperature pulses. In the current work, this “intrinsic heat treatment (IHT)” was exploited to induce the precipitation of NiAl nanoparticles in an Fe-19Ni-xAl (at%) model maraging steel, a system known for rapid clustering. We used Laser Metal Deposition (LMD) to synthesize compositionally graded specimens. This allowed for the efficient screening of effects associated with varying Al contents ranging from 0 to 25 at% and for identifying promising concentrations for further studies. Based on the existence of the desired martensitic matrix, an upper bound for the Al concentration of 15 at% was defined. Owing to the presence of NiAl precipitates as observed by Atom Probe Tomography (APT), a lower bound of 3–5 at% Al was established. Within this concentration window, increasing the Al concentration gave rise to an increase in hardness by 225 HV due to an exceptionally high number density of 1025 NiAl precipitates per m3, as measured by APT. This work demonstrates the possibility of exploiting the IHT of the LMD process for the production of samples that are precipitation strengthened during the additive manufacturing process without need for any further heat treatment.Peer ReviewedPostprint (author's final draft

Insight into grain boundaries with reduced liquid metal embrittlement susceptibility in a boron-added 3rd generation advanced high strength steel

The addition of boron to the advanced high-strength steels (AHSS) enhances grain boundary cohesion and can play a significant role in intergranular degradation phenomena such as Zn-assisted liquid metal embrittlement (LME). The objective of this study is to reveal a thorough understanding of the effect of B on LME behaviour, where the welding test results indicated substantial improvement even at relatively low B concentration. Here, we adopted the strategy of identifying LME-sensitive grain boundaries, followed by characterizing grain boundary chemistry. EBSD measurements and prior austenite grain (PAG) reconstruction provided strong evidence of intergranular LME crack propagation via PAGBs. Further transmission electron microscopy and energy dispersive X-ray spectroscopy investigations demonstrated an asymmetric Mn profile along the PAGB in B-free steel. Therefore, we propose that B addition and consequently, lower grain boundary energy appears to prevent Mn segregation. Based on our observation, we conclude that B seems to have the similar desired effect on Zn diffusion along the PAGBs and eventually mitigates Zn-assisted LME

Designing an Fe-Ni-Ti maraging steel tailor-made for laser additive manufacturing

Laser additive manufacturing (LAM) offers high flexibility in the production of customized and geometrically complex parts. The technique receives great interest from industry and academia but faces substantial challenges regarding processability and insufficient mechanical properties of LAM-produced material. One reason is that currently mainly conventional alloys are being used in LAM, which were developed for different processes such as casting. Since these alloys are not optimized for the specific process conditions encountered in LAM such as fast cooling and cyclic re-heating, they cannot be expected to perform ideally in such processes regarding processability and resulting mechanical properties. Here we present the development of a new, simple ternary Fe-Ni-Ti maraging-type alloy tailor-made for LAM. We used compositionally graded samples to screen Ti compositions from 0 to 21 at. % and efficiently identify promising microstructures and mechanical properties. Under LAM solidification conditions the desired mainly martensitic microstructure needed for a maraging steel formed at Ti compositions ranging from 0 to 7 at. %. Within this composition range, the intended microstructure is formed and additionally some unique process conditions of LAM such as cyclic re-heating can be exploited. Specifically, in-situ phase transformations can be controlled during LAM, via the thermal history. At higher Ti compositions two different eutectic microstructures with different primary phases were found that show a high hardness of up to 700 HV

Exploiting Metastability and Lattice Defects for Microstructural Engineering of Selective Laser Melted Ti-6Al-4V

Introduction Additive manufacturing (AM) is a technology of enormous potential, yet also of high complexity. Understanding and exploiting AM requires very interdisciplinary research efforts since material, processing, AM-design and engineering, build strategies, post-treatments and surface finishing are all interdependent. Particularly for class-one components in aerospace applications, achieving high material quality and performance in a robust AM process such as Selective Laser Melting (SLM) is a major hurdle. For the development of a manufacturing chain for a Ti-6Al-4V rocket engine turbo-pump impeller, different SLM processing and post-heat-treatment strategies for Ti-6Al-4V including intensified intrinsic heat treatment and in situ high-temperature build space heating to obtain stabilized -microstructures with fine -particles and films have been studied. Moreover, the transfer from coupon level to a complex part will be discussed. Methods In this study a SLM Solutions 280HL machine with a custom high-temperature build space heating was used. Results Heat treatments act to stabilize the initial as-built microstructures by decomposition of the acicular martensite in conjunction with pronounced element partitioning. New -phase nucleates at prior lattice defects in the as-built microstructures, leading to favorable materials properties. The interplay between SLM process parameters and the part geometry was studied using High Energy X-ray Diffraction in a section of an impeller manufactured with the optimized SLM bulk parameters. A mapping of the phase distribution shows that the local thermal history differs strongly in filigree and more massive sections and leads to substantial differences in the obtained microstructures and porosities compared to coupon specimen. Conclusions The process parameters and heat treatments were successfully optimized with coupon specimen to achieve stabilized -microstructures allowing for high strength and ductility. When used for manufacturing actual parts, however, the local geometry plays a key role: Results from studies carried out with cubes or cylinders can differ substantially from those in complex parts despite identical SLM parameters