Epitaxial laser treatment of single crystal nickel-base superalloys

The continued drive for increased efficiency, performance and reduced costs for aircraft and industrial gas turbines demands extended use of high temperature materials, such as single crystal nickel based superalloys. The cost for hot section components used in those applications is a primary factor driving a growing need for advanced blade repair procedures, which address the problem of recuparating damaged parts. In previous research at EPFL, it has been shown that single crystal deposition for turbine blade repair is possible by an Epitaxial Laser Metal Forming (E-LMF) technique. In this process, metal powder is injected into a molten pool formed by controlled laser heating with the aim of producing a single crystal deposit on a single crystal substrate. It is a near net-shape process for rapid prototyping or repair engineering of single crystal high pressure/high temperature gas turbine blades. Single crystal repair using E-LMF requires controlled solidification conditions in order to prevent the nucleation and growth of crystals ahead of the columnar dendritic front, i.e. to ensure epitaxial growth and to avoid the columnar to equiaxed transition. The feasability of the E-LMF process has been demonstrated in the laboratory on simple geometry substrates and platforms of turbine blades. Significant efforts are still required to up-scale this process for the repair of real, complex shaped parts on an industrial scale as solidification defects are often encountered when non-ideal processing conditions are used. Epitaxial laser treatment of single crystal nickel-base superalloys The aim of the present research is to study the microstucture development during the laser assisted deposition of Ni-based superalloys on single crystal substrates with the same composition and to gain an understanding of defect formation mechanisms, this in view of a complete control of the E-LMF process. As a follow-up to previous research, particular emphasis is placed on non ideal conditions by taking into account important aspects of dendritic solidification. In particular, the major defects encountered during single crystal laser deposition, i.e. loss of the crystal orientation of the substrate, are described taking into account the following aspects : off-axis heat flux dendritic growth, fragmentation, grain growth competition, and loss of epitaxy due to branching difficulties of cellular-dendritic structures. In this work, critical parameters, which affect epitaxy, the nucleation of spurious grains and grain structure, will be discussed and new results, which are fundamental to the full process control of E-LMF, will be presented. By contributing to the understanding of the phenomena which are responsible for loss of single crystallinity and through the definition of proper processing windows for single crystal generation and repair by laser deposition, this work will constitute a sound basis to guide future industrial practice.

Microstructure of single crystal Ni-based superalloys during blade manufacturing process and creep deformation

Stéphane Pierret

The aim of the thesis is to obtain more understanding about the influence of plastic deformation on the microstructural changes observed in single crystal (SX) Ni-based superalloys using diffraction techniques. This work was organised around two main problematics: (1) rafting in turbine blades after the manufacturing process and (2) rafting and mosaicity evolution in two SX alloys with different composition during creep deformation. SX Ni-based superalloys are first choice materials for turbine blade application in land-based gas turbines and aero-engines. Superior mechanical properties of these materials at high temperatures are achieved due to the optimisation of the alloying composition and the microstructure. SX Ni-based superalloy turbine blades solidify by dendritic solidification with dendrites aligned along the directions of lowest Young modulus (interdendritic spacing around 300μm). At the end of the manufacturing process, the microstructure consists of high volume fraction of cubic γ’ precipitates (around 500nm) coherently embedded in the γ matrix. However specific parts of engine-ready turbine blades exhibit directionally coarsened γ’ precipitates in a preferred direction (rafting) without the influence of an applied load. It is of prime importance to understand the origin of rafting prior to the introduction of the blades in the engine since rafting can alter the mechanical properties of the alloy. Chemical segregations and the introduction of plastic deformation prior to thermal treatments were identified as potential driving force for rafting during load-free high temperature annealing. Most of the SX Ni-based superalloys used in the current blade production in Alstom contain significant amount of refractory elements which are expensive and increase the density of the alloy. To answer the demand of cheaper and lighter components, Alstom launched a development program to design novel SX superalloys. One of these projects led to the production of a Re-free superalloy, so-called MD2. Extensive mechanical investigations on MD2 are on-going and compared to the behaviour of the MK4HC alloy (3wt% Re) currently used in turbine blade production. One of the major differences in terms of mechanical properties between Re-containing and Re-free SX alloys is the faster kinetics of coarsening and rafting during creep deformation at high temperature in the latter alloys. It prevented so far their application in the hottest areas in gas turbines. It is therefore of utmost importance to acquire more knowledge about the mechanisms associated with the microstructure evolution during creep of the new MD2 SX alloy in comparison with the MK4HC alloy. The evolution of the lattice parameter misfit in different directions with respect to the applied load during creep deformation was associated with the evolution of the microstructure from cubic to elongated γ’ precipitates It could be shown that rafting occurs after the cooling stage following the solution heat treatment (SHT) during the manufacturing of the blade with investigations of the microstructure. The influence of chemical segregations on rafting was excluded with the investigation of the blade alloy composition in different parts of the blade cooled from the SHT and diffraction measurements in stress-relieved combs prepared from the same blade. The formation of a rafted microstructure during the manufacturing process was associated with the formation of plastic strains during cooling after the SHT. This was confirmed after gathering results from investigations of the microstructure, simulation of the cooling stage and neutron diffraction measurements on the blade. The investigations of the microstructure performed on the engine-ready blade revealed an inversion of the γ’ raft orientation below (parallel to [001]) and above the platform of the blade (perpendicular to [001]). This was attributed to an inversion of the sign of residual strain between the previously mentioned positions as revealed by neutron diffraction measurements performed along the airfoil of the blade cooled after the SHT. Microscopy also showed that the rafted microstructure was more pronounced below the platform (long γ’ rafts) than above the platform (shorter γ’ rafts with retained cubic precipitates). The concept of the threshold plastic strain necessary to induce rafting during annealing subsequent to plastic deformation was used to describe the difference in rafted microstructure. It is suggested that plastic strains largely exceed the threshold below the platform leading to fully formed rafted microstructure. On the other side, plastic strains of the order to the threshold strain are suggested above the platform in order to explain the partially formed rafted microstructure Neutron diffraction measurements revealed that the lattice parameter misfit was larger in the MK4HC than in the MD2 alloy at 20°C in the unloaded state. The larger misfit in the MK4HC alloy was attributed to the larger concentration of elements dissolving in the γ phase and to the presence of Re. During creep deformation, coherency stresses are relieved at the γ/γ’ interface in the γ channels perpendicular to the applied load by dislocations. On the other hand, dislocations increase the internal stress at the γ/γ’ interface in the γ channels parallel to the applied load. This leads to the rafting of the γ’ precipitates perpendicular to the applied load. This was observed in the MK4HC. The absence of clear misfit evolution in the MD2 alloy was attributed to the anomalous microstructure at the beginning of the creep deformation. The mosaicity of the MK4HC and MD2 alloys revealed crystallographic misorientation between neighbouring dendrites up to 0.5°. The misorientation was associated with the presence of dislocations in the regions between the dendrites formed during the dendritic solidification of the alloys from existing work. It was shown that the composition of the SX alloys and short periods at high temperature in the unloaded state had only little influence on the misorientation between neighbouring dendrites. Upon creep loading to 180MPa, the mosaicity markedly changed in the two SX alloys independent of the creep temperature. In particular, the number of single misoriented domains measured by X-ray diffraction decreased. This was attributed to the fast propagation of the dislocations present in the interdendritic structure inducing a rotation of the individual dendrites. During creep deformation, the mosaicity increased as visible from the broadening of the maximum spread in misorientation. The formation of inhomogeneous plastic strain fields in the γ/γ’ microstructure is believed to create new mosaic blocks in the dendritic structure of the alloys leading to the observed broadening of the mosaicity. The influence of plastic deformation on the evolution of the mosaicity was demonstrated. The mosaicity significantly changed during creep of the MD2 at 1050°C/180MPa and 1000°C/180MPa whereas no significant evolution could be measured during creep at 950°C/180MPa. This was attributed to a temperature dependence of the yield strength of the MD2 alloy. It therefore induces difference in dislocation propagation at 950°C compared to 1050°C and 1000°C which retards the broadening of the mosaicity during creep at 950°C. Furthermore, the mosaicity formed during creep deformation was retained upon unloading and cooling of the SX samples to room temperature.

