Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The Ni- based superalloys are widely used as microstructural components of modern turbine engines due to its good high temperature strength, good fatigue and creep property and excellent hot-corrosion resistance. In order to increase their high temperature strength, more and more refractory elements, such as W and Mo, are added into these alloys while Cr content gradually decreases. During long-term aging, these alloys generally experience various microstructural changes, including coarsening of g' phase coarsening, formation of a continuous grain boundary (GB) carbide network, precipitation topologically close-packed (TCP) phase, and degeneration of MC carbide. However, there is limited available data about the effect of (W+Mo)/Cr ratios on the microstructural evolution of Ni- based superalloys. In this work, the cast Ni- based superalloys with different (W+Mo)/Cr ratios (mass ratios) are fabricated by vacuum induction furnace. After standard heat treated (1110 ℃, 4.5 h, air cooling+ 750 ℃, 10.5 h, air cooling), they are thermally exposed at 850 ℃ for different times. The stress-rupture tests are operated under the condition of 800 ℃, 294 MPa. Effects of (W+Mo)/Cr ratios on the microstructure evolutions and mechanical properties are investigated by the combination of OM, SEM, TEM and stress-rupture tests. The experiment results show that the (W+Mo)/Cr ratio has no obvious influence on the standard heat treated microstructure, which is mainly composed of g matrix, g' phase, MC carbide and secondary carbides distributing at grain boundaries. During long-term thermal exposure, the microstructure evolutions occur by g' phase coarsening, TCP phases formation, MC degeneration and grain boundary coarsening. The g' phase coarsening behavior is not affected obviously by the (W+Mo)/Cr ratio. However, the amount of TCP phases decreases significantly with decreasing of (W+Mo)/Cr ratio and the type of TCP phases transforms from m phase to coexist of m and s phases when (W+Mo)/Cr ratio decreases from 0.55 to 0.37. There are no TCP phases observed in the sample with (W+Mo)/Cr ratio of 0.22. The thermal stability of MC carbide is reduced obviously and the grain boundaries coarsen more severely by the decrease of (W+Mo)/Cr ratio. The degradation of stress-rupture property is attributed to the coarsening of g'phase and grain boundaries and the formation of TCP phases. Combined with the effect of (W+Mo)/Cr ratio on the solid solution strengthening, microstructure evolution and stress-rupture property, it can be concluded that the optimumstress-rupture property can be obtained when the (W+Mo)/Cr ratio is about 0.37.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： This paper reviews the research works of effects of elements on the microstructure and mechanical properties of Nb-Si alloys conducted by the authors in recent years, including effect of Hf on Ni-16Si, Hf and Sn on Ni-20Ti-5Cr-3Al-18Si, Zr on Ni-22Ti-16Si, Ta on Nb-22Ti-16Si-7Cr-3Al, and rare earth elements Dy and Ho on Ni-23Ti-10Ta-2Cr-18Si and Ni-22Ti-16Si-7Cr-3Al-3Ta-2Hf, respectively. The addition of elements Hf, Zr, Sn+Hf, Ta, Dy and Ho in Nb-Si binary system and multi-component system enhances room and high temperature strength, plasticity and fracture toughness obviously. The enhancement of strength is related with the solution strengthening of elements, and the improvement of plasticity and ductility is related with the fining of microstructure and the increase of particles which exceed the critical size of (Nb, Ti)ss.