| Summary: | 碩士 === 逢甲大學 === 材料科學與工程學系 === 103 === In this study, the effects of Ni content on crystal structure,
microstructure, mechanical property, and corrosive property in
CoCrFeNixTi0.3 (x = 0.3, 0.5, 0.7, 1.0, 1.5, and 1.7, in molar ratio)
high-entropy alloys denoted as Ni-0.3, Ni-0.5, Ni-0.7, Ni-1.0, Ni-1.5, and
Ni-1.7 were investigated. In addition, the as-cast alloys were performed
aging treatments at 500~1000℃ for 24, 48, 72, and 144 hours to analyze
the variations of phase constitution, microstructure, and hardness.
With the increases of Ni content from 0.3 to 1.7 moles, the ζ phase in
interdendrite gradually decreases, and another TCP phase is formed. This
TCP phase in Ni-0.3 and Ni-0.5 alloys is Laves phase, and Ni-1.0 to
Ni-1.7 alloys is R phase. The hardness of alloys decreases with the
increase of Ni content due to the decrease of volume fraction of the TCP
phases in interdendrite and the decrease of the content of large atom Ti.
These weaken both effects of second-phase strengthening and solid
solution strengthening, resulting in the decrease of alloy hardness.
The yield strength (ζy) and compressive strength (ζmax) of as-cast
alloys decrease with the increase of Ni content. The ζy decreases from
945 MPa (Ni-0.3) to 550 MPa (Ni-1.7). Both Ni-0.3 and Ni-0.5 alloys are
early failure during compressive tests before reaching their ζmax. For
Ni-0.7 to Ni-1.7 alloys, the ζmax decreases from 1458MPa to 1301MPa.
However, the fracture strain (εf) increases from 0.12 to 0.72 for Ni-0.3 to
Ni-1.7 alloys. The results of electrochemical corrosion tests show that the
CoCrFeNixTi0.3 高熵合金微結構與性質研究
VI
逢甲大學e-Theses & Dissertations（103 學年度）
corrosion resistance of alloy increases with the increase of Ni content,
and no pitting occurs on the surface of all alloys after tests.
After aging at 700℃, the hardness of Ni-0.3 alloy increases sharply
due to the precipitation of abundant ζ+FCC eutectic phases in
interdendrite. After 800℃ aging, although the ζ phase slightly decreases,
the dendrite still possesses the solution strengthening effect caused by the
element Ti. This makes the alloy hardness similar to 700℃ aging. After
900-1000 ℃ aging, the hardness decreases greatly because of the
dissolution of ζ phase.
For Ni-0.5 and Ni-0.7 alloys after 800 ℃ aging, the hardness
increases dramatically due to the precipitation of profuse needle-like ζ
phase in matrix. After 900-1000℃ aging, the hardness decreases because
both the R and ζ phase coarsen and dissolve into matrix. However, a
phase transformation of Laves phase to R phase appears for Ni-0.5 and
Ni-0.7 alloys at 800℃ and 700℃ aging, respectively.
For Ni-1.0 alloy after 500-700℃ aging, the hardness increases with
the increase of temperature due to the precipitation of R+ζ phases in
interdendrite. After 800-1000℃ aging, the hardness decreases because
the R+ζ phases coarsen and then dissolve into matrix.
For Ni-1.5 and Ni-1.7 alloy after 500-700℃ aging, the hardness
increases with the increase of temperature due to the precipitation of R
phase in interdendrite. After 800-1000℃ aging, the hardness decreases
because the R phase coarsens and then dissolves into matrix.
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