Suppressing damage in dual phase steel: Insights from micromechanics

Single crystalline ferrite and martensite islands were extracted from two different dual phase (DP) steel grades by focused ion beam milling (FIB) and tested by in situ pillar compression. Three slip plane families {110}, {112}, {123} in bcc ferrite are all observed to be active and their corresponding mean critical resolved shear stress (CRSS) of 3µm pillars are found to be nearly identical, i.e. 147 ± 6, 143 ± 9, 146 ± 4 MPa. Non Schmid contributions either due to tension-compression anisotropy or a size dependent breakdown of Schmid’s law plays a minor role in our case. Martensite pillars contain several interfaces which makes them deform isotropically and without distinct slip traces. The pillars exhibit a high mean compressive yield strength up to 2880±49 MPa and a low scatter of 188±17 MPa. By comparing two DP steels possessing identical ultimate tensile strength, the sample with softer ferrite phase and a harder martensite yields at a lower stress and shows a larger fracture elongation. The post mortem investigation of the macroscopic sample indicates that a larger mechanical heterogeneity between ferrite and martensite increases the amount of crack initiation sites but the improved local strain hardening capability imposed by a softer ferrite matrix suppresses catastrophic failure considerably. We thank Gerhard Dehm (MPIE) for discussions and the DFG for financial support via TRR188 project

The fracture toughness of martensite islands in dual-phase DP800 steel

In situ microcantilever bending tests were performed on martensite islands in a dual-phase (DP) steel to extract the fracture toughness of martensite at the microscale and to understand damage initiation during forming of DP steels. All microcantilevers were produced through FIB milling. The martensite islands do not exhibit linear elastic brittle fracture; instead, significant ductile tearing is observed. The conditional fracture initiation toughness extracted by definition and by Pippan’s transfer criterion is K$_{i}$ = 6.5 ± 0.4 MPa m$^{1/2}$ and K_{i,2%} = 10.1 ± 0.3 MPa m$^{1/2}$, respectively. The obtained value is well-represented by the strength-toughness trend of other ferritic steel grades. Considering the yield stress of the same martensite island, we found that crack initiation can occur only in very large martensite islands or in a banded or agglomerated martensite structure

The impact of grain boundary character on the size dependence of Bi- crystals

The deformation behavior of metallic single crystals is size dependent, as shown by several studies during the last decade [1]. Nevertheless, real structures exhibit different interfaces like grain, twin or phase boundaries. Due to the possibly higher stresses at the micron scale, the poor availability of dislocation sources and the importance of diffusion in small dimensions the mechanical behavior of samples containing interfaces can considerable differ from bulk materials. Within this study we will show the size scaling behavior of general high angle grain boundaries in copper. The first boundary presented is believed to show extensive dislocation slip transmission at bulk dimensions. The second example acts as perfect obstacle for dislocation slip transfer. In the talk results from in situ scanning electron microscopy (SEM) and in situ µLaue diffraction will be shown. While the SEM data is used to proof slip transmission, µLaue is probing the occurrence of dislocation pile-ups at the grain boundary. The results show that at low plastic strains the size scaling behavior of single and bi-crystalline samples is identical in cases where the grain size is assumed as the critical length scale [2]. It can therefore be concluded that the initial number and size of dislocation sources is dominating not only the deformation behavior of single crystalline pillars, but also for bi-crystals (at low plastic strains) (see Fig. 1a). Thus, the character of the boundary does not play any role for the mechanical properties at the onset of yield! Please click Additional Files below to see the full abstract

On microstructural constraints for slip transfer in nanotwinned silver

Micromechanics of bicrystals with selected grain boundary (GB) types is an effective method of understanding dislocation – GB interactions. Micropillar compression tests on bicrystals containing a coherent twin boundary (CTB) [1,2], show that CTBs act as a weak obstacle for slip transfer unlike general high angle grain boundaries. Perfect slip transmission of dislocations through CTBs has been found to be similar to cross slip in fcc metals, and therefore denotes as cross-slip-like slip transfer [2,3]. Please click Additional Files below to see the full abstract

Development of protocols to quantify the twinning stress of a CoCrFeMnNi high entropy alloy108

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Challenges in the phase identification of steels using unsupervised clustering of nanoindentation data

