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Grain structure control is essential in metal additive manufacturing. It is used to avoid anisotropic mechanical properties, hot cracking and to increase the strength via the Hall-Petch effect. Here we demonstrate the use of grain refining particles for grain control in ferritic alloys, specifically Fe-Ti model alloys. We introduce the particles in three different ways, namely (i) by using oxygen-rich raw powders, (ii) by reaction of the molten material with the process gas atmosphere and (iii) by direct addition of the refining ceramic phase in powder form. Samples are produced by Laser Powder Bed Fusion with various concentrations of Ti and under Ar and N2 atmosphere. The resulting microstructures are analyzed using optical and electron microscopy. We demonstrate a transition from a microstructure containing columnar grains >100 μm in length to a strongly grain refined microstructure with equiaxed grains of approx. 1 μm in size. The refining sub-micron-sized particles in all cases are cubic Ti(O,N). We discuss the findings in the light of thermodynamic calculations as well as established grain refinement models.

In the development of high-strength aluminium alloys tailored specifically to additive manufacturing (AM), L12-Al3X-forming elements have been proven to be particularly effective alloying additions, reducing the susceptibility of aluminium alloys to solidification cracking by grain refinement and increasing the strength by forming secondary nanoscale precipitates. In the present work, we employ laser powder bed fusion ( L -PBF) to examine the feasibility of using Ti as the main strengthening, L12-forming element for the design of a well-processable, precipitation-strengthened model alloy. Al-1.76Ti and Al-2.51Ti (wt%) alloys with and without ternary additions of Ni and Si are fabricated during processing from powder mixtures. Crack-free microstructures are produced, which display alternating fine- and coarse-grained regions. During heat treatment, one of the investigated alloys, Al-2.51Ti-0.7Si, exhibits an ageing response due to the formation of nanoscale, metastable L12-(Al,Si)3Ti precipitates, reaching a peak hardness of 97 HV. This demonstrates that L -PBF-processed Ti-containing aluminium alloys can exhibit a precipitation hardening response similar to alloys containing Sc or Zr. In the other Al-Ti-based materials tested, no significant hardness increase was found. This discrepancy is ascribed to the effects Si exerts on the precipitation process. First, the addition of Si increases the thermodynamic driving force for homogeneous nucleation of L12-Al3Ti precipitates. Secondly, Si-Ti co-clustering is observed by atom probe tomography, which is expected to enhance the otherwise sluggish diffusion of Ti. The Al-2.51Ti-0.7Si alloy developed in this work can in the future be further strengthened by adding additional alloying elements and by optimising the heat treatment.

In this study, we investigate the fundamentals of deformation-driven cementite decomposition during rolling contact fatigue in a binary Fe-0.74C (wt.%) steel with pearlitic microstructure. To reveal the involved nano-scale mechanisms, we apply transmission electron microscopy and atom probe tomography, as well as the combination of both techniques on the same probe volume for correlative structural and chemical analysis. After ~32,500 individual ball contacts under a nominal Hertzian contact stress of ~1,250 MPa, cementite lamellae are consistently depleted in carbon (C) down to ~20 at.%, which is a reduction of 20% in comparison to the original stoichiometric composition of 25 at.% before deformation. By electron diffraction on atom probe tip volumes, we show that this under-stoichiometric cementite still maintains its crystal structure. The potential effect of temperature increase during rolling contact fatigue on cementite decomposition is addressed by carrying out annealing experiments at 150°C and 250°C for 30 minutes after rolling contact fatigue. Our results show that slip transmission across cementite lamellae is primarily responsible for the observed C-depletion in cementite. We quantitatively discuss slip transmission from ferrite to cementite using slip trace analysis and correlate the amount of slip activity with the observed C-depletion. In this way, we explain the formation of under-stoichiometric cementite by combined slip transmission and solute drag by dislocations, where C is transported out of cementite by dislocation gliding through cementite lamellae. Only afterward, cementite decomposition in terms of phase dissolution is suggested to occur by the dislocation-shuffle mechanism.

