Lijing Zheng

High temperature tensile properties of directionally solidified Ni-43Ti-4Al-2Nb-2Hf alloy

By liquid metal cooling (LMC) process, the Ni-43Ti-4Al-2Nb-2Hf (%, atomic fraction) alloy was directionally solidified (DS). The microstructure and tensile properties at room and elevated temperature were investigated. It was found that the DS process significantly improves the room temperature tensile strength, increasing by 70% compared with the as-cast alloy. After appropriate heat treatment (HT), the average tensile strength reaches above 1900 MPa, nearly twice of the as-cast one. At 800 and 900 °C, the tensile strengths are about 308 and 169 MPa, respectively.

Microstructural evolution of a PM TiAl alloy during heat treatment in α+γ phase field

In this study, the effect of temperatures and cooling rates of heat treatment on the microstructure of a powder metallurgy (PM) Ti-46Al-2Cr-2Nb-(B,W) (at.%) alloy was studied. Depending on the cooling rate and temperature, the different structures were obtained from the initial near-γ (NG) microstructures by heat treatment in the α+γ field. The results show that the microstructures of samples after furnace cooling (FC) consist primarily of equiaxed γ and α2 grains, with a few grains containing lamellae. Duplex microstructures consist mainly of γ grains and lamellar colonies were obtained in the quenching into another furnace at 900°C (QFC) samples. However, further increasing of the cooling rate to air cooling (AC) induces the transformation of α→α2 and results in a microstructure with equiaxed γ and α2 grains, and no lamellar colonies are found.

Microstructural characteristics of directionally solidified Ni-43Ti-7Al alloys

Abstract Ni–43Ti–7Al (at-%) alloy was directionally solidified at different withdrawal rates (2, 20 and 100 μm s−1) and a constant temperature of 1550°C by liquid metal cooling method. Results show that as the withdrawal rate decreases from 100 to 2 μm s−1, the cellular arm spacing increases from 39·5 to 126 μm, the size of Ti2Ni and the stability of the liquid/solid interface also increase, while the volume fraction of Ti2Ni decreases from 3·1 to 0·9%. Moreover, microstructural analysis reveals that a NiTi+Ti2Ni anomalous eutectic structure is formed in intercellular regions of directionally solidified samples withdrawn at 20 and 100 μm s−1. However, in the sample withdrawn at 2 μm s−1, Ti2Ni phases represent strip and liquid droplet morphologies in the intercellular region. Finally, the possible explanation to the change of microstructure is discussed.

Microstructure and mechanical properties of NiTi-Al based alloys with addition of Fe

Abstract The influences of Fe on the microstructure and mechanical properties of Ni–43Ti–4Al–2Nb–2Hf–xFe (x = 0, 1, 3, 5 at-%) alloys were investigated, prepared by vacuum non-consumable arc-melting method. The results show that the microstructure of the alloys is refined gradually with increasing Fe content. The appropriate addition of Fe produces an improvement of the tensile strength and ductility of the alloy. The highest tensile strength and elongation of the study’s alloys was seen with the 3 at-%Fe addition, resulting in strength and elongation of up to 1308 MPa and 4·15%. The fracture characterisation changes from a quasi-cleavage fracture to a mixed fracture of quasi-cleavage and dimple. Further increasing the Fe content to 5 at-%, the mechanical properties deteriorated.

Precipitation behaviour of oxide in directionally solidified aerospace bearing steel

The directional solidification enables independently control of the holding temperature and withdrawal rate over a given range. This technology was used to investigate the precipitation behaviour of oxide in aerospace bearing steel M50NiL. Most of the oxides are Si–Al–Mn–Ti–O complex inclusions with spherical shape. With the increase of withdrawal rate, the size and volume fraction of oxides decrease. In contrast, the diameter and volume fraction increase remarkably with the increase of holding temperature. The composition of oxides with different sizes varies with holding temperature. The holding temperature of 1550°C accompanied with a cooling rate over 95.7°C min−1 is deemed as the optimal solidification parameters for M50NiL steel, which can significantly reduce the size and volume fraction of oxides.