ZEISS Xradia CrystalCT

09.15 – 10.00

In this session, we‘ll explore the emerging research applications, along with underlying technology and methodology, stemming from 3D X-ray microscopy (XRM). As a nondestructive characterization method, XRM allows us to uniquely evaluate the internal structures of our samples and specimens at sub-micron resolution, covering multiple contrast mechanisms and length scales. Moreover, 4D investigation of structures via in situ or ex-situ repeated imaging provides new opportunities for understanding materials evolution/degradation processes, and correlative workflows linking XRM with other modalities such as EM or FIB-SEM offer the chance to easily span across a range of length scales. Several examples will be presented, emphasising the latest developments and an outlook towards the future.

10.00 – 10.20

A major limitation of additively manufactured metal parts using laser powder bed fusion (LPBF) processes is their low fatigue resistance vis-á-vis the parts manufactured using conventional processes. Where porosity, inevitable in the parts produced by powder-based processes, is the cause of the poor performance. In this work, the effects of process parameters such as laser power, layer thickness, and scan rotation on the defect formation and their influence on the fatigue performance of LPBF Ti-6Al-4V, the most widely used aerospace grade Ti alloy, were investigated using 3D tomography. The possibility of enhancing fatigue resistance via post-processing heat treatment and subsequent shot peening is also examined. A fracture mechanics-based failure envelope was proposed as a guide for the use of LPBF Ti64 in fatigue loading applications, assuming the presence of pores is inevitable.

11.00 – 11.20

The unnotched fatigue behavior of the 17-4 PH martensitic (α’) stainless steel, additively manufactured using the binder jet printing technique (BJP), with varying porosity levels and in two different aging conditions are investigated and are compared to that of the conventionally manufactured (CM) alloy. As expected, fatigue strength is enhanced by the reduction in porosity, with hot isostatic pressing of the BJP alloy resulting in a fatigue strength that is similar to the CM alloy. Over-aging of the alloy improves its fatigue resistance further. These variations are rationalized by recourse to the analysis of the microstructure-defect interactions based on microscopy and x-ray tomography. Furthermore, it was found that the combination of plasticity induced crack closure mechanisms and transformation induced plasticity are dominant at the fatigue crack tip of the over-aged alloy. Fracture mechanics-based Kitagawa and Takahashi diagrams were utilized to illustrate the significance of pore size on the unnotched fatigue resistance of BJP 17-4 PH alloys. Lastly, pore distribution played a critical role in the propagation of fatigue cracks, which eventually affected the fatigue life. These results are discussed in terms of designing alloy fabrication using BJP specifically for cyclic loading conditions.

11.20 – 12.00

Correlative microscopy is a powerful and sophisticated microscopy approach that enables precise subsurface sample preparation and analysis. As metallurgy research advances, there is a need to understand the effect of microstructure and properties of a sample at different locations and length scales. In this presentation, we explore a correlative microscopy workflow that combines the capabilities of both the X-Ray Microscope (XRM) and the Focused Ion Beam SEM (FIB-SEM) in various metallurgy studies. The use of X-ray microscopy allows the precise identification of subsurface defects or Region-of-Interest (ROI) without compromising the integrity of the samples. The identified subsurface ROI can be accessed with FIB-SEM to prepare high-quality sample surfaces for subsequent advanced analysis. We will present a number of use cases on Metals Additive Manufactured parts where these printed parts or materials have been characterized and analyzed with the correlative microscopy workflow.

14.00 – 14.30

Determining the crystallographic microstructure of a given material in 2D can be challenging. Further extending such an investigation to 3D on meaningful volumes (and without sample sectioning) can be even more so. Yet reaching this insight holds tremendous value for 3D materials science since the properties and performance of materials are intricately linked to microstructural morphology, including crystal orientation. Laboratory diffraction contrast tomography (LabDCT) technique implemented on a lab X-ray microscope opens up a whole new range of possibilities for studies of the effect of 3D crystallography on materials performance in the laboratory. Grain morphology, orientation and boundaries of metals, alloys or ceramics can be characterized fully in 3D. Recent advances in LabDCT allow the recording and reconstruction of larger representative sample volumes seamlessly, meeting the demand for statistically relevant characterization of polycrystalline grain microstructure. We will present a number of use cases covering a variety of polycrystalline materials that have been characterized and analyzed using LabDCT providing deeper insights into the three-dimensional microstructure of materials.

14.30 – 15.00

In this study, we investigated the corrosion and stress corrosion cracking behaviours of 17-4 precipitation hardening stainless steel, comparing samples produced through binder jet printing and conventional manufacturing methods. X-Ray Computed Tomogram (XCT) was employed to analyse the corrosion morphology, including identifying crack initiation sites and the paths of crack propagation. The XCT results revealed that pores induced by binder jet printing served as crack initiation sites. In contrast, in specimens from conventional manufacturing, where almost no pores were detected, the crack initiation site was notably different, with grain boundaries identified as potential initiation sites. Regarding crack propagation paths, specimens from conventional manufacturing exhibited long and sharp cracks, while those from binder jet printing displayed blunt pit-like cracks. Within the cracks observed in binder jet-printed specimens, XCT revealed the presence of strips that did not dissolve. This observation suggests that these strips are more resistant to corrosion, effectively halting the propagation of cracks.

15.00 – 15.30

Electron channelling contrast imaging (ECCI) is an SEM-based technique for observing extended crystal lattice defects like dislocations and stacking faults. It exploits the dependence of the backscatter electron intensity on crystal orientation and atomic order. The basic principle of contrast formation is that electrons channel into a crystal lattice when the incident beam hits the lattice along the Bragg angle of a set of crystal planes. In this case, very few electrons are backscattered, and the observed crystal appears dark. In this case, every defect that disturbs the order of the lattice planes leads to backscattering and is visible in the ECC image as bright features in a dark grain. Dislocations, for example, appear as bright lines, stacking faults as bright areas. The technique can be used very similar to transmission electron microscopy (TEM), however with the advantage that a bulk sample is observed and not a thin foil. This enables observation of much larger samples, simplifies sample preparation, and facilitates in-situ experiments like deformation, heating, or gas reaction observations. In this presentation, the basic principles of the technique are explained and illustrated, along with application examples of ECCI on superalloys and high-strength steels.