Understanding the transition pathway toward metastable phases in SiGe alloys is a key issue for future generations of semiconductor and energy-related technology. High pressure techniques have proven to be a very efficient method of exploring new phases and properties in germanium and silicon, leading to the discovery of a large number of metastable phases with exotic properties. However, there is only a few experimental reports on the high-pressure behaviour of Si_x Ge_(1-x) alloys, mainly due to the difficulty to synthesize homogeneous SiGe alloys of good quality. In this study, we first present synthesis pathways toward homogeneous polycrystalline SiGe alloys of different compositions using arc melting and high temperature treatments. Then, we report the high pressure behaviour of Ge and SiGe alloys studied using Paris-Edinburgh press and diamond anvil cells coupled with in-situ characterization techniques such as Raman spectroscopy and X-ray diffraction. Upon compression, the behaviour of the stable SiGe diamond phase is reported along with transition toward the β-metallic phase. During decompression, the formation of new metastable structures named SiGe-R8 and SiGe-BC8 is observed in SiGe alloys. Temperature is discussed as a key parameter to drive the diamond to β transition in large volume presses, possibly allowing synthesis of large quantities of metastable phases. The experimental results are discussed in comparison to those reported in the literature and using DFT simulations.