Formation, Magnetic Properties And Phase Transition Of Gd-Al-TM Bulk Metallic Glasses

Turnbull and his co-workers suggested that the supercooled liquids could affect the formation of metal glasses and thus predicted the possible formation of metallic glasses based on their research work on the glasses since 1958. Amorphous alloys have the typical structure of long-range disordered but short-range ordered atomic characteristics. And therefore, the materials have attracted increasing interests due to their unique properties including ultra-high strength, hardness, toughness, wear resistance, corrosion resistance, high electric resistance as well as excellent soft magnetic properties, superconductivity and low magnetic loss. Amongst these materials, the RE-TM metallic glasses have been widely used as new magnetism, stores hydrogen, high temperature superconductivity and other magnetic functional materials due to their unique properties.As an important magnetic material, Gd and Gd-based alloys exhibit excellent magnetic caloric effect (MCE). The alloys with giant MCE (especially at room temperature) could be applied as functional materials because magnetic refrigeration technology based on MCE is regarded as an environment-safe technology and a promising alternative to conventional vapor-cycle refrigeration. In 2001, Ames laboratory had firstly reported the room temperature magnetism refrigerator by using Gd as magnetism refrigeration substance under a permanent-magnet (PM) circumstance. Recent research work on this topic mostly focus on the high MCE of single-phase intermetallic compounds as RA₁₂(R=Er,Ho,Dy), RNi₂(R=Gd,Dy,Ho), RAlNi, Gd₅(SiGe)₄, La(Fe,P,Si)₁₂ and so on. However, many urgent problems including easy oxidation and hard preparation should be solved before these single-phase intermetallic compounds are employed in magnetic refrigeration. Hence, for further application, several features of these materials, such as large total angular momentum number, small fraction of lattice entropy, large electric resistance and fine molding and processing behaviors, are required. Amorphous materials with high glass forming ability (GFA) and MCE, which have unique properties such as high corrosion resistance and electric resistivity, can be easily prepared into bulk samples by copper mold casting and could be attractive for application as magnetic refrigerants.In this work, the glass forming ability, thermal stability, phase transformation and magnetic caloric effects of the Gd-Al-TM bulk metallic glassy system alloy was investigated. The details of our research work are as follow:1. We studied the glass forming ability of Gd-Al-TM multi-component alloy systemsby using the parameterγwe defined recently from binary alloy systems for theprediction of the compositional range of better glass formers. The predictions have been proved to be useful for the multi-component alloy systems under rapid quenching. Gd₅₅Al₂₀Ni₂₅, Gd₅₅Co₂₀Fe₅Al₂₀ and other bulk metallic glasses with good glass formation ability were obtained.2. Vitrification induced high MCE in Gd-Al-TM multi-component alloy systems. We have found that vitrification of Gd₅₅Al₂₀Ni₂₅ glass forming alloy could obviously enhance the Curie temperature and magneto-entropy changes by comparing the magnetic properties of Gd₅₅Al₂₀Ni₂₅ BMG with that of the corresponding crystalline samples. And the Gd₅₅Co₂₀Fe₅Al₂₀ BMG was found to have the highest magneto-entropy changes amongst other BMGs under 2T magnetic filed till now. Further investigations have found that the Fe additions could obviously improved the Curie temperature and temperature range of the magneto-entropy changes of Gd-Al-Ni-Fe. At thus, we can obtain the controllable magneto-entropy changes and Curie temperatures of Gd-based BMGs alloy by adjusting their compositions conveniently.3. Thermal stability and phase change of Gd₅₅Al₂₀Ni₂₅ BMG alloys were studied. Long-term thermal stability of Gd₅₅Al₂₀Ni₂₅ BMG was investigated by Kissinger and Vogel-Fulcher-Tammann (VFT) analyses respectively. The heating rate dependence of crystallization temperature of the BMG was found to follow the VFT non-linear relationship rather than the Kissinger's linear fittings. It is suggested that the continuous heating crystallization diagrams obtained from VFT equation should be more adequate to estimate the long-term thermal stability of Gd₅₅Al₂₀Ni₂₅ BMG according to the time-temperature-transformation diagram based on isothermal annealing. In addition, the formation of metastable phases and its effect on the magneto-entropy changes of partially crystalline Gd₅₅Al₂₀Ni₂₅ BMG were also investigated.