The Exploration Of Metal Materials With Negative Thermal Expansion

In this study, NiTi-based and FeNiCo-based alloys were investigated to see their possibility as NTE (Negative thermal expansion) metals, with regards to their microstructure, phase transformation behaviors, mechanical and NTE properties. The obtained results in this study are helpful for the development of NTE mechanisms and will also provide practical guidance for exploring and developing new NTE materials with wide temperature range.The results showed that the Ti_46.3Ni_44.7Nb₉ alloy with the reduction of 5%, 7% and 9% by cold rolling at room temperature exhibited NTE effect obviously in the temperature range of -130-130℃along rolled direction. And the coefficient of thermal expansion (CTE) were -38.044×10^-6℃^-1, -22.921×10^-6℃^-1 and -22.684×10^-6℃^-1, respectively. However, this sample exhibited the normal thermal expansion with a CTE of 7.378×10^-6℃^-1 after hot rolling. Ti₄₄Ni₄₇Nb₉ alloy with the reduction of 5%, 7% and 9% by cold rolling at liquid nitrogen or solid-state carbon dioxide temperature showed obvious shrinkage during heating from -140℃to 60℃. And the CTE reached -15.100×10^-6℃^-1. In the TiNiNb alloy, the internal stress field caused by stress induced martensites after cold rolling efficiently controlled the phase transitions from B2 to B19'.It was also found that Ni_50.6Ti_49.4 alloy aged at 350℃for 20h exhibited the normal thermal expansion with a CTE of 7.6×10^-6℃^-1 from -50℃to 200℃at a constraint stress of 200 MPa. When the constraint stress was up to 400MPa, the similar result was obtained except the temperature range of 60-75℃. The reasons may be that the specific orientation R phase or B19' phase, which was controlled by aligned particles of Ti₃Ni₄ precipitate, was absent due to the formation of the internal stress field caused by reorientation of the matensitic with a constraint stress. Therefore, this alloy didn't exhibit NTE behavior after constraint aging.The NTE behaviors of FeNiCo-based alloys, including Fe_64.8Ni₃₁Co_4.2, Fe_63.5Ni_32.5Co₄, Fe_63.3Ni_32.5Co₄Nb_0.2, Fe_63.8Ni₃₂Co₄Ta_0.2 and Fe_63.8Ni₃₂Co₄Zr_0.2 were also investigated. All these alloys exhibit NTE behaviors with a CTE about (-0.218--3.506)×10^-6℃^-1 in the temperature range of 30- 100℃. The reasons may be that the normal thermal expansion which was caused by the atomic thermal vibration was offseted by the magneto-volume effect. In addition, it was also found that the NTE properties of FeNiCo-based alloys can be affected by several other factors, such as cooling speed, carbide forming elements, hydrogen and so on.