The Influence Of Additional Elements On The Microstructures And Mechanical Properties Of Zr-Cu-base Alloys

Recently, the ZrCu (B2) austenite and ZrCu martensite phases have been observed in a number of Zr-Cu-base multi-component alloys during rapid cooling and strain, and have been regarded as reinforcement in the Zr-Cu-base alloys to improve the ductility or toughness of the materials. Generally, the Zr-Cu-base alloy system is very useful for investigating the formation of metastable phase. The glass forming ability (GFA), austenitic stability and microstructure of these alloys strongly depend on the composition and cooling rate. With the increase in the Al content, the GFA and austenitic stability of these alloys increase. The different martensitic transformations (MTs) were found in the alloys with different Al content. The distortions with different values induced by solid solution Al atoms result in the different shuffling in martensite phase, changing the symmetry to P21/m or Cm. The Al element plays a decisive role in controlling the formation and microstructures of the martensite phases under non-equilibrium solidification conditions. Phase stability, MT and electronic structure of Al-doped ZrCu intermetallics were investigated by experiments and first-principles calculations. When the Al content is smaller than 9.375 at.%, the austenite phase could form a martensite base structure during quenching or straining. When Al content is no smaller than 9.375 at.%, it forms a superstructure. The electronic-structure-dependent total energy difference between austenite and martensite plays an important role in the MT. The Ni addition affects the GFA of these alloys slightly. With the increase in the Ni content, the calculated formation energy shows that the structural stability of martensite phase increases, and the total energy difference between austenite and martensite also increases. Thus, with the Ni addition, the austenitic stability decreases. With the increase in the Ni content, the size of the ZrCu martensite becomes remarkably finer, and the lattice of the ZrCu martensite deflates. The decrease in the lattice volume of the ZrCu martensite weakens the internal strain caused by the martensitic transformation. The smaller internal stress can result in the lower density of dislocation in as-cast sample. For Zr-Cu alloys, the Ti addition improves the GFA and austenitic stability of these alloys remarkably. With the increase in the Ti content, volume fraction of martensite decreases, and that of ZrCu (B2) austenite increases. When the Ti content reaches 9 at.%, the alloys can form an amorphous phase. It has been demonstrated that oxygen impurity at a level of 0.3 at.% dramatically reduces the GFA, and the crystalline phases in copper-mold-cooled bulk samples increase with increasing oxygen content. The nose of the crystalline phase stability region in time-temperature-transformation (TTT) curve shifts to higher temperatures and shorter times with the increase in the oxygen content. It can be deduced that the oxygen addition would induce crystallization, expand the crystalline phase stability region and stabilize the crystalline phase under the copper-mold-cooling condition.The crystalline phase and grain size selections are governed by the cooling rate. In copper-mold-cooled Zr-Cu-Ni-Al-O alloy, near the surface, equiaxed nano-scale ZrCu (B2) grains forms, without undergoing a MT. Towards the center, the microstructure is characterized by the transformation of an austenite to martensite, which changes from the martensite base structure to the superstructure with the position closer to the center. Meanwhile, the grain size increases from a nanometer scale to a micrometer scale. The release of the crystallization latent heat influences the cooling condition, inducing a larger cooling rate in the core area than that in the area between the surface and the core in the temperature range of 715~135 oC, which determines the MT of the ZrCu (B2) austenite to the martensite base structure [in the area between the surface and the core] and the martensite superstructure [in the core area]. In brief, the melt could evolve into a glassy structure at the largest cooling rate. At a larg cooling rate, the melt could form a microstructure consisting of the ZrCu (B2) and Zr2Cu phases, without undergoing a MT. For a relatively low cooling rate, the ZrCu (B2) phase is formed and MT occurs. For even smaller cooling rates, the ZrCu (B2) phase could decompose into the Cu10Zr7 and Zr2Cu phases.In the Zr-Cu-based multiphase alloys containing ZrCu (B2) austenite phases, the strong work-hardening effect results from the continuous transformation of the ZrCu (B2) austenite phase to the harder martensite phase under deforming. As the austenite phase undergoes a MT, the volume fraction of the B2 phase decreases, resulting in the decrease in work-hardening rate. The increasing strain brings an increase in dislocation density in the retained crystalline phases. The complex interactions among slip dislocations, twinned martensites and interfaces also contribute to the work-hardening. Meanwhile, in the Zr-Cu-based glassy composites containing ZrCu (B2) austenite or martensite phases, the higher volume fraction of the crystalline phases in the alloys leads to a stronger work-hardening effect. The work-hardening effect results from the continuous MT, the dislocation multiplication in the crystalline phase and the interactions of shear bands during deformation. It has been confirmed that the ductile Zr-Cu-based glassy composites and multiphase alloys containing ZrCu (B2) austenite or martensite phases can effectively release the local stress concentration around the shear transformation zone and in front of microcrack, leading to the improvement of plasticity. The fracture strain of the alloy can be controlled by the local stress relaxation attributed to the martensite phases and the MT of the austenite phases. The work-hardening effect, and the enhanced plasticity and strength result from the transformation of the metastable ZrCu (B2) phase to the ZrCu martensite.