Atheoreticalmodelthattakesintoaccountthefree-volumeaidedcooperativeshearingofsheartransformationzones(STZs)isdevelopedtoquantitativelyunderstandtheductile-to-brittletransition(DBT)ofmetallicglasses.TheSTZdilatationalstrainisdefinedastheratioofSTZ-activatedfreevolumetoSTZvolumeitself.ThemodeldemonstratesthattheSTZdilatationalstrainwillincreasedrasticallyandexceedthecharacteristicshearstrainofSTZastemperaturedecreasesbelowacriticalvalue.ThiscriticaltemperatureisingoodagreementwiththeexperimentallymeasuredDBTtemperature.OurresultssuggestthattheDBTofmetallicglassesisunderpinnedbythetransitionofatomic-clustermotionsfromSTZ-typerearrangementstodilatationalprocesses(termedtensiontransformationzones(TTZs)).
