Preparation Of NiZrXNbAl (X=Ti,Cu) Alloys By Crystallization Of Amorphous Phase And Their Mechanical Behaviors

Composites based on bulk amorphous alloys (CBBAAs) possess wide applicationprospect because of their high strength, good wear and corrosion resistance as well as manyother excellent performances. In this work, Ni57Zr22Ti8Nb8Al5(Ti8), Ni57Zr22Cu8Nb8Al5(Cu8)and Ni20Fe20Co20Cr20Al20(NiFeCoCrAl) alloy system were selected as research objects.Amorphous alloy powders were prepared by mechanical alloying (MA). The effect ofsubstitution elements Ti and Cu on the glass-forming ability (GFA), thermal stability andcrystallization mechanisms of the prepared Ti8and Cu8amorphous alloy powders wasinvestigated. The high-strength Ti8and Cu8and the NiFeCoCrAl ultrafine-grained (UFG)CBBAAs with high strength and large ductility were prepared by spark plasma sintering (SPS)and crystallization of amorphous phase. Composition design rules and preparation prosess ofNi-containing alloys with high strength and large ductility was revealed during SPS andcrystallization of amorphous phase.Firstly, the Ti8amorphous alloy powder and the Cu8amorphous/nanocrystallinecomposite powder were synthesized by MA after35h and38h milling, respectively.According to the results of TEM and three main indicators of GFA, The Ti8compositionpossesses better GFA than that of the Cu8composition. This is attributed to the larger atomicsize differences and more negetive mixed enthalpy in the Ni-Ti atomic pair than those of theNi-Cu atomic pair. Besides, the crystallization activation energy of atomic pair is obviouslyhigher than that of the Cu8amorphous/nanocrystalline composite powder according to theKissinger’s equation. This suggests that the Ti8powder possesses better thermal stability thanthat of the Cu8powder. Based on the JMA theory, the Avrami exponent n of the Ti8and Cu8alloy powders are2.44and1.89, respectively. This indicates that crystallization mechanismsof Ni-Zr-Nb-Al amorphous powders are affected by the minor substitutions of Ti and Cu. Thecrystallization mechanisms of the Ti8powder is governed by typical volumediffusion-controlled three-dimensional growth of neclei at a nearly constant nucleation rate,while the crystallization mechanisms of the Cu8powder is governed by typical volumediffusion-controlled two-dimensional growth of nuclei at a decreasing nucleation rate.Subsequently, Ni-based UFG bulk alloys were frabricated by SPS of amorphous alloy powders above their crystallization temperatures (Tx) under50MPa. The minor substitutions(Ti and Cu) in the Ni-Zr-Nb-Al alloy aystem obviously affect the microstructures andmechanical properties of the fabricated Ni-based UFG alloys. The main crystallized phases ofthe Ti8amorphous powder are Ni10Zr7, NiZr and NiTi, while the Cu8amorphous compositepowder possesses crystallized phases of Ni10Zr7and NiZr. With the increasing sinteringtemperature and holding time, the density, microhardness, fracture strength and elasticmodulus of the fabricated Ti8and Cu8bulk alloys increases gradually. The Ti8UFG bulkalloy fabricated by heating to1223K at164K/min and holding for15min shows the highestfracture strength of2092MPa and an elastic modulus of88.8GPa. Meanwhile, under heatingto1243K at164K/min and holding for10min, the fracture stress of the fabricated Cu8UFGbulk alloy reaches a maximum of2388MPa and an elastic modulus of81.8GPa. Both thefabricated Ti8and Cu8UFG bulk alloys exhibit brittle fracture due to the poor plasticity ofthe crystallized Ni10Zr7matrix phases.Finally, the amorphous/nanocrystalline NiFeCoCrAl alloy powder was prepared by MAafter35h milling. Its microstructure is amorphous matrix embedded by about10nm BCCphase. The NiFeCoCrAl UFG bulk composites with high strength and large ductility werefrabricated by SPS and crystallization of amorphous phase. The microstructure of thefabricated UFG bulk composites is composed of Ni/Al-riched BCC solid solution matrix andFe/Cr-riched FCC reinforced phase when sintering temperature is between1273~1523Kunder50MPa. With the increasing of sintering temperature, the volume fraction of BCCphase and the lattice parameter of BCC increase gradually. The fracture strength and strainincrease and the yield strength decreases. When heated to1523K at100K/min and holdingfor10min, the NiFeCoCrAl UFG bulk composite exhibits the best comprehensivemechanical property. Its yield stress, ultimate compression stress, elastic modulus and fracturestrain are1272MPa,2950MPa,50.2GPa and30.1%, respectively. The high strength andlarge plasticity is explained by the microstructure of the UFG reinforced FCC phasedispersedly distributing in continuously ductile BCC matrix.