Processing dense hetero-nanostructured metallic materials by spark plasma sintering

doi:10.1016/j.scriptamat.2007.05.022

Scripta Materialia

Volume 57, Issue 6, September 2007, Pages 525-528

https://doi.org/10.1016/j.scriptamat.2007.05.022 Get rights and content

Fully dense hetero-nanostructured (i.e. containing intermixed nano and micrometric grains) metallic (FeAl) materials can be successfully produced by spark plasma sintering (SPS) when mechanically milled nanostructured powders are used. This is due to the large temperature differences generated during the SPS runs, which modify locally the recrystallization response of the heavily deformed structure during sintering. Y₂O₃ addition delays the onset of recrystallization and, thereby, produces microstructures containing more than 80% of sub-500 nm grains.

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The strength and ductility trade-off is a long-standing issue of single face-center-cubic (FCC) high entropy alloys (HEAs). In the present work, the in-situ formation of heterogeneous grain structure coupled with the introduction of Y₂O₃ nanoparticles in FCC structure Ni₂₆Co₂₆Fe₂₅Cu₁₇Ti₆ HEA is proposed as a strategy to enhance the strength and ductility synergy. The oxide dispersion strengthened (ODS) Ni₂₆Co₂₆Fe₂₅Cu₁₇Ti₆ HEAs were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) methods and the influences of different Y₂O₃ additions on the evolutions of oxide precipitates, heterogeneous microstructure and mechanical properties of ODS-HEAs were systematically investigated. The results show that the introduction of Y₂O₃ facilitates the new precipitation of finer and semi-coherent Y₂Ti₂O₇ oxide nanoparticles, in addition to the protogenous TiO in the Ti-containing HEA matrix. All the studied ODS-HEAs display the bimodal grain microstructure, in which numerous oxide dispersoids are distributed exclusively in the fine-grained (FG) region. With the increase of Y₂O₃ addition, the average grain size of FG region first decreases and then increases in ODS-HEA, while the proportion of FG region shows the opposite trend. When the content of Y₂O₃ is 1.05 wt%, the yield strength, ultimate tensile strength and elongation to fracture of ODS-HEA at room temperature are 21%, 27% and 157%, respectively, higher than those of Ni₂₆Co₂₆Fe₂₅Cu₁₇Ti₆ base alloy. The simultaneous enhancement of strength and ductility in ODS-HEA by introducing a proper amount of Y₂O₃ is mainly attributed to the additional precipitation strengthening contribution of generated Y₂Ti₂O₇ oxide nanoparticles having semi-coherent interfaces with the FCC matrix.
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Citation Excerpt :
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The conflict between strength and ductility has been a longstanding dilemma for metallic alloys. Herein, we present a facile strategy to overcome the strength-ductility trade-off of face-center-cubic high-entropy alloys (HEAs) via the in-situ formation of heterogeneous grain structure, using the high local temperature gradients generated during spark plasma sintering coupled with the ‘sluggish diffusion’ of HEAs. The fabricated CoCrFeMnNi HEA has a heterogeneous structure over a large range of grain size from 140 nm to 5 μm, which enables a gigapascal yield strength and a uniform elongation of 9.5%. The mechanism responsible for the associated mechanical properties was analyzed.
Fabrication and characterization of nano-Y<inf>2</inf>O<inf>3</inf>, Al<inf>2</inf>O<inf>3</inf>, La<inf>2</inf>O<inf>3</inf> dispersed mechanically alloyed and liquid phase sintered W[sbnd]Ni for structural application
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The investigation involves the fabrication of various nano oxide (Y₂O₃, Al₂O₃, La₂O₃) dispersed WNi alloys by mechanical alloying for 10 h and sintering in Ar atmosphere at 1400 °C, 1500 °C for 2 h. The selected composition for the study is W₈₉Ni₁₀(Y₂O₃)₁ (alloy A), W₈₉Ni₁₀(Al₂O₃)₁ (alloy B) and W₈₉Ni₁₀(La₂O₃)₁ (alloy C) (in weight%). Alloy A exhibit least crystallite size and maximum lattice strain at 10 h of milling. The lattice parameter of W exhibits expansion and contraction behavior during the milling. Microstructure of sintered alloys reveals the presence of both faceted and nearly spherical shaped grains. Bimodal grain size distribution is higher in alloy A at 1500 °C as compared to other alloys and 1400 °C sintering temperature. Texture study and Young's Modulus value reveals that hardness of alloy A is higher against alloy B and alloy C. Maximum % relative sintered density, mean hardness, compressive strength of 89%, 5.53 GPa, 2.25 GPa respectively has been achieved in alloy A at 1500 °C.
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Intermetallic iron aluminide alloys show various advantages such as low material cost and a good creep resistance that is superior to the P92 martensitic-ferritic steel at 650 °C, nominating them as potential candidates for steam turbine applications. However, cast preforms for hot forging show a rather coarse microstructure and limited workability. Recent research thus introduced Fe-25Al-Ta alloys, where tantalum prevents excessive grain growth during solidification. This paper, for the first time, investigates the possibility of producing the Fe-25Al-1.5Ta (at.%) alloy by spark plasma sintering (SPS) from pre-alloyed powder particles and investigates its deformation behavior under compression in the temperature range of 900–1100 °C using the concept of processing maps. SPS is mainly used to analyze the possibility of inheriting the size of the initial powder particles into the sintered material.
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