THERMAL STABILITY AND MECHANICAL PROPERTIES OF Fe-Cr NANOSTRUCTURES PREPARED BY MECHANICAL ALLOYING FOLLOWED BY SPARK PLASMA SINTERING

Abstract

ron (Fe)-based alloys, especially, Fe-Cr alloys have wide variety of applications in various industries such as nuclear, construction, automotive, oil, gas etc. due to their superior strength and formability, high corrosion resistance, minimum embrittlement and low maintenance cost. These alloys are also being used for high temperature applications primarily in energy conversion plants, refractory supports, hot acid containers, oven linings and heat exchangers. Demand to improve the efficiency as well as performance of the components in high temperature applications has increased enormously. Nowadays, it is well known that the nanocrystalline/ultrafine grained (nc/UFG) structure can provide significant strength and toughness to the materials, which have attracted considerable scientific interest in the recent past. However, the nc/UFG materials have high surface energy and high surface area/volume ratio. This makes them unstable and grain coarsening occurs as a result of sintering of these nc/UFG materials at medium to high temperature regime. Therefore, the nc/UFG Fe-Cr alloys must be stable to realize their unique mechanical properties. An appropriate type and quantity of insoluble oversize (compared to solvent atoms) solute atoms (e.g. Y, Nb, Zr, Hf etc.) dissolved in the matrix (e.g. Fe) could restrict grain growth successfully at elevated temperatures. First, the oversize insoluble solute atoms are brought into the solid solution of matrix by a non-equilibrium processing method, such as mechanical alloying (MA). The MA is preferred due to its effectiveness in preparing highly supersaturated solid solutions in non-equilibrium alloy system (e.g. Fe-Cr-Y/Zr/Nb) easily in large quantities. Then, the supersaturated solute atoms in the disordered solid solution are allowed to segregate along the matrix grain boundaries and/or precipitate out during annealing at suitable temperature/further processing. Thus, grain size stabilization at high temperature could be achieved by two approaches: first approach is the kinetic mechanism that involves precipitation of second-phase particles, which obstruct the mobility of grain boundary by Zener pinning. The second approach is the thermodynamic mechanism that involves segregation of oversize solute atoms along the matrix grain boundaries, which reduces grain boundary energy (γ) to zero (or close to zero). Hence, the stabilization of the nc/UFG Fe-Cr alloys (after addition of oversize solute atoms) could be attained through Zener pinning and/or thermodynamic approach depending on the phase evolution (i.e. solid solution/intermetallic compound etc.) during processing and/or vi annealing. Hence, the MA followed by spark plasma sintering (SPS) could be an effective route to produce stabilize Fe-Cr nc/UFG alloys for high temperature applications. First, four Fe-Cr alloys (Cr = 7, 11, 15 & 19 at.%) were developed by MA for 25 h and their thermal stability was investigated (all in at.%). It was found that the nc Fe-Cr alloys developed by MA dictated a poor thermal stability. Therefore, the Fe-7Cr (low Cr content) and Fe-15Cr (high Cr content) alloys were chosen to study their thermal stability further after addition of small quantity of insoluble solute atoms (i.e. 0.25, 0.5 & 1at.% of Y/Nb/Zr). Then, Fe-7Cr-X and Fe-15Cr-X (X=0.25, 0.5 & 1at.% of Y/Nb/Zr) alloys (after addition of suitable solute) were developed by MA under same conditions of milling (as done for Fe-Cr alloys). The feasibility of formation of the Fe-7Cr-X and Fe-15Cr-X disordered solid solutions was confirmed by XRD phase analysis & increase in the lattice parameter calculation, thermodynamic feasibility analysis (change in the Gibbs free energy estimation by Toop's model) and TEM-SAED analysis. The increase in the lattice parameter of the Fe-lattices (solvent) indicates the formation of ordered/disordered solid solution due to the dissolution of solute atoms (i.e. Cr and oversize solute atoms of Y/Nb/Zr) in Fe. Thermodynamic feasibility analysis as per Toop's model confirms that the energy barrier required to form the disordered solid solutions has been overcome by the total stored energy due to the grain boundary energy and lattice strain energy. Moreover, XRD phase analysis followed by TEM-SAED pattern analysis of the as-milled samples did not reveal any peak related to free Cr and Y/Nb/Zr or any intermetallic phase(s). This also confirmed the formation of the complete solid solutions of the Fe-7Cr-X and Fe-15Cr-X compositions by MA. Then, the as-milled powder samples were annealed in batches from 600-1200℃ under Ar+2%H2 atmosphere. During annealing, the Fe-Cr-Y & Fe-Cr-Zr samples were