In situ X-ray-based measurements of the laser powder bed fusion (LPBF) additive manufacturing process produce unique data for model validation and improved process understanding. Synchrotron X-ray imaging and diffraction provide high resolution, bulk sensitive information with sufficient sampling rates to probe melt pool dynamics as well as phase and microstructure evolution. Here, we describe a laboratory-scale LPBF test bed designed to accommodate diffraction and imaging experiments at a synchrotron X-ray source during LPBF operation. We also present experimental results using Ti-6Al-4V, a widely used aerospace alloy, as a model system. Both imaging and diffraction experiments were carried out at the Stanford Synchrotron Radiation Lightsource. Melt pool dynamics were imaged at frame rates up to 4 kHz with a ∼1.1 μm effective pixel size and revealed the formation of keyhole pores along the melt track due to vapor recoil forces. Diffraction experiments at sampling rates of 1 kHz captured phase evolution and lattice contraction during the rapid cooling present in LPBF within a ∼50 × 100 μm area. We also discuss the utility of these measurements for model validation and process improvement.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1437&context=ameslab_manuscripts

An instrument for in situ time-resolved X-ray imaging and diffraction of laser powder bed fusion additive manufacturing processes

We review the investigations on clathrate materials having C, Si, Ge or Sn framework atoms in which the pressure parameter plays an important role. In this article, we studied the synthesis of clathrates, superconducting properties and those studies relative to the structural cohesivity and phase diagrams. We have tried to extensively review these subjects. Key references on other important properties of the group IV clathrates are provided in the introductory sections. As a main result of this review, we note that pressure appears as a key parameter for the elaboration of present and future clathrates in particular those exhibiting superconducting properties. We show how high-pressure research has also played an important role in the understanding of the parameters governing clathrate superconductivity. In contrast, the study of the structural evolution of group IV clathrates (bulk modulus and stability) gives abundant clues for tailoring new materials with improved mechanical properties. Finally, practi...

High-pressure properties of group IV clathrates

A number of different families of nanowires which self-assemble on semiconductor surfaces have been identified in recent years. They are particularly interesting from the standpoint of nanoelectronics, which seeks non-lithographic ways of creating interconnects at the nm scale (though possibly for carrying signal rather than current), as well as from the standpoint of traditional materials science and surface science. We survey these families and consider their physical and electronic structure, as well as their formation and reactivity. Particular attention is paid to rare earth nanowires and the Bi nanoline, both of which self-assemble on Si(001).

Self-assembled nanowires on semiconductor surfaces

Single crystals of the Ba-containing silicon clathrate superconductor Ba776Si46 were prepared using a high-pressure and high-temperature condition (3 GPa, 1300 °C) It crystallized in the cubic space group Pm-3n with a = 103141(7) A and Z = 1 There are two crystallographically different types of Ba sites, at the centers of Si dodecahederal (Ba@Si20) and Si tetrakaidecahedral (Ba@Si24) cages On evacuation at 527 °C, a part of Ba atoms were removed from the Ba@Si20 sites The superconducting transition temperature (Tc) decreased from 90 to 60 K with the decrease of the Ba content from 776 to 663 Ba/Si46 The Ba deficient sites and the deficiency were determined by the structural refinement in the single-crystal X-ray analyses

Superconductivity of Metal Deficient Silicon Clathrate Compounds, Ba8-xSi46 (0 < x ≤ 1.4)

In the present review article, firstly, the experimental observations of the binary metallic glass formation by various glass-producing techniques are briefly summarized. Secondly, a detailed discussion is presented concerning the concepts of the glass-forming ability and glass-forming range (GFA/GFR) of a binary metal system. Meanwhile, some of the proposed empirical criteria or rules for predicting the binary metallic glass formation are discussed and compared with the experimental observations. Thirdly, it is proposed to take the interatomic potential of a binary metal system as the starting base to develop an atomistic theory capable of predicting the binary metallic glass formation. Accordingly, eight binary metal systems are selected as representatives to cover the various structural combinations as well as various thermodynamic characteristics. The n-body potentials of eight representative systems are then constructed with the aid of ab initio calculations. Applying the constructed and proven realistic n-body potentials, a series of molecular dynamics simulations are carried out. In the simulations, solid solution models are employed to compare the relative stability of the solid solution versus its competing disordered counterpart (i.e. the amorphous or metallic glass phase) as a function of the solute concentration. Finally, based on the realistic interatomic potentials, molecular dynamics simulations not only reveal the physical origin of the binary metallic glass formation, but also quantitatively determine, for each system, an alloy composition range, within which the disordered state is energetically favored, thus leading to establish the atomistic theory capable of predicting the GFR, i.e. the quantitative GFA, of the binary metal systems.

