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The SPHC/MgO composites were fabricated by friction stir welding (FSW) and then the mechanical property of the composites was investigated. MgO particles were flocculated on the advancing side in the stir zone (SZ) formed by one-pass processing. On the other hand, MgO particles were more homogenously distributed in the SZ formed by two-pass processing. Compared to the SZ without MgO particles, the hardness profile of the SZ with MgO particles was increased to 20–30 Hv due to refined grain by pinning effect. Grain growth in the SZ without MgO particles was observed at elevated temperature. On the other hand, the fine grain produced with MgO particles was maintained at elevated temperature (∼900℃) due to pinning effect by MgO particles. [ABSTRACT FROM AUTHOR]