Evolution of the Structural and Phase Composition of Ni–Ti–Cu Alloy Produced via Spark Plasma Sintering After Aging

Aubakirova D. Sagymbekova E. Kozhakhmetov Y. Dauletkhanov Y. Urkunbay A. Yerbolat D. Kowalewski P. Tabiyeva Y.
November 2025 Multidisciplinary Digital Publishing Institute (MDPI)

Crystals
2025 #15 Issue 11

This study investigates the control of the phase-structural state in Ni–45Ti–xCu (x = 5, 7 at.%) shape memory alloys fabricated via a shortened powder metallurgy route: mechanical activation → spark plasma sintering (SPS) → heat treatment. Compact samples were produced from mechanically alloyed powders (650–750 rpm, up to 5 h) and sintered at 900 °C. The structure and microstructure were characterized using X-ray diffraction (to identify B2/B19′/Ni₄Ti₃ phases and assess ordering) and SEM–BSE/EDS (to analyze morphology, porosity, and Ni-rich precipitates). Two post-processing treatments were applied: single-stage annealing (500 °C, 2 h) and a three-stage treatment (900 °C/30 min → water quenching → 300 °C/20 min). Mechanical alloying transformed the initial elemental powder mixture (fcc-Ni, hcp-Ti, fcc-Cu) into a supersaturated fcc-(Ni, Cu, Ti) solid solution with emerging NiTi phases, with a minimum particle size achieved after ~300 min at 750 rpm. SPS compaction yielded a high-density matrix consisting predominantly of the B2 phase. Single-stage annealing preserved B19′ martensite and Ni₄Ti₃ precipitates, particularly in the 5 at.% Cu alloy. In contrast, the three-stage treatment dissolved the Ni₄Ti₃ precipitates, suppressed the formation of B19′ and R phases, and stabilized a highly ordered B2 matrix. Increasing the Cu content from 5 to 7 at.% significantly enhanced the B2 phase fraction, reduced secondary nickel-rich phases, and improved structural homogeneity, evidenced by a continuous neck network and closed porosity. The optimized condition—7 at.% Cu combined with the three-stage annealing—produced a microstructure with >95% B2 phase, <1% Ni₄Ti₃, and ~98% relative density. This forms the prerequisite microstructural state for a narrow transformation hysteresis and high functional cyclic stability.

aging treatment , mechanosynthesis , microstructure , Ni–Ti–Cu , phase composition , powder metallurgy , shape memory , sintering , spark plasma sintering (SPS) , thermal hysteresis

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Center of Excellence “VERITAS”, D. Serikbayev East Kazakhstan Technical University, Ust-Kamenogorsk, 070004, Kazakhstan
Faculty of Mechanical Engineering, Department of Fundamentals of Machine Design and Mechatronic Systems, Wroclaw University of Science and Technology, Wroclaw, 50-370, Poland

Center of Excellence “VERITAS”
Faculty of Mechanical Engineering

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