found to decompose partially to form dilute quantity of Fe17Y2 & Fe2Zr intermetallic phases, respectively, in the matrix of nanocrystalline solid solutions; whereas, the Fe-Cr-Nb alloys retained their complete solid solubility after the annealing (up to 1200°C). This is confirmed by XRD phase analysis followed by TEM-SAED pattern analysis. After the annealing at 1000°C, TEM grain size was found to retain within 100 nm for all the three alloys (i.e. 50, 53 and 55 nm for the Fe-15Cr-1Nb, Fe-15Cr-1Y and Fe-15Cr-1Zr alloys) with a corresponding hardness value of 8.3, 8.1 and 8.4 GPa, respectively. The estimated yield strength (YS) (as per Cahoon model) of the annealed Fe-15Cr-1Nb sample (at 1000°C) is found to be quite attractive (i.e. YS:1754 MPa) and correlated well with the hardness value (8.3 GPa) and its microstructural vii features. Overall, it is found that the Fe-15Cr-1Nb, Fe-15Cr-1Y and Fe-15Cr-1Zr alloys showed excellent thermal stability in terms of the grain size stability (<100 nm) and retaining a quite high microhardness values (8-8.5 GPa). On the basis of the thermal stability, the suitable compositions, such as Fe-7Cr-X & Fe-15Cr-X (X: Y-1; Nb: 0.25, 1 and Zr: 1at.%) were chosen and consolidated by SPS at 800, 900 & 1000℃. Microstructural features of the SPSed samples characterized by optical and SEM were found to corroborate well with the sintered density obtained by Archimedes method. The relative sinter density of the SPSed (at 1000°C) Fe-15Cr-1Y, Fe-15Cr-1Nb & Fe-15Cr-1Zr alloys was estimated to be in the range of 97-98%. XRD phase analysis followed by TEM-SAED pattern analysis of the SPSed samples also confirmed the formation of Fe17Y2 & Fe2Zr intermetallic phases in the Fe-Cr-Y and Fe-Cr-Zr alloys; whereas, the Fe-Cr-Nb alloys retained the complete solid solubility after the SPS. TEM grain size of the SPSed (at 1000°C) samples was found to stabilize within 100 nm (49, 72 and 61 nm, respectively, for Fe-15Cr-1Nb, Fe-15Cr-1Y and Fe-15Cr-1Zr alloys). The corresponding microhardness values were estimated to be 9.5, 9.6 and 9.45 GPa, respectively. The experimental compressive strength (UCS-2400 MPa, YS-1800 MPa for Fe-15Cr-1Nb; UCS-2600 MPa, YS-2000 MPa for Fe-15Cr-1Y and UCS-2200 MPa, YS-1600 MPa for Fe-15Cr-1Zr) of the SPSed (at 1000°C) samples depicted a very good correlation with the corresponding density, hardness and microstructural features. Analysis of various strengthening mechanisms of the SPSed Fe-15Cr-1Nb alloy was performed in the light of solid solution strengthening, grain size strengthening and dislocation strengthening and their quantitative contribution to the total YS was estimated. The estimated total YS (i.e. YS:1744 MPa) was found to correlate well with experimentally measured compressive YS (i.e. 1800 MPa) of the corresponding sample. The best combination of hardness, wear resistance and corrosion behavior was achieved for the samples sintered at 1000°C. The high hardness, minimum coefficient of friction and extremely low wear volume & low corrosion rate obtained have been discussed in the light of solid solution strengthening, grain size strengthening, grain boundary segregation, densification and diffusion bonding and precipitation hardening by intermetallic phase (Fe17Y2 & Fe2Zr) in the alloy matrix. The wear and corrosion resistance are found to be almost similar for the Fe-15Cr-1Y, Fe-15Cr-1Nb and Fe-15Cr-1Zr SPSed samples (e.g. wear volume and CR: 0.00196×10-2 mm3 & 3.43 mpy for Fe-15Cr-1Y, 0.00169×10-2 mm3 & 4.12 mpy for Fe-15Cr-1Nb and 0.00186×10-2 mm3 & 4.02 mpy for Fe-15Cr-1Zr); but these are much superior as compared to viii that of the other SPSed samples studied (0.0024×10-2 mm3 & 3.46 mpy for Fe-7Cr-1Y, 0.0023×10-2 mm3 & 6.03 mpy for Fe-7Cr-1Nb, 0.0025×10-2 mm3 & 4.89 mpy for Fe-7Cr-1Zr). The SEM analysis of the worn surface and corroded features corroborated well with the wear resistance and corrosion behavior of the corresponding samples. Overall, the Fe-15Cr-1Y/1Nb/1Zr alloys showed superior thermal stability and the SPSed samples of these alloys revealed highly attractive mechanical properties (hardness of 9-10 GPa), excellent wear resistance and outstanding compressive YS (1600-2000 MPa). Moreover, the corrosion resistance of these SPSed samples was found highly significant in aerated 3.5% NaCl solution. Therefore, the bulk-size samples of the Fe-Cr-Y/Nb/Zr alloys are expected to be highly suitable for applications in coal mining machinery parts (e.g. mining sliding shoes), cement industry (e.g. cement roller press), nuclear first wall reactor (750-950°C), automotive (e.g. exhaust system operating at 800-850°C), chemical processing unit (e.g. acid carrying pipes 320-450°C, petrochemical industries U-tube) etc., where high wear resistance and good corrosion resistance are essential.