| 其他摘要 | The ordered intermetallic compound NiAl has been paid more attention to as a potential candidate for high temperature structural utilizations because of its high melting point (Tm=1921 K), substantially lower density (5.9 g/cm3) than commercial Ni-based superalloys (about 8 g/cm3), high thermal conductivity (above 6 W/m•K), and excellent oxidation resistance at temperature above 1273 K. However, the industrial applications of NiAl alloy are limited by two major drawbacks. One is poor strength and creep resistance at high temperature; the other one is the serious scarcity in fracture toughness and ductility at room temperature. Therefore, much effort has been devoted to solve the above two problems. It has been established that alloying and directional solidification are effective ways to improve mechanical proverties of intermetallics. So the paper systematically investigate the effects of alloying and directional solidification on microstructure and mechanical properties of NiAl alloys by means of X-ray diffaction, scanning electron microscope and transmission electron microscope,etc.
The multi-phases NiAl alloy with nominal composition Ni-26.6Al-13.4Cr-8.1Co- -4.3Ti-1.3W-0.9Mo (at. %) was fabricated from superalloy K444 and Al element using vacuum induction and casting technique. Investigations to this alloy reveal that a new phase Cr3Ni2 possessing low melting point and poor ductility is formed, which is distributed as a network along NiAl matrix grain boundaries. Subsequent solution are carried out and lead to microstructural changes to various extents, such as the partial dissolve of Cr3Ni2 phase. Rapid cooling (water quenching) after solution at1523 K for 20 hours gives rise to macrocracks in the specimen while slow cooling (furnace cooling) after the same treatment results in the formation of spheric α-Cr solid solution (d, 300-1000 nm) and needle-like Ni3Al phase (l, the order of 1μm), which are embedded in NiAl matrix. During aging treatment, needle-like Ni3Al (l, the order of 10 μm) and small spherical α-Cr particles were precipitated from NiAl matrix which owns orientation relationships with these precipitates such as [00 ]β∥[ 10]γ′ and (110)β∥(111)γ′. The multi-phases NiAl alloy after aging treatment possessed the perfect combination of compression properties and fracture toughness at room temperature due to the dissolution of brittle Cr3Ni2 phase network and the precipitation of ductile Ni3Al phase. Then the long-term aging exposure resulted in the dissolution of the Ni3Al precipitates which worsen the room temperature mechanical properties and the coarsening of α-Cr particles that improved the compressive strength at 1273 K.
The effect of Sc addition on the microstructure and mechanical properties of cast NiAl and NiAl-Cr(Mo)-0.15Hf alloys was studied. The solid solubility of Sc in NiAl alloy is about 0.05-0.06 (at. %). A Sc-rich second phase precipitates in the NiAl alloys doped with more than 0.10 wt. % Sc. The strength and microhardness of NiAl alloys increase with increasing Sc content. As for the NiAl-Cr(Mo)-0.15Hf alloy, the interlamellar spacing and the intercellular spacing decrease with the increasing Sc content when the additional Sc content is less than 0.1 wt. %. The microstructural refinement leads to the improvement of room temperature compressive properties. Excess Sc breaks the typical NiAl/Cr(Mo) eutectic cell structure and plays a decisive role on the deterioration of compressive properties at room temperature. NiAl-Cr(Mo)-0.15Hf alloy doped with 0.10 wt. % Sc possesses the best ductility of about 35% and the highest compressive stress of 1600 MPa at room temperature. Therefore, 0.10 wt. % Sc addition is speculated as the appropriate amount to improve the room temperature ductility of NiAl-Cr(Mo)-0.15Hf alloy. However, the positive effect of Sc addition is negligible at 1273 K.
The hypoeutectic alloy with nominal composition NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho (at. %) was directionally solidified at three different withdrawal rates by liquid metal (Sn) cooling (LMC) process and conventional radiation solidification process (HRS) in order to choose appropriate withdrawal rate and assess the benefits of liquid metal cooling technique. The hypoeutectic alloy is composed of primary NiAl, NiAl/Cr(Mo) eutectic cell and Hfss. Additional trace elements Hf and Ho led to the appearance of primary NiAl. Moreover, the volume fraction of primary dendritic NiAl increases from 21.1% to 25.9% with increasing withdrawal rate from 3 mm/min to 15 mm/min. In view of the increasing withdrawal rates, the microstructures including the NiAl primary dendrites become fine. The longitudinal tensile strength at RT and 1373 K and RT fracture toughness presents a low value in the DS alloy at 8 mm/min withdrawal rate. Compared by HRS process, LMC process can provide higher thermal gradient and higher cooling rate, which results in the microstructural improvement and the capability for faster withdrawal rate. The higher gradient widens the composition range of coupled zone, therefore, decreases the volume fraction of primary dendritic NiAl. The higher cooling rate restrains the diffusion and promotes refinement of the microstructures including the size of NiAl/Cr(Mo) eutectic cell, the size of primary dendritic NiAl and the arm spacing of the primary dendritic NiAl. The content of contaminative element from the metal-mold reaction also decreases in LMC process. In addition, casting defects such as freckles, misoriented primary dendritic NiAl grains and discontinuity of primary dendritic NiAl grains decrease or even totally disappear in the DS alloy processed by LMC. The microstructural improvement caused by LMC process leads to the improvement of the tensile properties both at room temperature and high temperature. Moreover, the alloy processed by LMC grown at the fastest rate possesses the best tensile properties.
To inhibitate the primary NiAl, the compostion was adjusted as Ni-30.4Al-34Cr-4.3
Mo-0.1Hf-0.05Ho(at. %). The eutectic DS alloy processed by LMC is composed of NiAl/Cr(Mo) eutectic cell and a spot of Hfss. In virtue of the disappearance of primary dendritic phase, the mechanical properties of DS alloys were better than that of as-cast alloy and improved with increasing withdrawal rate from 8 mm/min to 15 mm/min. Compared to the unadjusted hypoeutectic DS alloy with nominal composition NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho (at. %), the eutectic DS alloys possess optimized microstructures and better mechanical properties except high temperature tensile ductibility because of the disappearance of primary dendritic NiAl. |
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