Microstructure and temperature effects on submicron plasticity of bcc metals

Body centred cubic (bcc) metals are commonly used at elevated operation temperatures, where their mechanical bulk properties change significantly. Moreover, miniaturization of engineering parts in the submicron regime leads to size effects on strength. Therefore, the combination of thermal activation and miniaturization becomes increasingly relevant, for instance to optimize an engineering design.

In this project, a new techniques for site-specific micro-sample preparation and experiments to explore the temperature- and size-dependent deformation behaviour in terms of flow stress, hardening behaviour and fracture properties of several bcc metals are developed.

Regarding sample preparation, a cross section polisher using low energy ion beams is used to preform thin lamellas, on which micron-sized samples are machined by the FIB (LEO 1540XB). This drastically decreases subsequent FIB preparation times. A detailed approach can be found here.

To address the impact of sample size and microstructure, different sized pillars with single crystalline and ufg microstructure were compressed in-situ in the SEM, as shown in the still image below. The single crystal pillar on the left shows discrete slip, while the ufg sample on the right deforms in a bulk-like manner at much higher flow stresses. More details about the size-affected strength scaling behaviour in ufg bcc metals can be found here.

In-situ SEM compression test of two ~4 µm sized Cr pillars. The left sample is single crystalline while the right one shows an ufg microstructure. The insets show the corresponding stress-strain data.

To gain insights in temperature-dependent deformation mechanisms, a commercial high temperature nanoindenter is used. To get detailed insights into the deformation at elevated temperatures, also a custom-made in-situ high temperature unit was developed based on the ASMEC UNAT. By separately heating and controlling the temperature of sample and indenter tip, a broad variety of miniaturized testing techniques (tensile, compression, fatigue, beam bending) can be applied up to 300°C and directly observed in situ in the SEM while keeping the influence of thermal drift at a minimum.

Installed in-situ heating device in the SEM

To address the interplay between the ufg microstructure and an increasing amount of free surfaces on the deformation behaviour, several experiments were performed at vastly different length scales and between ambient and elevated temperatures using advanced nanoindentation techniques, miniaturized pillar compression experiments, and macroscopic compression tests. This serves to investigate the transition from bulk to single crystalline mechanisms and surface-affected deformation behaviour, respectively.

To relate the behaviour of the high lattice friction bcc metals to their low lattice friction, face centred cubic (fcc) counterparts, high temperature micro-compression experiments on copper single crystals were conducted in collaboration with EMPA Thun.

Lastly, to differentiate between grain boundary effects and lattice friction contributions, the influence of grain boundaries on the rate dependent properties was studied with respect to single crystal materials using rate- and temperature dependent nanoindentation and published for fcc gold and bcc chromium.

Besides plastic deformation, also failure properties of bcc materials at small scales are of prime concern. To address the fracture properties of such miniaturized structures as well as the influence of temperature and microstructure on fracture mechanisms, respectively, we established notched cantilever beam bending experiments (single-end and double-clamped bending) in-situ in the TEM and SEM, respectively. A corresponding publication regarding small-scale fracture properties can be found here.

Experimental setup during in-situ TEM loading. Scale bar is 100 nm

Recent publications are provided below. Note that use of the manuscripts is for academic purpose only.

 

Acknowledgement: This work is supported by the FWF (P-25325: Microstructure and temperature effects on (sub-)micron plasticity of bcc metals). Further financial support by the Austrian Federal Government (837900) within the framework of the COMET Funding Programme “Materials, Processing and Product Engineering” (MPPE, A7.19) is appreciated.