Atomistic theory for predicting the binary metallic glass formation

Characterizing of directional solidification mode of Fe-Cr-Ni alloys during welding was performed using intense synchrotron radiation probe. Consequently, the crystal growth in the rapid solidification process was revealed in detail and the peak profile was systematically analyzed in order to acquire the essential information for controlling the weld microstructure. Then, the crystallization timing of the primary and secondary phases was discussed. Furthermore, the possibility of the lattice rotation to commensurate (i) each dendrite and (ii) crystallites in the dendrites is suggested.

In-Situ Observation for Weld Solidification in Stainless Steels Using Time-Resolved X-ray Diffraction

We present a joint experimental and theoretical study of the stability and structural properties of Ba-doped silicon type-I clathrates Ba 8Si46 synthesized under high pressures. The thermodynamic stability of Ba8Si46 under high pressure has been discussed from the total energy calculations of some barium silicides within the local density approximation ~LDA!. We have theoretically found that pressure favors the formation of the clathrate phase as experimentally observed. We have also performed a synchrotron x-ray-diffraction experiment of Ba8Si46 prepared under high pressures. Some of the missing endohedral Ba elements in the small cage of Si~20! have been observed by x-ray crystallography, while big cages of Si~24! are found to be completely occupied by Ba elements. The stabilization energies of Ba atoms in the endohedral sites estimated within the present LDA calculation suggest that this is presumably attributed to the energetical site preference of Ba atoms between d(6) and a(2) sites. In addition, the isothermal parameter of Ba in the big cage of Si ~24! has been found to be larger than that in the small Si~20! unit, which is consistent with some theoretical predictions in earlier works.

/pdf/structural-properties-and-thermodynamic-stability-of-ba-2gbahndcx8.pdf

Structural properties and thermodynamic stability of Ba-doped silicon type-I clathrates synthesized under high pressure

We demonstrate, using x-ray diffraction, that we have taken one-dimensional Bi nanolines fabricated on a Si(001) surface, and buried them in crystalline silicon while retaining both their one-dimensional characters and important aspects of their structure. In particular, after burial, the nanolines retain the two-by periodicity associated with their surface structure along their length. We have used density functional theory calculations to model a structure for these buried nanolines, whose minimum length can be estimated to be 100 nm from the coherence length of the x-ray measurements.

Encapsulation of atomic-scale Bi wires in epitaxial silicon without loss of structure

PROBLEM TO BE SOLVED: To solve problems in a conventional scanning surface probe microscope (SPM) method or an X-ray diffraction (XRD) method that it requires a diffractometer (multiaxis diffractometer) with a plurality (normally, four axes or more) of rotational axes and a table for rough adjustment on a specimen since the SPM method provides no information on crystal structure, etc. while providing information on the shape of an ultrafine structure from unevenness information on a substance surface, and further, the XRD method requires that the angle of a specimen is precisely adjusted with respect to an incident X-ray beam so as to cause Bragg diffraction from the ultrafine structure and that the angle is adjusted to the direction of diffraction X rays generated in an X-ray detector. SOLUTION: Short-wavelength X rays of 0.1 nm or less are let into an ultrafine structure, such as an ultrafine nanowire developed on a surface of a substrate crystal, a buried ultrafine nanowire put between a substrate crystal and a cap layer for covering its surface, or a thin film crystal, from its surface at an angle of several degrees or less. Diffraction X rays generated are recorded in a two-dimensional X-ray detector at a stroke in a short period of time, thereby visualizing the intensity of diffraction X rays from the ultrafine structure in a reciprocal lattice space to rapidly analyze its structure. COPYRIGHT: (C)2006,JPO&NCIPI

Rapid x-ray structure analysis method for ultrafine structure

A nondestructive method has been developed for quickly characterizing a 1 D crystalline structure. It required brilliant synchrotron x-rays in grazing incidence and an x-ray 2 D detector. X-ray patterns recorded on the detector showed anisotropically sheet-shape diffraction from NiO nanowires epitaxially grown on an ultra-smooth sapphire (0 0 0 1). Other shots obtained from Bi atomic wires embedded in Si (0 0 1) showed that the wires have a 2 x n superstructure, which indicated that the Bi-dimer bonds were parallel to the wires.

https://link.springer.com/content/pdf/10.1557/PROC-840-Q6.4.pdf

Akiko Kitano

Papers

In-Situ Observation for Weld Solidification in Stainless Steels Using Time-Resolved X-ray Diffraction

Structural properties and thermodynamic stability of Ba-doped silicon type-I clathrates synthesized under high pressure

Encapsulation of atomic-scale Bi wires in epitaxial silicon without loss of structure

Rapid x-ray structure analysis method for ultrafine structure

Reciprocal-Lattice Space Imaging of X-Ray Intensities Diffracted from Nanowires