Abstract
Generally, the evolution of metallic single crystals is based on crystal growth. The single crystal is either produced by growing a seed single crystal or by sophisticated grain selection processes followed by crystal growth. Here, we describe for the first time a fully new mechanism to generate single crystals based on thermo-mechanically induced texture formation during additive manufacturing. The single crystal develops due to two different mechanisms. The first step is a standard grain selection process due to directional solidification, leading to a pronounced fiber texture. The second and new mechanism bases on successive thermo-mechanically induced plastic deformations and texture formation in FCC crystals under compression. During this second step, the columnar grain structure transforms into a single crystal by rotation of individual grains. Thus, the single crystal forms step by step by merging the originally columnar grain structure. This novel, stress induced mechanism opens up completely new perspectives to fabricate single crystalline components and to accurately adjust the orientation according to the load.
Highlights
The evolution of metallic single crystals is based on crystal growth
Single crystals from alloys or semiconductors are normally produced by crystal growth, e.g. the Czochralski method1, the Bridgman method2,3 or zone melting4 or by means of recrystallization as a consequence of abnormal grain growth5
Single crystalline components made from nickelbase superalloys are used in the hottest sections of stationary gas turbines or aircraft turbines6
Summary
The evolution of metallic single crystals is based on crystal growth. The single crystal is either produced by growing a seed single crystal or by sophisticated grain selection processes followed by crystal growth. The second and new mechanism bases on successive thermo-mechanically induced plastic deformations and texture formation in FCC crystals under compression During this second step, the columnar grain structure transforms into a single crystal by rotation of individual grains. Single crystalline components made from nickelbase superalloys are used in the hottest sections of stationary gas turbines or aircraft turbines6 These components develop in a Bridgman process where competitive grain selection during directional solidification and a geometric selection process (spiral selector) are combined to select one grain which eventually forms the c omponent. Due to the inherent high solidification velocities, combined with steep thermal gradients, the dendrite arm spacing is about two orders of magnitude smaller compared to the classical Bridgman process14,15 As a result, these single crystals are homogenized within m inutes and the solidification porosity scales with the dendrite arm s pacing. Based on this new insight, the potential of additive manufacturing to fabricate single crystalline components manifests
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