Analytical TEM for Product-related Material Development

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Fig. 1: SiC nanoparticles, encapsulated with amorphous and graphitic carbon (TEM).

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Fig. 2: High resolution TEM image of a Zr/Al RMC with (upper left) and without (lower right) diffusion barrier.

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Fig. 3: Micro hardness depth profiles of different precipitation-hardened steels after surface age hardening.

Introduction

Materials and material parameters form the background for many technological innovations. The desired material properties depend, to a large extent, on their microscopic structure, making micro analytical tools a necessity. Analytical transmission electron microscopy (TEM) plays a key role as it is the only method that provides complete structural characterization down to the atomic level. TEM combines the three fundamental analytical methods of imaging, diffraction and spectroscopy in the same tool, allowing for comprehensive analyses.

The analytical TEM has decisively contributed to tailoring the structure of materials with regard to their application-relevant properties. The method is not only advantageous to address basic research questions but can also be used:

-         for improving product related material properties (e.g. process technologies used to design surface-near microstructures),

-         to further develop material-dependant manufacturing processes (e.g. joining),

-         for failure analysis and

-         to evaluate the quality of manufacturing processes.

In this respect it is the goal of Fraunhofer IWS engineers to utilize the technique for product-related materials development and also to offer this particular epxertise to its customers and their process-driven research issues. At Fraunhofer IWS the analytical TEM is combined with comprehensive metallographic capacities, scanning electron microscopy (SEM), focused ion beam facilities and materials testing capabilities. The following results present an overview of Fraunhofer IWS research activities in the field of material-related process and product design.

 

Results

Current research is devoted to the synthesis of silicon carbon nanoparticles in a so-called “core and shell” arrangement. This material is designated as an electrode material in lithium ion batteries. TEM investigations provide information such as the structure, size and distribution of the nanoparticles and information about the condition of the carbon shell as a function of the synthesis conditions (Fig. 1). This information in return provides the opportunity to optimize synthesis parameters.

Another research topic at Fraunhofer IWS is the development of reactive multilayer coatings (RMC), which are applied in low heat impact joining processes for various material combinations by delivering heat energy precisely and reproducibly to the contact zone. RMCs consist of nanometer multilayers with hundreds or thousands of individual films. The exact knowledge of the periodic thickness allows for a precise property design of the RMCs. Another important aspect of the development is to avoid diffusion within the coating stack during fabrication. TEM analysis demonstrated that specific barrier coatings help avoid this undesired diffusion effect (Fig. 2).

New materials and material combinations require efficient methods for the realization of dissimilar joints. In this respect IWS focuses on such promising new technologies as laser welding, electromagnetic pulse welding, friction stir welding and laser induction roll plating. TEM analysis is once again one of the key methods to study the microstructural changes as the process parameters re changed and hence analyzing the development of undesired phase seams that may form at the interface between two materials during such joining processes. Friction stir welding, laser induction roll plating and electromagnetic pulse welding all achieve subcritical phase seam thicknesses of less than 1 μm. In the case where aluminum alloys are the base material, mechanical tests revealed that failure always occurred in the base metal and not in the joint. The low phase seam thickness also reduces contact resistance, which was much lower for these three techniques when compared to laser welding and conventional screw connections. TEM analysis provided additional valuable information about the growth process and the nature of the forming phases (Fig. 4).

Precipitation-hardened martensitic stainless steels offer a good combination of strength, ductility and toughness, good fabrication characteristics and corrosion resistance as well as superior fatigue performance. Therefore these steels are extensively used for structural components in aircraft, chemical, naval, nuclear and power generation industries. However, if the application requires high wear resistance, common surface heat treatments are not applicable for this kind of material. To overcome this drawback an effective surface modification technique has been developed at Fraunhofer IWS Dresden, which allows the selective generation of wear resistant surface regions up to several millimeters in depth without altering mechanical properties in the bulk of the material. This technique consists of selective short-time laser surface solution annealing (austenitization) at unusually high temperatures to completely dissolve the precipitations in the near surface region and maintain a solid solution condition upon rapid self-quenching and phase transformation to lath martensite and an aging treatment at relatively low temperatures to optimally strengthen the solution-annealed surface regions. By choosing the appropriate laser and aging treatment, the hardness of precipitation-hardened steels (15-5 PH, 17-4 PH and PH 13-8) can be increased by more than 150 HV up to a depth of 4 mm (Fig. 3). SEM and TEM analyses verified that the improved properties of the age hardened surface are due to a more homogeneous and finer precipitation arrangement. The property-determining Cu and Ni3Al precipitations in the age-hardened surface are generally smaller than 10 nm and are thus much smaller than in the bulk (Fig. 5). Further positive effects of the laser surface annealing are the dissolution of coarse carbides and a substantially higher thermal stability of the precipitations.

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Fig. 4: TEM image of the phase seam formed during laser induction roll plating of Al and Cu.

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Fig. 5: TEM dark field image of Ni3Al precipitations in the surface age hardened region of PH 13-8 steel.