Abstract The solidification process of Ti45Al2Mn2Nb1B (Ti4522-1B) was studied together with the baseline alloy Ti45Al2Mn2Nb (Ti4522) through directional solidification. Boron addition at 1 at% was found to have a strong limiting effect on beta dendrite growth, resulting much finer beta dendrites than in the boron-free alloy. Boron addition at that level was also found to have changed the orientation relationship of peritectic alpha with beta dendrites. The peritectic alpha has the Burgers orientation relationship with the beta dendrites in Ti4522 whilst is randomly oriented in Ti4522-1B. The peritectic alpha in Ti4522-1B was likely to be inoculated by boride precipitates at the beta/liquid interfaces. Formation of randomly oriented peritectic alpha during solidification, boride precipitate-induced randomly oriented alpha during beta-to-alpha transformation and fine beta dendrites contribute to grain refinement in Ti4522-1B.