A Hypothesis for the Mechanism that Produces the Utsuri Pattern on Japanese Swords

High-quality Japanese swords display a pattern just below the spine called the utsuri pattern. The physical mechanism causing this pattern remains a mystery. An hypothesis is presented here postulating that the pattern is produced by a transition in the microstructure from pearlite to the product microstructure of the divorced eutectoid transformation, DET. Experiments are presented in which such a transition was produced on small blade pieces. It is demonstrated that the contrast on these blades is adequate to produce the utsuri patterns.