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CS8.5 Pressure Induced Bcc to Fcc Phase Transformation in Cold Spraying

Wednesday, May 23, 2012: 9:40 AM
Room 335 AB (Hilton Americas Houston )
Dr. Frank Gaertner , Helmut Schmidt University, University of the Federal Armed Forces Hamburg, Hamburg, Germany
Dr. Christine Borchers , Georg-August-University Goettingen, Goettingen, Germany
Dr. Tobias Schmidt , Helmut Schmidt University, University of the Federal Armed Forces Hamburg, Hamburg, Germany
Dr. Thorsten Stoltenhoff , Helmut Schmidt University, University of the Federal Armed Forces Hamburg, Hamburg, Germany
Dr. Hamid Assadi , Tarbiat Modares University, Tehran, Iran
Dr. Heinrich Kreye , Helmut Schmidt University, University of the Federal Armed Forces Hamburg, Hamburg, Germany
Thomas Klassen , Helmut Schmidt University, University of the Federal Armed Forces Hamburg, Hamburg, Germany
The present contribution demonstrates that during cold spraying bcc powder structures of 316L and NiCoCrAlY alloys completely transform to fcc phases, independent of process conditions. The nature of transformation is explored by X-ray diffraction, microstructural investigations by optical microcopy and scanning electron microscopy as well as by transmission electron microscopy. For a deeper understanding, also explosive powder compaction and high energy ball-milling were employed to provoke such bcc to fcc phase transformations.

As compared to normal thermal spray processes, cold spraying or kinetic spraying exposes the feedstock powder only to moderate temperatures below the melting point of the respective material. Thus, phase transformations are usually prevented by the sluggish kinetics. However, the situation could become different for alloys that allow composition invariant, polymorphous transformations.

By the high quench rate during inert gas atomization, 316L and NiCoCrAlY alloys can be solidified as metastable bcc structures. Nevertheless, even for transformations from metastable to stable structures, still the nucleation barriers for creating new interphases and working against distortion stresses must be negotiated. For the transformation from a metastable bcc to the thermodynamically more stable fcc structure during cold spray spraying of 316L and NiCoCrAlY alloys, the necessary additional driving force is most prominently supplied by the pressure pulse during the solid impact, favoring the denser fcc structure. In addition, the transformation might be promoted by the better plastic deformability of the fcc phase.

In view of cold spray applications, such transformations could be of interest to reduce critical conditions for coating formation. For the example of the NiCoCrAlY alloy, high hardness and low deformability of the bcc powder would require harsher impact conditions for achieving shear instabilities and bonding, than those required for bonding of transformed fcc structures. Such concepts might be followed for cold spray powder designs to enhance bonding.

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