July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 1 7 HIGH-RESOLUTION MICROSCOPY AND ITS IMPORTANCE Additive manufacturing is a layer-by-layer deposition pro- cess where each layer melts and welds to the previous layer to construct a 3D structure. In this process, there is a large heat transfer across the interfaces of various layers resulting in com- positional and microstructural complexity both inside the grain and at the grain boundary [5] . High-resolution microscopy in- vestigations for AM components primarily utilize scanning elec- tron microscopy (SEM), trans- mission electron microscopy (TEM), and atomic probe tomog- raphy (APT), which cover the full range of microstructure length- scales from micro to the atom- ic scale. A holistic approach is presented in Fig. 3, demonstrat- ing the use of several micros- copy techniques to understand microstructural aspects in AM processed HEAs. While various imaging techniques (secondary electron and backscattered) in a SEM are critical for microscale examinations of the quality of AM components, the grain ori- entation mapping using electron backscattered diffraction (EBSD) and elemental mapping by ener- gy dispersive spectroscopy (EDS) in SEM ensure the homogene- ity in the structure for expected properties. The grain morphol- ogy of different regions within a molten pool in the AM process significantly varies from each other depending on the ratio of a temperature gradient to crys- tal growth velocity, as shown in Fig. 3a for CoCrFeMnNi [6] . EBSD and EDS critically assist in identi- fying the nature (inhomogeneity in grain morphology, phase for- mation, and elemental segrega- tion) of interfaces between two subsequent layers. The ability to show morphology and defects Fig. 2 — Different phases of HEAs processed via additive manufacturing, where the nominal composition varies from x=0.2 to x=6. The majority of the HEAs exhibit the FCC and duplex phase, followed by the BCC phase. Other phases, such as BCC1+BCC2, BCC+B2, FCC+BCC+Laves Phase, are also observed in AM processed HEAs. (a) Fig. 3 — An overview schematic illustrating various microscopic characterization techniques used to analyze HEAs processed by AMmethods. (a) SEMmicrographs showing the microstructural variations inside a molten pool of CoCrFeMnNi [6] ; (b) the correlated backscattered images and EBSD phase maps in Al x CrCuFeNi 2 from x=0.8 t0 x=1.0 [7] ; (c) TEM image with the selected area diffraction pattern of Al 0.8 CrCuFeNi 2 , showing the formation of FCC and B2 phase [8] ; (d) elemental distribution maps in CoCrCuFeNiAl using TEM-EDS from the corresponding region shown in high angle annular dark field [9] ; (e) APT map showing the compositional variation of Al 0.3 CoCrFeNi HEA [12] . (b) (c) (d) (e)