Precipitation and dissolution peaks of hydride in Zr–2.5Nb during quasistatic thermal cycles

Two internal friction peaks and corresponding discontinuity (`kneeʹ) points of elastic modulus have been observed in hydrideforming metals upon heating and cooling, respectively. In the present work, measurements of Youngʹs modulus as functions of temperature and hold time during a quasistatic thermal cycle were made in Zr–2.5Nb samples containing hydrogen using a composite oscillator technique. The increment of modulus during an isothermal hold is proportional to the decrease in hydrogen concentration in solid solution of the Zr alloy. As a result, elastic modulus measurements provide a means to determine the amount of transformed hydride during the transition. It is confirmed experimentally that the two peaks reflect the variation of the hydride transformation rate during heating or cooling. It is demonstrated by the present work that the maximum-slope point of each peak, on the high temperature side, is coincident with the knee point on the curve of modulus versus temperature and this point provides the most reliable and physically sensible indicator of the end of hydride dissolution during heating, or the beginning of hydride precipitation during cooling.