Internal Interfaces in Severely Deformed Metals and Alloys: Coupling of Kinetics, Structure and Strain with Properties and Performance

Severe plastic deformation drives bulk materials far away from equilibrium and thus opens up a new opportunity to explore hitherto uncharted regions of structure-property correlations with respect to grain size, strain and defect density under extreme conditions. This allows addressing long-standing issues in materials research, such as the validity of “effective temperature” concept and provides a set-screw for exploring the possible levels of defect design and property tuning by deformation processing. As a pre-requisite, basic issues concerning the interaction of defects of different dimensionality during deformation and their interrelation with fluxes of solutes or with segregation fields, resulting in chemo-mechanical coupling effects need to be analyzed quantitatively. In the following review, we address issues related to internal interfaces in severely deformed metals and alloys by highlighting recent observations.

Gerhard Wilde, Harald Rösner, Sergiy Divinski

Coherent twin-oriented Al3Sc-based precipitates in Al matrix

Al3(Sc,Zr,Ti) nanoparticles with an ideal twin-type orientation relationship to Al host matrix were found in cold-rolled and subsequently annealed Al-based alloy. Atomic-scale investigations using high-resolution scanning transmission electron microscopy identified particles that form prominent coherent (111) twin-type interfaces along their longer facets and semi-coherent twin interfaces on their shorter facets. Ab-initio calculations showed that a coherent Al/Al3Sc twin-like phase boundary corresponds to a local energy minimum. A model is proposed explaining the formation of the twin orientation relationship of an Al3Sc nanoparticle with the Al host matrix.

Sankaran Shanmugam, Martin Peterlechner, Mohamad Riza Iskandar, Ujjal Saikia, Vladislav Kulitckii, Marta Lipińska-Chwałek, Joachim Mayer, Harald Rösner, Tilmann Hickel, Sergiy V Divinski, Gerhard Wilde

Concentration dependence of the crystal nucleation kinetics in undercooled Cu–Ge melts

The crystallization temperature of deeply undercooled Cu–Ge alloy melts is repeatedly measured. A statistical analysis is applied on the undercooling distributions obtained from nine different compositions, ranging from the pure semimetal (Ge) to the pure metal (Cu). By considering each undercooling distribution as an inhomogeneous Poisson process, the nucleation rates for every composition are calculated. The Thompson–Spaepen model for homogeneous nucleation in binary alloys is applied, enabling the estimation of nucleation parameters, such as kinetic pre-factors and interfacial energies, as a function of composition. Furthermore, the Turnbull coefficient α, a dimensionless solid–liquid interfacial energy constant, is also calculated as a function of alloy constitution, suggesting a dependence on the liquid composition. The composition-dependent changes of α are of considerable importance, since the α is originally defined for pure systems as a quantity dependent on crystal structure, and is nevertheless used for describing nucleation kinetics of binary and glass forming multi-component alloy systems.

 M. W. da Silva Pinto, M. Peterlechner, G. Wilde

Dislocation exhaustion and ultra-hardening of nanograined metals by phase transformation at grain boundaries

The development of high-strength metals has driven the endeavor of pushing the limit of grain size (d) reduction according to the Hall-Petch law. But the continuous grain refinement is particularly challenging, raising also the problem of inverse Hall-Petch effect. Here, we show that the nanograined metals (NMs) with d of tens of nanometers could be strengthened to the level comparable to or even beyond that of the extremely-fine NMs (d ~ 5 nm) attributing to the dislocation exhaustion. We design the Fe-Ni NM with intergranular Ni enrichment. The results show triggering of structural transformation at grain boundaries (GBs) at low temperature, which consumes lattice dislocations significantly. Therefore, the plasticity in the dislocation-exhausted NMs is suggested to be dominated by the activation of GB dislocation sources, leading to the ultra-hardening effect. This approach demonstrates a new pathway to explore NMs with desired properties by tailoring phase transformations via GB physico-chemical engineering.

Shangshu Wu, Zongde Kou, Qingquan Lai, Si Lan, Shyam Swaroop Katnagallu, Horst Hahn, Shabnam Taheriniya, Gerhard Wilde, Herbert Gleiter & Tao Feng

Evidence for strain and a structural reset in Pd₄₀Ni₄₀P₂₀ bulk metallic glass

The amorphous structure of a Pd-based metallic bulk glass after controlled heating into/cooling from the super-cooled liquid was investigated by fluctuation electron microscopy.

The results indicate that former activated α and/or β relaxation processes are deleted and thus a a defined structural reset was achieved.

