DEVELOPMENT OF HIGH STRENGTH Mg-Ni-Gd AND Mg (Ni, Zn)-Gd-Li ALLOYS

Abstract

Mg-alloys, in spite of being the lightest structural metal, finds sparse industrial application due to low strength resulting from dismal values of critical resolved shear stress (CRSS) coupled with poor workability caused by its hcp structure devoid of sufficient independent slip systems. Grain refinement, alloying additions, and synthesizing composites aid in boosting mechanical properties. It is known that the Li addition to magnesium improves the formability of the alloy but at the cost of diminishing strength. The addition of transition elements along with rare earth elements formed a new type of phases called long period stacking ordered (LPSO) phases that effectively improved the strength without compromising the ductility of the alloys. Hence, in this thesis, efforts were made to synthesize low-density, high-strength, and ductile Mg-alloys using Li, Ni, Zn, and Gd as alloying elements via different processing routes, i.e., casting and powder metallurgy. Ni and Gd were selected as alloying additions due to their prolificacy towards forming LPSO phases. The effect of Li on the microstructural and mechanical properties of Mg-0.5Ni 2.5Gd (at. %) alloy was studied by varying Li content, from 0-25 at.%. The alloys were processed through casting in vacuum induction melting furnace and then hot extruded. In as-cast state, a eutectic phase consisting of α-Mg and Mg3Gd phase is present along the dendritic boundaries of α-Mg, Mg3Gd, and LPSO phases. In addition, high Li-containing alloys such as 15 and 25 at. % Li alloys contain β-Li phase as well, and its volume fraction is a direct function of Li content. The volume fraction of the LPSO initially increases up to 5 at. % Li and subsequently reduces. Extrusion at 400 ℃ led to grain size refinement due to dynamic recrystallization, eliminating the dendritic and eutectic network and forming lamellar LPSO/ blocky LPSO and Mg3Gd particles. High yield strength of 302 MPa, ultimate strength (UTS) of 347MPa, and 5% elongation was achieved in the 5 at. % Li alloy whereas a YS of 167 MPa, UTS of 188 MPa, and a tensile ductility of 37.5% were attained in 25Li alloy. To further enhance the precipitation/formation of LPSO phases even at high Li contents of 15, 23, and 30 at. %, a small amount of Zn was added, which induced formation of lamellar LPSO phases in 15 and 23 at. % Li alloys, whereas some blocky LPSO phase fraction was observed in the 30 at. % Li alloy. The alloys were cast, and solutionized at 510 ℃ for 48 h, which increased the amount of the LPSO phase compared to the cast alloys. Hot extrusion at 200 or 300oC led to viii dynamic recrystallization of matrix grains and refinement and uniform distribution of LPSO and Mg3Gd phases. A high UTS of 320 MPa and 18% elongation is attained with 15Li alloy extruded at 200 ℃. The alloys extruded at 200 ℃ performed better than the ones extruded at 300 ℃ due to the fine grain size and uniformly distributed second-phase particles. 30Li alloys exhibited initial strain hardening and after a critical amount, strain softening behavior at both the extrusion temperatures. This can be attributed to the dislocation annihilation by a dynamic recovery, owing to the presence of soft β-Li phase and lack of sufficient distribution of precipitates in the matrix. This investigation aims to achieve a YS of >600 MPa for a magnesium alloy. Introducing thermally stable and coherent secondary phases would boost the strength at elevated temperatures. A master alloy comprised of Ni and Gd was cast, and the compositions of Mg98.82Ni0.59Gd0.59 and Mg97.56Ni1.22Gd1.22 (at. %) were formulated using ball milling for 150 hours. Consolidation through sintering with 5, 7, and 9 h of exposure at 550 ℃ and subsequent extrusion at 500 ℃ resulted in the formation of Mg5Gd, Mg2Ni, Gd2O3 and MgO phases. The extruded samples possessed a high strength of 804 MPa and 3.75% elongation which can be attributed to ultra-fine grains and dispersoid strengthening by homogeneously distributed second-phase particles in the 100–200 nm range. The development of a high-specific strength Mg alloy has been attained via different processing routes and an exhaustive evaluation of the microstructure and mechanical properties has been carried out.