A Novel Insight Toward Zr Poisoning on Grain Refinement of Al–5Ti–1B and Its Solution

Adding Al–5Ti–1B to the melt is a universal practice to refine the microstructure of aluminum alloys. However, its major problem is susceptibility to the appearance of element Zr. A novel insight toward Zr poisoning effect on grain refinement of Al–5Ti–1B as well as its solving method is proposed in this work. For an Al–Zr melt with the existence of solute Ti and TiB₂, solutes Zr and Ti show almost the same effect on the kinetic behaviors of TiB₂ or (Ti, Zr)B₂. Solute Zr leads to the formation of Zr-contained multi-layers (ZCMLs) on the TiB₂ surface in the form of (Ti, Zr)B₂. Different from solute Ti, solute Zr shows no segregation tendency to the Al(L)/TiB₂ interface and decreases the interfacial atomic ordering level. Zr poisoning effect is determined by the Zr mole fraction \(X_{{{\text{Zr}}}}^{{{\text{In}}}}\) $X_{\text{Zr}}^{\text{In}}$ in the interfacial monolayer between ZCMLs and Al melt. For a given Zr concentration, \(X_{{{\text{Zr}}}}^{{{\text{In}}}}\) $X_{\text{Zr}}^{\text{In}}$ and Zr poisoning effect decreases with Ti concentration of the melt. Zr poisoning effect can be vanished using hundreds of ppm of solute Ti within its allowed limit in most Al alloys while keeping the addition level of Al–5Ti–B unchanged.