The study suggests controlled heating into/cooling from the SCL as a possible standard protocol for investigations of metallic glass as it would afford a better reproducibility of experiments, independent of the fabrication, and/or thermal history of the samples.

Mark Stringe, Harald Rösner, Gerhard Wilde

Observing Dislocations Transported by Twin Boundaries in Al Thin Film: Unusual Pathways for Dislocation–Twin Boundary Interactions

Twins are generally regarded as obstacles to dislocations in face-centered cubic metals and can modify individual dislocations by locking them in twin boundaries or obliging them to dissociate. Through in situ tensile experiments on Al thin film in a transmission electron microscope, we report a dynamic process of dislocations being transported by twin lamella via periodic twinning and detwinning at the atomic scale. Following this process, a 60° dislocation first transforms into a sessile step of the twin boundary, then migrates under stress as a step and finally reverts back into a 60° dislocation. Our results reveal a novel evolution route of dislocations by a dislocation–twin interaction where the twins act as transport vehicles rather than as obstacles. The potential implications of this mechanism on toughening are also discussed. 

Zongde Kou, Tao Feng, Si Lan, Song Tang, Lixia Yang, Yanqing Yang, and Gerhard Wilde 

In Situ Generated Shear Bands in Metallic Glass Investigated by Atomic Force and Analytical Transmission Electron Microscopy

In this study, we investigated in situ generated shear bands in a metallic glass formed under tension in the transmission electron microscope and show, by accompanying thickness and surface measurements,
that their contrasts are related to thickness reduction, unlike ex situ samples where no change in thickness was observed.

Harald Rösner, Christian Kübel, Stefan Ostendorp and Gerhard Wilde

Effects of solutes on thermal stability, microstructure and mechanical properties in CrMnFeCoNi based alloys after high pressure torsion

The equiatomic Cantor alloy and Ni-enriched derivates of it ((CrMnFeCo)(x)Ni1-x with x = 0.8, 0.4, 0.08 and 0) were deformed by high pressure torsion to the saturation regime and subsequently annealed in a wide temperature range. The microstructural stability of the alloys was investigated in terms of grain growth and decomposition tendencies with an emphasis on Ni92 and Ni60 that are marking a transition from dilute solid solutions to HEA alloys. Ni92 and Ni100 show a larger grain size after HPT than the HEA type alloys Ni20 and Ni60, yet, they do exhibit a lower resistance to grain growth. Both HEA type alloys exhibit decomposition tendencies in the form of secondary phases (Ni20) or nanosized Cr-rich precipitates and Mn segregations to grain boundaries (Ni60), that reduce the grain boundary mobility. A comparison of the activation energies for grain growth and diffusion coefficients of the alloys shows a good qualitative agreement. The differences in microstructural stability are also reflected by the mechanical properties. All alloys show a small amount of hardening after annealing, followed by a softening for higher temperatures in case of dilute solid solutions. In contrast, the hardness of the HEA type alloys remains constant up to 450 degrees C (Ni60) or even increases up to 500 degrees C (Ni20) followed by a softening at higher annealing temperatures. The second phase formation with limited volume fractions has no effect on the modulus.

(C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Tom Keil, Shabnam Taheriniya, Enrico Bruder, Gerhard Wilde, Karsten Durst

Decoupled alpha and beta relaxation kinetics in a thermally cycled bulk Pd40Ni40P20 glass

Reproducible thermodynamic sample states of a Pd40Ni40P20 bulk metallic glass are realized via differential scanning calorimetry by repeated quenching from the supercooled liquid state to temperatures well below the glass transition. Annealing treatments at 0.81 Tg and 0.96 Tg are embedded in the calorimetric method, changing the energetical state of the system. Varying the annealing times, a detailed and reproducible picture of the reversible relaxation dynamics with separated alpha- and beta-relaxation is obtained. An endothermic signature before Tg can either be provoked or depressed depending on the annealing temperature. The activation energy related to this process is obtained via Kissinger analyses yielding about 30 RTg. A large number of annealing cycles at 0.96 Tg irreversibly alters the response of the alpha-relaxation, while the mechanism of beta-relaxation is interestingly not influenced by this alternation. In order to extend the calorimetric response of the relaxation spectra to spatial resolution, the sample states were additionally analyzed using electron correlation microscopy providing information on the glass dynamics on an atomistic scale. The thus obtained kinetic parameters of local dynamics do not show an alteration of room temperature dynamics for different levels of alpha-relaxation, which is consistent with the results obtained via kinetic analyses of calorimetric data.

Mark Stringe, Katharina Spangenberg, Manoel Wilker da Silva Pinto, Martin Peterlechner, Gerhard Wilde