Cluster analysis tools are used in data interpretation to separate information without the bias of a user. In the current study we investigate two techniques, the elbow method and K-means clustering to achieve a phase classification for a dual phase (DP) and a high strength low alloy (HSLA) steel by using hardness and reduced Young’s modulus from nanoindentation tests as input variables. For the DP steel the contrast in hardness of the two phases ferrite and martensite is high, while for the HSLA steel the hardness contrast between ferrite and bainite is small, as seen from the corresponding load-displacement curves (Fig. 1). Please click Download on the upper right corner to see the full abstract

Quantitative insights into the dislocation source behavior of twin boundaries suggest a new dislocation source mechanism

Pop-in statistics from nanoindentation with spherical indenters are used to determine the stress required to activate dislocation sources in twin boundaries (TBs) in copper and its alloys. The TB source activation stress is smaller than that needed for bulk single crystals, irrespective of the indenter size, dislocation density and stacking fault energy. Because an array of pre-existing Frank partial dislocations is present at a TB, we propose that dislocation emission from the TB occurs by the Frank partials splitting into Shockley partials moving along the TB plane and perfect lattice dislocations, both of which are mobile. The proposed mechanism is supported by recent high resolution transmission electron microscopy images in deformed nanotwinned (NT) metals and may help to explain some of the superior properties of nanotwinned metals (e.g. high strength and good ductility), as well as the process of detwinning by the collective formation and motion of Shockley partial dislocations along TBs. Graphic abstract: [Figure not available: see fulltext.] © 2021, The Author(s)

Micropillar compression of hexagonal and cubic NbCo2 Laves phases

Transition metal-based Laves phases have high strength and good creep resistance which make them potential candidates for high-temperature applications. On the other hand, they exhibit pronounced brittleness at low temperature. Due to their brittleness, the difficulty of producing sufficiently large and flaw-less bulk samples for conventional mechanical testing limits the study of their mechanical properties. Therefore, today there is only very little knowledge on their mechanical behavior and their deformation mechanism. The existing literature indicates that the mechanical properties of transition metal-based Laves phases significantly depend on their composition. The underlying mechanism is not yet understood. Laves phases with AB2 stoichiometry may have three structure types, cubic C15 (MgCu2-type), hexagonal C14 (MgZn2-type) and hexagonal C36 (MgNi2-type). All three types of Laves phases exist as stable phases in the Co-Nb system and the C15-NbCo2 Laves phase has a large composition range of 26.0±0.5 − 35.3±0.3 at.% Nb. It makes the NbCo2 Laves phases a perfect candidate to study not only the deformation behavior of transition metal-based Laves phases but also the influence of composition and crystal structure on their strength. To circumvent the difficulties in preparing large flaw-less samples, we grew the NbCo2 Laves phases with diffusion couples and studied their mechanical properties by micropillar compression tests. After proper heat treatment, extended diffusion layers of the three NbCo2 Laves phases with coarse grains were obtained. As there are concentration gradients in the diffusion layers, micropillars with different compositions can be obtained by focused ion beam (FIB) milling at selected positions in the diffusion layers. Please click Additional Files below to see the full abstract

Insights into dislocation grain-boundary interaction by X-ray μLaue diffraction

The deformation behavior of metallic single crystals is size dependent, as shown by several studies during the last decade. Nevertheless, real structures exhibit different interfaces like grain, twin or phase boundaries. Due to the possibly higher stresses at the micron scale, the poor availability of dislocation sources and the importance of diffusion in small dimensions the mechanical behavior of samples containing interfaces can considerable differ from bulk material. In the talk we show the first in situ µLaue compression experiments on micron sized, bi-crystalline samples. Three different grain-boundary types will be presented and discussed (i) Large Angle grain Boundaries (LAGBs) acting as strong obstacle for dislocation slip transfer; (ii) LAGBs allowing for easy slip transfer and (iii) coherent sigma 3 twin-boundaries. The talk will focus on pile-up of dislocations, slip transfer mechanisms, storage of dislocations and dislocation networks at the LAGB

How to avoid FIB-milling artefacts in micro fracture? A new geometry for interface fracture

Focused ion beam (FIB) based small-scale fracture studies have been well established in recent years despite the ongoing discussion of possible artefacts caused by FIB milling. Stable crack growth geometries—where the FIB-prepared notch stably propagates through the sample—have the potential to ameliorate some of the FIB-based challenges. In this work, we propose a new sample geometry for testing interface toughness at the micron scale which results in intrinsically stable crack growth. This geometry is straightforward to fabricate using established FIB-based methods and testing setups. We prove the stability of crack growth by finite element modelling and by experimentally applying the approach on a hard coating–silicon interface. We observe that even with small imperfections, the FIB-milled notch propagates towards the interface and the natural crack stably grows along the interface