Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Fe-based oxide dispersion strengthened (ODS) alloys are widely used in advanced aircrafts and gas turbine engines due to their good high temperature strength, creep properties and hot-corrosion resistance. Traditionally, ODS alloys are prepared by internal oxidation and mechanical alloying. However, internal oxidation cannot be applied to multi-component alloys. It is difficult to guarantee other elements from being oxidized. On the other hand, the use of mechanical alloying will bring in impurities in the process of ball milling which will compromise the purification of alloy particles surface. In this work, TiO2 xerogel prepared by using sol-gel method was added to the thermite powder mixture and the mixture was then ignited by using a tungsten filament. It solidified rapidly after the molten metal flowed into the bottom of the graphite mold because of the gravity field. It was found that Al2O3 and NiAl were formed in situ in the molten metal. Therefore, Al2O3 nanoparticles and NiAl reinforced Febased ODS alloy could be prepared by using this method. The phase composition and morphology of the Fe-based ODS alloy were investigated by using the combination of OM, SEM, TEM, XRD. The size of Al2O3 nanoparticles and the influence of Brownian motion and interface energy on the distribution and movement of the Al2O3 nanoparticles were investigated. The mechanical properties of the Fe-based ODS alloy with different contents of TiO2 xerogel was investigated by using mechanical properties testing machine. The experimental results show that the Febased ODS alloy consists of ferrite a-FeNiCrAl, NiAl, and Al2O3 nanoparticles. The diameter of Al2O3 nanoparticles is approximately 10 nm. Both Brownian motion and interface energy affect the motion of Al2O3 nanoparticles during the solidification, however, interface energy is dominant. The interface energy between Al2O3 nanoparticles and NiAl is lower than that of Al2O3 and ferrite a-FeNiCrAl. Therefore, nearly all the Al2O3 nanoparticles are connected with the NiAl phase. Higher TiO2 xerogel additions increase the tensile strengthen and elongation of the Febased ODS alloy. When the content of TiO2 xerogel is 1.24%, the tensile strength of the Fe-based ODS alloy attains 849 MPa and the elongation is 13%. Continuing adding the TiO2 xerogel results in the release of large quantities of gas which produces holes in the Fe-based ODS alloy and these holes decrease the mechanical properties of the alloy.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The excellent thermal conductivity, low thermal expansion and high oxidation resistance of ferritic FeNiCrAl alloys, provide them with the potential to be replacements for nickel-based superalloys in high-temperature applications. However, their usage is limited, because of their poor high- temperature mechanical properties. The high melting point of NiAl intermetallic compounds, together with their excellent high temperature stability and similar lattice parameters to a-Fe, allow them to be used to coherently strengthen ferritic FeNiCrAl alloys to extend their high-temperature performance. Traditionally, these Fe(Ni, Cr)/NiAl alloys are prepared by vacuum reaction melting followed by an aging process. But the aging process has drawbacks including excessive cost, the length of aging time required and coarsening of the NiAl phase at high temperature. A more cost-effective thermite reaction process, was tried to prepare the Fe(Ni, Cr)/NiAl alloys. In this route, ferrite FeNiCrAl alloys were strengthened by a high volume fraction nanoscale-NiAl phase which was achieved without using the aging process. Several types of thermites were designed and studies were conducted to explore the transformations of the alloy microstructures and the changes of the tensile properties with the various thermite compositions. The microstructures of these thermites synthesized Fe(Ni, Cr)/NiAl alloys were investigated using XRD, SEM, EDS, TEM and SAED. The effect of Al content in the thermites on the microstructures of the alloys was studied. Experimental results showed that when the thermites contained no more than 25.4% (mass fraction) of Al, the synthesized Fe(Ni, Cr)/Ni-Al alloys were composed primarily of an austenite phase. The main component phase of the alloy composites was transformed into ferrite when the mass fraction of Al in the thermites was 26.6%, meanwhile the NiAl particle precipitates arose. As the Al content of the mixture was further increased, the NiAl precipitates were gradually replaced by an intertexture structure. The intertexture structure was totally dominant when the mass fraction of Al in the thermites was 31.4%. Experimental results showed that this intertexture microstructure material was composed of a ferritic FeNiCrAl matrix with a width of 80~100 nm and NiAl precipitates with a width of about 50 nm, and the two phases matched coherently. This microstructure resulted from liquid spinodal decomposition. The effect of Al content on the mechanical properties of the alloys was also investigated. The increase of the Al content in the thermites resulted in a decrease of the elongation of the alloys, which varied from 25.5% to 1.7% when the mass fraction of Al ranged from 24.2% to 29.0%. When the thermites contained 26.6% mass fraction of Al, the tensile strength of the alloy achieved its maximum value of 640.87 MPa.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： According to the requirement of high-pressure turbine guide vane during service, the aim of this work is to design a single crystal Ni3Al-based alloy named IC21 with low density, low cost, and high strength which can be used as high-pressure turbine guide vane material. The mass fraction of the Re has been limited less than 1.5% on purpose. The single crystal bars of IC21 were prepared by high rate solidification method. The density of IC21 is 8.0 g/cm3 and the incipient melting temperature was identified by metallography. After standard heat treatment, the distribution of the g' precipitates is uniform with the average size of about 420 nm, and volume fraction of 80%. The tensile and yield strengths at 1100 ℃ are 490 and 470 MPa, respectively. Moreover, IC21 shows superior creep properties, the stress-rupture life at 1100 ℃,140 MPa is 170.5 h and at 1150 ℃, 100 MPa still remains 110.0 h. The microstructure stability of IC21 alloy at 1080 ℃ for as long as 1000 h were evaluated. The results show that no precipitated phase exists during thermal exposure at 1080 ℃, which exhibits good stability. The oxidation kinetic curves of IC21 alloy follows a parabolic rate law in different oxidation stage during cycle oxidation for 100 h in air. IC21 alloy has a good high temperature oxidation resistance, the strengthening mechanism are attributed to high volume fraction of g' phase, large negative misfit and well-established interface networks.
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