Observation of X-ray diffuse scattering preceding for Martensitic transformation in Ag-Cd alloy

对β相银镉合金进行了x射线漫射散射测量。用两种仪器测量了x射线漫射散射强度。其中一个是奈良女子大学的理学AFC-5四圆衍射仪与传统的x射线发生器相结合。另一台是安装在高能物理国家实验室光子工厂BL-4C的Huber 5020.4四圆衍射仪。散射强度的分布有以下显著特征：(1)沿[11^^-0]方向穿过基本反射点的漫反射条纹，均匀平行于（111）平面的漫反射片，(3)沿[11^^-0]轴q=(1/2)[11^^-0]位置附近的漫反射峰。在175K左右，随着温度的降低，q=(1/2)[11^^-0]峰附近的强度急剧增加。另一方面，[11^^-0]条纹的强度在此温度下减小。如前所述，对于某些类型的β相合金，反常的扩散散射(1)和(2)是由于β相固有的不稳定性。Ag-Cd合金在低温下转变为M2H莫氏体。因此，在q=(1/2)[110]位置附近的漫射峰被认为是由于晶格调制波导致马氏体向M2H结构转变。

X-ray diffuse scattering measurements have been carried out on beta -phase Ag-Cd alloys. Two instruments were used to measure the X-ray diffuse scattering intensity. One was a Rigaku AFC-5 four-circle diffractometer combined with a conventional X-ray generator at Nara Women's University. Another was Huber 5020.4 four-circle diffractometer installed at BL-4C at the Photon Factory of the National Laboratory for High Energy Physics.Following notable features were found in the distributions of diffuse scattering intensity; (1) diffuse streaks running through the fundamental reflection points in the [11^^-0] directions, diffuse sheets extending uniformly parallel to the (111) planes, (3) diffuse peaks near q=(1/2)[11^^-0] positions along the [11^^-0] axis.Furthermore, the intensity near q=(1/2)[11^^-0]-peak increases drastically at about 175K with decreasing temperature. On the other hand, the intensity of the [11^^-0]-streak decreases at this temperature.As already reported for some kinds of beta phase alloys, anomalous diffuse scattering (1) and (2) are due to an inherent instability of the beta-phase. beta Ag-Cd alloy transforms into the M2H mertensite at low temperature. Thus, the diffuse peaks near q=(1/2)[110] position are considered to be due to a lattice modulation wave which causes the martensitic transition with the M2H structure.