The effect of (KyNa1-y)NbO3(KyNN) addition on the ferroelectric stability and consequent changes in the electrical properties of lead-free Bi0.5(Na0.80K0.20)0.5TiO3(BNKT20) piezoceramics were systematically studied. Results showed that the KyNN substitution into BNKT20 induces a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic with a significant disruption of the long-range ferroelectric order, and correspondingly adjusts the ferroelectric-relaxor transition point TF-Rto room temperature. Accordingly, a large electric-field-induced strain of 0.33%-0.46% (Smax/Emax= 413-575 pm/V), which is derived from a reversible field-induced ergodic relaxor to ferroelectric phase transformation, was obtained in KyNN-modified compositions near the phase boundary. Moreover, a relationship between the position of the ferroelectric-relaxor phase boundary and the tolerance factor t of the end member KyNN was found in BNKT20-xKyNN system, which is expected to provide a guideline for searching the new ferroelectric-relaxor phase boundary where the strain response is consistently derivable in (Bi0.5Na0.5)TiO3-based materials. © 2014 AIP Publishing LLC.