Premature failure of bearings during rolling contact fatigue is often associated with the formation of white etching cracks (WECs). Crack surface rubbing of WECs transforms the original bainitic/martensitic microstructure into white etching areas (WEAs), comprised of nanocrystalline ferrite. The grain size and carbon content vary within the WEA. Here, we show by atom probe tomography and scanning electron microscopy, that there is an inversely proportional relationship between grain size and carbon content in WEAs formed in 100Cr6 bearings that failed by WECs in service. We explain this phenomenon by the reduction of grain boundary energy through carbon segregation. Depending on the carbon content, this reduces the driving force for recrystallization and grain coarsening, thereby stabilizing the nanocrystalline microstructure. No such effect is observed for the substitutional element chromium. The smallest grain size (

Since white etching crack (WEC) phenomena primarily occur in high carbon steels, we elucidate the role of carbon in this failure mechanism in bearings. The nano-crystalline ferritic regions that make up the white etching area (WEA) are formed by crack surface rubbing leading to complete decomposition of the initial microstructure by severe plastic deformation. In order to analyze local carbon compositions on µm-nm length scales, we employ electron probe microanalysis, transmission electron microscopy and atom probe tomography. We focus on a 100Cr6 wind turbine gearbox bearing which failed in service due to extensive formation of WEC networks below the raceway surface and subsequent spalling. Our results show a significantly lower carbon content in the WEA as compared to the nominal alloy composition. At the same time, we find carbon deposits with a carbon content of > 85 at%, which are heterogeneously distributed across WEAs. We explain this observation by assuming segregation of excess carbon from the WEA to the open crack surfaces during crack surface rubbing. Further, the presence of a “lubricating” carbon film at the WEC surfaces might explain the accelerated failure by WECs as compared to classical rolling contact fatigue.

Twinning induced plasticity (TWIP) and phase transformation induced plasticity (TRIP) are two effective mechanisms for achieving good combination of strength and ductility in metallic materials, such as steels and high entropy alloys (HEAs). A further enhancement of the strength-ductility combination can be achieved by grain refinement owing to the grain boundary strengthening effect. In this work, we investigated the microstructure and mechanical properties of a TRIP HEA (Cr20Mn24Fe30Co20Ni6, at.%) and a TWIP HEA (Cr20Mn15Fe34Co20Ni11, at.%) after grain refinement by cold-rolling and short-time annealing. The results show a simultaneous increase of strength and ductility for the Cr20Mn15Fe34Co20Ni11 alloy after grain refinement, maintaining the TWIP effect. However, the Cr20Mn24Fe30Co20Ni6 alloy displays a significant enhancement of strength accompanied by a loss of ductility. Detailed microstructure analysis shows that nano-sized σ precipitates are formed in the Cr20Mn24Fe30Co20Ni6 alloy during the grain refinement process. While the nano-precipitates contributes significantly to the strength, they also lead to a spread of micro-voids and early fracture upon deformation. Our work provides some insights in balancing different strengthening mechanisms in alloy design and microstructural manipulation.

White etching cracks (WECs) and the associated white etching areas (WEAs) are responsible for failure of widely spread engineering applications such as bearings and railways. Although the phenomenon is known for more than 100 years, the underlying mechanisms are still a matter of debate. In this work, we thoroughly investigate a 100Cr6 wind turbine gearbox bearing after failure in service operation. Based on our findings from detailed microstructure characterization on multiple length scales we formulate a new consistent explanation for the formation of WEAs during rolling contact fatigue. We propose a mechanism of moving WECs - not only in terms of conventional crack propagation but also as a movement of the crack normal to its plane. During cyclic loading the crack continuously changes its position and leaves behind a severely plastically deformed area consisting of ferritic nano-grains, i.e. the WEAs. The atomic-scale delocalization of the crack plane in a single loading cycle adds up to micron-sized WEAs during repetitive loading/unloading. After the initial formation of a fatigue crack around inclusions, crack face rubbing occurs during compressive loading cycles. This leads to the formation of WEA by local severe plastic deformation. It also leads to partial cohesion of the abutting crack faces and material transport between them. As a result, the WEC opens at a slightly shifted position with respect to its former location during unloading.