Mechanical alloying (MA) is well known processing technique to synthesise materials with various structures, starting usually from the powder mixtures of pure elements. If the powders subjected to milling are alloyed powders, the process is called mechanical milling (MM). Both processing methods (MA and MM) induce phase and structural transformations in the processed materials.
The aim of this paper was to study phase and structural changes in high speed steel powders subjected to ball milling. Two types of materials were studied: commercial powders of M2 steel and the chips obtained by machining of the rod of the same composition. For comparison MA process was also performed, using the elemental powders of metals and graphite. Fritsch P5 planetary ball mill was used for milling experiments.
Particle size analysis revealed that milling of chips resulted in very fine final particle size (below 0.3 micrometer), while milling of commercial powders led to the final particle size of about 10 micrometers. The particle size as well as the morphology was also examined by scanning electron microscopy (SEM) observations.
X-ray diffraction (XRD) investigations showed the dissolution of carbides and formation of supersaturated ferrite. The mean crystallite size of ferrite was in the range of several tens of nanometers and lattice strain was up to 1%. MA resulted in very similar structure. The changes of lattice parameter of ferrite were also analysed.
Mössbauer spectroscopy measurements allowed to distinguish between ordinary" ferrite and alloyed" ferrite in the structure after ball milling, as well as to detect the residual carbides content at the level of 3%.
Differential scanning calorimetry (DSC) heating experiments showed two types of thermal effects, attributed to crystallite size increase and lattice strain decrease (at lower temperatures), and, to carbides precipitation at higher temperatures range.