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： NiAl base eutectic alloy is an attractive material and promising to use in high temperature environment. However, the inadequate high temperature strength limits its application. In order to improve its strength, Zr was added in the Ti and Hf doped NiAl/Cr(Mo) base eutectic alloys and the effect of Zr addition on microstructure and mechanical properties of the eutectic alloy was investigated in this work. The results show that small addition of Zr can refine the NiAl/Cr(Mo) lamella inside the eutectic cell and optimize NiAl and Cr(Mo) phase morphology in the intercellular zone. Moreover, the Zr addition promotes the precipitation of bulk Heusler phase along eutectic cell boundary.With the increase of Zr addition, the eutectic cell of the alloys becomes fine, but the NiAl and Cr(Mo) phases in the intercellular zone become coarse and the Heusler phases exhibits semi-continuously distribution along the eutectic cell boundary. When the Zr content increases to 1% (atomic fraction), the NiAl and Cr(Mo) phases in eutectic cell and intercellular zone are all coarsened obviously. Additionally, coarse Cr- rich phases precipitate in the intercellular zone and Heusler phase forms the continuous network along eutectic cell boundary. The addition of Zr promotes the precipitation of coarse b-NiAl and a-Cr phase in Cr(Mo) phase and NiAl phases, respectively. Moreover, the segregation of Heusler phase forming elements along the precipitate interface leads to the formation of a large number of interfacial dislocations. In addition, the addition of Zr results in the precipitation of fine Heusler particles in NiAl phase. It is shown that appropriate addition of Zr can improve the compression strength of Ni-33Al-28Cr-5.5Mo-1.0Ti-0.3Hf eutectic alloys significantly at room temperature and high temperature without reducing its compression plasticity, but more addition of Zr reduces the compressive plastic of the alloy inevitably.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-18 Cooperative journals: 《材料研究学报》
Abstract： The corrosion wear properties of NiAl-2.5Ta-7.5Cr-x(0, 1)B alloys in 5% H2SO4 solution have been investigated at room temperature. The results show that in comparison with NiAl-2.5Ta-7.5Cr alloy, NiAl-2.5Ta-7.5Cr-1B alloy exhibited better corrosion resistance to 5% H2SO4 solution with passive current density 0.299(μA·cm-2)and free corrosion potential -0.213 V. For which the key factor is that a passive film could form on the surface of NiAl-2.5Ta-7.5Cr-1B alloy. The corrosion wear rate of NiAl-2.5Ta-7.5Cr-1B alloy decreased 2-8 times compared with NiAl-2.5Ta-7.5Cr alloy. The abrasion erosion mechanism for NiAl-7.5Cr-2.5Ta-1B alloy was dominated by the synergistic effect of corrosion wear and protectiveness of passive film. While that for NiAl-7.5Cr-2.5Ta alloy was mainly corrosion wear and abrasive wear.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：对合金长期时效过程中析出σ相的析出规律和机制进行了综述，研究了不同热处理条件下σ相的回溶规律及其对合金持久性能的影响.研究发现，在800-900℃范围内经过最长到1×104h时效后，合金中产生的σ相率先在枝晶干的M23C6碳化物附近形成，之后扩展到枝晶间；随着时效温度的升高，σ相形成速度加快，σ相形核的孕育时间缩短.激活能计算结果对比表明，σ相形成初期与Co、Cr的扩散相关，稳态阶段与Mo的扩散相关；长期时效后合金在1000-1170℃固溶时，σ相都可以回溶到基体，且固溶温度越高，σ相回溶越快.σ相的回溶动力学研究表明，σ相的回溶速度受Co的扩散过程控制.对比持久实验结果表明，合金中的σ相并不能使合金变脆；经过恢复热处理，长期时效过程中析出的σ相回溶，持久寿命提高.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-04 Cooperative journals: 《金属学报》
Abstract：本文研究了两种Ti/Al比对新型Ni-Fe-Cr基合金GH984G在长达上万小时高温时效过程中γ′沉淀相的粗化行为及其力学性能的影响规律. 结果表明：随时效温度从700℃升高至800℃，球形γ′沉淀相的粗化速率明显增大. 在700℃和750℃长期时效过程中，高Ti/Al比和低Ti/Al比合金γ′沉淀相的粗化行为均符合Lifshitz-Slyozof-Wagner (LSW) 理论，受扩散过程控制，高Ti/Al比合金中γ′沉淀相的粗化速率较高. 然而，800℃长期时效过程中，两种Ti/Al比合金γ′沉淀相的粗化行为偏离LSW理论，此外，时效时间小于3000h时，高Ti/Al合金的γ′沉淀相长大较快，进一步延长时效时间，低Ti/Al比合金的γ′沉淀相长大速率较快. Ti/Al比对合金标准热处理态和700~800℃一万小时时效后合金的700℃拉伸性能无明显影响. 通过选取合适的Ti/Al比，可以控制γ′沉淀相的粗化行为，增强合金组织稳定性.
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