EPFL2012

Recrystallization and grain boundary formation in an 18-carat gold alloy studied by mechanical spectroscopy

Daniele Mari, Robert Schaller, Iva Tkalcec Vâju

Heavily deformed 18-carat yellow gold samples show a recrystallization peak at 700 K during the first heating. The mechanical loss spectrum of polycrystals shows a relaxation peak at about 780 K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal from 2 % to 10 % causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak. At 8 % deformation the high temperature peak disappears and the peak that is normally observed in polycrystals appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the high temperature peak is attributed to a relaxation mechanism in the sub grain boundaries. The peak of polycrystalline samples located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. The peak temperature can be related to the mean grain size.

Trans Tech Publications Ltd2013

Grain Boundary Relaxation in 18-carat Gold Alloys

Ann-Kathrin Audren

Grain boundaries (GBs) play an important role for the mechanical properties of metals. In addition to dislocation motion inside the grains, some metal alloys deform along the GBs at high temperatures. GBs can also be the cause of a brittle behaviour of some metallic alloys, where crack propagation along the GBs is observed. This thesis aims to understand the role of GBs on the thermo-mechanical behaviour of metals and especially to define the microscopic mechanism, which leads to a deformation at the GBs. In this thesis, polycrystals, single crystals and bi-crystals of a yellow gold alloy are studied in detail, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies different anelastic relaxations (internal energy dissipation processes), that produce peaks in the mechanical loss spectrum. The mechanical loss spectrum of polycrystals shows a relaxation peak P2 at about 780K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak (P3). Above a critical deformation, P3 disappears and the peak P2, which is normally observed in polycrystals, appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the peak P3 is attributed to a dislocation relaxation mechanism in the sub-grain boundaries. The peak P2 located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. It is shown that the relaxation time is proportional to the grain size d. Such a grains size dependence is in agreement with the Zener model based on geometrical considerations of an assembly of elastic grains, which can slide against each other. The investigations on bi-crystals containing a single GB with a specific misorientation between crystal lattices and a specific boundary plane orientation show that the relaxation peak P2 is closely related to a mechanism taking place at the boundary. The peak height is proportional to the GB density in a bi-crystal, whereas a single crystalline part cut from a bi-crystal does not show a relaxation peak. Molecular dynamics simulations are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak P2 in polycrystals. A Sigma5 grain boundary is submitted to a shear deformation parallel to the boundary plane. The grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700K. Above 1000K, only grain boundary sliding is observed. Two models are developed that provide expressions for the relaxation strength and the relaxation time that are compared to experimental measurements performed on polycrystals. The observed grain size dependence favours the sliding model over the migration model. Measurements as a function of stress allow a refinement of the sliding model. The stress amplitude dependence of the peak P2 indicates a depinning mechanism, which is interpreted as due to GB dislocations between zones where sliding of different amount occurred. Obstacles to the sliding movement like steps or extended coincidence sites in the GB plane act as pinning points. The pinning points can be overcome at high temperatures close to the melting point, which results in the onset of local microscopic creep.