STUDY OF THE MECHANICAL BEHAVIOUR OF A FE-MO ALLOY WITH ULTRA-FINE GRAINS PRODUCED BY SPS OF BALL MILLED POWDER

Ultra-fine grained and nanostructured alloys are materials of great interest due to their outstanding mechanical properties. According to the Hall-Petch equation, the decrease of grain size leads to an increase of hardness and yield strength; the DBTT (ductile to brittle transition temperature) decreases too [1-4]. Among the different techniques used to manufacture such materials [5], Powder Metallurgy is very promising because: - 1- it is a 'near net-shape' technology, - 2- a nanostructured powder can be easily manufactured by high energy ball milling of commercially available powders [6]. Nanostructured powders must be sintered by limiting crystallite growth, therefore the process should involve lower sintering temperature and/or shorter treatment time. SPS (Spark Plasma Sintering) is very promising for such application. Some of the authors have already investigated SPS to manufacture nanostructured materials using Al [7] and WC nanostructured powders [8]. This paper presents part of the results of an extensive research program aimed to produce nanostructured Fe alloys. The main target was to get a full dense material with crystallite size in the sub-micron range. The samples were produced by using a pre-alloyed commercial Fe-1.5 wt.% Mo (FeMo) powder with mean particle size of 90 μm. Some samples were reinforced by 1.5 wt.% of nanometric (10 nm) SiO 2particles (purity > 99.5%). SiO 2particles were added to activate grain boundary pinning and control the crystallite growth during sintering. The powders were milled 20 hours in a planetary mill Fritsch "Pulverisette 6". Sintering was performed with a DR.SINTER® SPS1050 (Sumitomo Coal & Mining, now SPS Syntex Inc.) apparatus using graphite punches and dies. Cylindrical samples (diameter 30 mm, height 5 mm) were produced. The mechanical behaviour has been characterized by tensile and hardness tests. The probes for tensile tests, which had a non standard shape, were cut by EDM from the cylindrical samples. In addition, FIMEC (Flat-top Cylinder Indenter for Mechanical Characterisation), an instrumented indentation test employing a cylindrical punch [9-15], has been used for the first time on sintered materials for determining the yield strength. The technique also provides indications about the DBTT by performing the tests at decreasing temperatures (up to -196°C). The main characteristics of the ball milled powders are summarized in Tab. 1. Density, micro-hardness and mean size of crystallites of sintered samples are reported in Tabs. 2 and 3. The mean size of crystallites was measured by TEM (Transmission Electron Microscopy) [16]. The two different powders reach the full density (>99% of the theoretical density) at nearly the same temperature but the alloy reinforced with SiO 2nanoparticles displays a mean size of crystallites quite smaller (fig 1). The different thermal stability of the two materials was confirmed by XRD (X-ray Diffractometry) and DSC (Differential Scanning Calorimetry) [17-18]. The measured mechanical properties are reported in Tab. 4. The pressure-depth curves of FIMEC reveal a first elastic stage up to a pressure p L. This stage is followed by three plastic stages. The first one has a linear trend and ends at the pressure p y, when a macroscopic deformation occurs. The second plastic stage is characterized by a strong change of the slope corresponding to the start of material protrusion around the imprint. The third plastic stage shows a nearly constant slope. The yield strength of the metal is determined by the following relationship: σ y FIMEC≅p y/3. In tensile tests the Fe-Mo alloy does not show an uniform plastic deformation and the yield strength matches the ultimate strength [19]. The Fe-Mo + SiO 2alloy with the smallest crystallite size (sintered at 800°C and 808°C) exhibits brittle fracture whereas, in the other cases, a uniform deformation with hardening is observed [19]. The comparison between yield strength data from tensile and FIMEC tests evidences that the relative difference is lower than 7 % when the material has elastoplastic behaviour and the density is close to 100 % [15]. The deformation behaviour of the alloy reinforced by ceramic nanoparticles can be ascribed to the particular microstructure shown in Fig. 2. The TEM micrograph shows an inhomogeneous microstructure with white areas inside a darker matrix; the image analysis [16] evidenced a bimodal distribution of crystallite sizes. In particular, while dark areas consist of crystallites of ̃ 400 nm, the white ones correspond to larger dimensions (several micrometers). The bimodal distribution induces plastic stability because the larger crystals react to the stress with the typical mechanisms of plastic deformation leading to hardening [20]. FIMEC tests were performed at different temperatures (down to -196°C) to study the ductile to brittle transition. Fig. 3 shows FIMEC curves of the same sample at +25 and -196 °C: the slopes (Δp/Δh) of the third plastic stage are rather different since at the lower temperature corresponds the higher slope. The plot of the slopes vs. the test temperature gives a trend similar (but reversed) to that achieved by resilience tests. So, with the same procedure, it is possible to determine the DBTT. The DBTT values of martensitic and ferritic steels evaluated by FIMEC are very close to those obtained by resilience tests using Charpy probes of reduced size (KLST) [14]. Moreover, FIMEC permitted to measure σ yvalues at low temperatures (see for example fig 4a and b): ? yvalues increase as test temperature decreases, from 720 MPa (+25 °C) to 1100 MPa (-196 °C)). Fig. 5 and 6 display the slope of the third plastic stage vs. test temperature of the FeMo + SiO 2alloy sintered at 850 and 900 °C. The mean crystallite size of the alloy is 2 μm and 540 nm in samples sintered at 900 °C and 850 °C respectively. DBTT is higher for the material with larger crystallites. In conclusion, the material prepared by a powder mixture with ceramic particles shows a higher yield strength; owing to the bimodal distribution of crystallite sizes, it exhibits good hardening and uniform deformation. FIMEC is suitable to determine the yield strength of the material if that has a density close to the theoretical one. Such technique was also employed to get information about the DBTT and showed that the structure with ultra-fine grains guarantees an enough good toughness even at low temperatures.