Massive reorientations of bulk single and oligocrystals via solid state processing
Graphical abstract
Introduction
Over the years, single crystals have played a vital role in a variety of areas including microelectronics [1,2], optical crystals [1,2], magnetic devices [3,4], solar cells [2,3,5], piezoelectric components [3,6] and multifunctional alloys [7], [8], [9], [10]. For instance, single crystalline silicon wafers have revolutionized the development of microelectronic devices [11], [12], [13]. Similarly, single crystalline nickel-based superalloys, with high temperature strength, creep and fatigue life [14], [15], [16], are used in jet engine turbine blades, which enabled wide-spread commercial aviation in the last century. However, for most applications, the ability to simply grow a single crystal is not sufficient. It is vital to control their size, shape and crystallographic orientation in order to exploit their desired properties. For example, for nickel-based superalloys, the rupture life at 750 to 870°C is significantly higher when operating stresses are applied along orientations near [001] and [111], than along orientations near [011] [17]. Similarly, differences in charge mobility of monocrystalline silicon wafers along the <100> and <110> directions greatly affect the performance of positive and negative channel solid oxide semiconductor devices [18].
Aside from the orientation, many single crystal applications depend on the ability to produce single crystals with large dimensions. Larger single crystals can increase the efficiency of crystalline photovoltaic (PV) cells [5], provide better performance in large-scale integrated circuits [18], and are expected to enable new large-scale applications in civil engineering as seismic structural elements in bridges and buildings [19], [20], [21].
Nonetheless, for all single crystal growth techniques, controlling crystal orientation and efficiently producing large single crystals remain as major challenges [1,3]. For melt growth techniques (such as Bridgman and Czochralski processes), the ability to obtain large crystals with a preferred orientation relies on the availability of proper seed crystals and the precise control of nucleation and thermal profile during the processing. Seed crystals must be free of defects and flaws and have the proper crystallographic habit planes in order to avoid strain evolution during crystal growth and achieve growth rates fast enough to be economical. This is often difficult because desired habit planes do not often correspond to the fastest growth direction [22]. Rapid growth rates require a high degree of supersaturation; however, when the degree of supersaturation is very high, crystal defects and seeds are formed [22]. Hence, it is often difficult to produce a high quality large single crystal with the preferred orientation using melt growth methods.
Solid-state crystal growth (SSCG) techniques (such as strain annealing, discontinuous grain growth, or secondary recrystallization), on the other hand, do not require complex and expensive equipment, are more versatile, and can achieve better chemical homogeneity than melt growth techniques [3,22,23]. However, it is still difficult to control the crystal orientation and produce large single crystals using available SSCG methods. This difficulty is mainly due to the fact that optimum conditions for required prior treatments (e.g. pre-deformation, annealing) vary depending on the chemical purity and the initial microstructure of the materials [22]. For example, in the strain annealing method, if the grains of the initial polycrystalline matrix are randomly oriented, after application of a critical deformation strain, the range of potential crystal orientations forming can be quite large and the orientation of the final crystal is usually random. A strong initial texture can overcome this problem, but this limits the orientations that can be obtained as well as the size and the shape of the specimens [22]. Until recently, it was not possible to obtain crystal sizes larger than a few millimeters using traditional SSCG techniques. In 2013, Omori et al. [20] introduced a new abnormal grain growth (AGG) method to control grain size in CuAlMn alloys. This new method combines thermal cycling and solid-state diffusional phase transformations to trigger AGG in alloys [19,20]. This technique has thus far produced large single crystal bars up to 70 cm in length [19].
Traditional SSCG techniques require that one grain grows preferentially at the expense of neighboring grains. This growth depends on the grain boundary energy and mobility, which are strongly influenced by the grain boundary curvature, grain boundary plane, grain misorientation, and pinning sources in the microstructure [24], [25], [26], [27]. However, the driving force for grain growth naturally diminishes with increasing grain size, eventually halting the grain growth due to grain boundary roughening [28], reduction in the capillary forces [26] or thermal grooves [29]. Therefore, traditional SSCG methods are not effective in creating very large grains. The crystal growth technique developed by Omori et al. [20] overcomes this limitation. In this method, the temperature is cycled between a high temperature, where the material is in a single phase state, and a lower temperature, where it is in a two phase state. This cycling results in the repeated formation and dissolution of semi-coherent second-phase precipitates. These precipitates dissolve upon heating to the single-phase region and leave a subgrain structure behind with the subgrain boundaries decorated by defects and dislocations. Repetition of this heating-cooling cycle triggers AGG, which results in large single crystals [19,21,30]. Although the mechanism behind this unique AGG is still subject to debate, the subgrain boundary energy is believed to be the main driving force for continuous AGG.
Although these advances in production of large single crystals are promising, controlling grain orientation still remains as a challenge in solid state single crystal growth. In the present work, we have discovered that during this AGG, a new mechanism emerges that has not been recognized before and leads to unexpected abrupt and dramatic changes in grain orientations. This mechanism, which we have observed in two materials systems, FeMnAlNi and CuMnAl, can be used to alter the orientation of a large single crystal without the need to re-melt the crystal. Below, we describe how this new mechanism operates in these alloy systems that can be used to massively change the crystal orientations using simple solid-state heat treatments.
Section snippets
FeMnAlNi
As cast ingots with a nominal composition of Fe-34Mn-15Al-7.5Ni (at.%) were hot rolled at 1200°C with an area reduction of 60%. Hot rolled samples were then annealed in an ultra-high purity argon atmosphere at 900 °C for 1h followed by water quenching to create ductile second phase particles that are necessary for cold rolling. Ductile samples were cold rolled into 1.5 mm sheets with a 75% area reduction at room temperature. Samples of 3mm x 1.5 mm x 10 mm dimensions were cut from the rolled
The repetitive massive crystal reorientation mechanism in FeMnAlNi alloys
Our work with an FeMnAlNi alloy, with a composition of Fe43.5Mn34Al15Ni7.5 (at.%), demonstrates the physics behind this mechanism. In this alloy, at temperatures below 1157°C, precipitates with a face centered cubic (FCC) structure form in the matrix of a body centered cubic (BCC) structure [30]. When the alloy is subjected to repeated thermal cycling between the single-phase (BCC) (>1157°C) and the two-phase (BCC+FCC) regions (by cooling to room temperature (RT), or slow cooling to 900°C, and
Summary and conclusions
This study establishes that the orientation of bulk single and oligocrystals can be manipulated completely in solid state. We show that orientation tailoring can be accomplished by simple heat treatments that result in massive orientation changes and help to achieve hard-to access microstructures/orientations. The large crystal rotations observed in this study are also the first experimental evidence that grain rotations have a significant influence on microstructural evolution and that they
Data Availability Statement
All data are available in the main text or the supplementary materials.
Funding
This work was supported by the Transportation Consortium of South-Central States through a grant from the U.S. Department of Transportation/University Transportation Center Program [Grant No. 69A3551747106].
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would also like to acknowledge the TAMU Materials Characterization Facility where the authors conducted the microstructural characterization.
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