Improvement of High Temperature Strength of Al and Co Alloy by L12 Type Coherent Precipitates

Abstract

The present work aims at developing a new class of high temperature alloys based on ordered intermetallic compound that forms coherently with the matrix during solid state transformation. The chosen intermetallics have L12 ordered structure, which is a derivative of fcc unit cell. Most popular example of this fcc derivative is Ni3Al that is critical in developing high strength at high temperatures (~900°C) in commercially successful Ni based superalloys. Similar ordered structures form either in stable or metastable form can act as a main strengthening constituent in Al and Co matrices. For example Al3Sc, Al3Zr, Al3Hf can be dispersed in fcc Al matrix that are stable at temperatures ~ 400°C due to very low diffusivity of transition metals (Sc, Zr, Hf etc.) in the matrix. However, due to low solid solubility of these transition metals, the obtained volume fraction of these precipitates in the matrix is not sufficient to provide adequate room temperature strength. In fcc Co matrix, stable Co3Ti phase with L12 ordered structure forms with cuboidal morphology. However, besides having lower melting point, the precipitates have large misfit that lowers thermal stability at high temperatures. Recently, addition of Al and W with a proper ratio in Co is reported to lead the formation of metastable Co3(Al,W) L12 ordered phase in fcc α-Co matrix. This provides significant strength at high temperatures (~ 900°C). The main drawback for these alloys is their high densities (9.6 to 10.5 gm.cm-3) due to the requirement of compulsory addition of W (~ 15 to 25 wt%) for stabilising the ordered phase. In the present work, these problems are overcome leading to the development of new class of Al and Co alloys. The thesis is organized in three parts. In the first part, the principles of strengthening that can be optimized to develop newer high temperature high strength alloys are reviewed. The ordered L12 structure, which is the mainstay of the current effort of new alloy development, is elaborated. In the second part we present the results of our effort to the development a new class of high strength high temperature Al alloys. A new approach has been adopted to get a microstructure that contains both high temperature stable and room temperature strengthening precipitates. This has been illustrated by two Al rich compositions, Al-2Cu-0.1Nb-0.15Zr and Al-2Cu-0.1Hf-0.15Zr (at% unless stated otherwise). Addition of Nb/Zr or Hf/Zr in Al alloys leads to the formation of high temperature stable L12 ordered spherical coherent precipitates in the fcc Al matrix. Cu addition gives room temperature strengthening θ’ and θ” precipitates. The arc melted alloys were chill cast (suction cast) in the form of 3 mm rods followed by a novel three stage heat treatment process, as shown below. In the case of Al-2Cu-0.1Nb-0.15Zr alloy, the chill cast structure consists of Cu rich phase at the boundaries along the α-Al dendrites while Zr and Nb partition inside the α-Al dendrites. Aging at 400°C leads to an increase in the hardness of the cast alloy due to the precipitation of coherent L12 ordered Al3(Zr,Nb) spherical precipitates (~5nm) in the α-Al dendrites. Zr strongly partitions to the L12 ordered precipitate relative to the matrix. Nb exhibits weak partitioning in the precipitate. Further solutionising was optimized at 535°C for 30 minutes such that the segregation of Cu in the chill cast samples can be eliminated. The WDS mapping shows that Cu dissolved uniformly in the α-matrix while the Zr/Nb enriched α-Al dendrites are still present. The L12 ordered precipitates are mostly found in these Zr/Nb enriched dendrites formed during solidification. The precipitates sizes are finer (~5 nm) in dendrites and larger in the interdendritic region. The Nb partitioning increases in the ordered L12 precipitates relative to the matrix after solutionising. On aging at 190°C, fine θ” precipitates nucleate on prior Al3(Zr,Nb) precipitates present in α-Al dendrites while the interdendritic regions contain coarser θ’ nucleated on larger size L12 precipitates. The θ”/θ’ are much finer and higher in number density for the quaternary alloy compared to binary Al-2Cu alloy subjected to conventional heat treatment. The quaternary alloy show higher peak hardness of 1500 ± 8 MPa after 5 hours of aging at 190°C compared to binary Al-2Cu alloy with peak hardness of 1260 ± 11 MPa.