Transformation Plasticity in (Gd_xDy_1−x)PO₄ Fiber Coatings During Fiber Push Out

Abstract

The reduction in fiber push‐out stress by transformation plasticity in xenotime rare‐earth orthophosphate fiber–matrix interphases was demonstrated. Processing methods for transformable xenotime coatings were explored. For conversion to xenotime during processing, (GdxDy1−x)PO4 solid solutions had to be more Dy‐rich than those for pellets. Single‐crystal alumina fibers were coated with 10–20 μm of (Gd0.4Dy0.6)PO4 xenotime and incorporated into polycrystalline alumina matrices. Coated fiber push‐out stresses were between 10 and 80 MPa, significantly lower than those for fibers with other rare‐earth orthophosphates coatings. Phase transformations and deformation mechanisms were characterized by SEM and TEM in fiber coatings after push out. Bands of deformed coating several micrometers in width formed during fiber push out. Cataclastic flow with fracture, granulation, translation, rotation, and intense plastic deformation of coating grains was observed. Three phase transformations may occur in heavily deformed particles in the deformation band: xenotime → monazite, xenotime → anhydrite, and anhydrite → monazite. Anhydrite was abundant as a fine lamellar phase on (100) planes in xenotime. Selected area electron diffraction and high‐resolution TEM confirmed formation of monazite in a variety of heavily deformed particles. Issues for use of rare‐earth orthophosphate transformation plasticity to lower fiber pull‐out stress in ceramic matrix composites are discussed.

Indentation recovery in GdPO4 and observation of deformation twinning

“…12 In addition to the twin, numerous stacking faults, all along (100) planes, were observed in the deformed region (see supplementary material for addition TEM images). There was no evidence of a phase transformation in the deformed material; GdPO 4 is known to be stable in its monazite form under high pressures [2][3][4] and ambient conditions, and therefore, is unlikely to transform whereas some xenotime REPO 4 's can transform under pressure. 2,4,13 High shear stresses underneath the indentation likely promote twinning, but GdPO 4 is quite sensitive to the presence of shear stresses even in high-pressure diamond anvil cell experiments.…”

“…1 As fiber coatings for CMCs, these materials readily absorb energy by plastically deforming via a variety of mechanisms including dislocation activity, fracture, and twinning, as well as phase transformations for some xenotime materials. 3,6,7 Previous studies in xenotime REPO 4 's and silicon have shown anomalous nanoindentation curve behavior -i.e., large elastic recovery in the unloading portion. 2,8 Hay et al completed nanoindentation studies highlighting this behavior in TbPO 4 , where a distinct variation (elbow) in the slope was observed upon unloading.…”

In situ Raman spectroscopy of pressure‐induced phase transformations in polycrystalline TbPO4, DyPO4, and GdxDy(1−x)PO4

“…Tb 0.5 Gd 0.5 PO 4 has a cation effective rare‐earth radius above that of TbPO 4 but it has a similar monazite transformation pressure of 10.5 GPa . In nonhydrostatic testing, the REPO 4 solid solution Gd 0.4 Dy 0.6 PO 4 was shown to have a lower fiber push‐out stress than DyPO 4 , which also coincides with the solid solution having a larger effective rare‐earth radius . But the hydrostatic transformation behavior and pressures of Gd x Dy (1− x ) PO 4 solid solutions and pure DyPO 4 have not been experimentally documented.…”

“…Certain REPO 4 fiber coatings with the xenotime structure (tetragonal, I 4 1 / amd ) have demonstrated a reduction in fiber push‐out stress compared to other xenotime‐ and monazite‐structured (monoclinic, P 2 1 / n ) REPO 4 fiber coatings. Specifically, Gd 0.4 Dy 0.6 PO 4 fiber coatings demonstrated significantly lower fiber push‐out stresses than DyPO 4 and LaPO 4 fiber coatings . Hay et al.…”

The Effect of Milling Time on the Microstructural Characteristics and Strengthening Mechanisms of NiMo-SiC Alloys Prepared via Powder Metallurgy

“…The zone axis for the Ni matrix is identified as [130] (Figure 9d), while the spots for the nano-precipitate are close to the simulation patterns of Ni 3 Si in the zone axis of [123] (Figure 9e). The simulation patterns for Ni and Ni 3 Si were performed using Singlecrystal™ software (CrystalMaker Software Limited, Oxfordshire, UK) [22,23]. Figure 9f presents the simulation of the combined NBD patterns for NiMo matrix and Ni 3 Si nano-particle in the zone axis of [130] and [132] respectively.…”