Metal Casting Technologies : MCT-MARCH-2014
METAL casting Technologies March 2014 25 24 www.metals.rala.com.au Introduction reep behaviour describes the processes by which gradual plastic (non-reversible) deformation takes place when a material is subject to loading for a prolonged period of time. Following the initial elastic strain which is produced on application of a stress creep causes progressive plastic deformation with time and this eventually leads to complete failure - usually at stress levels well below the tensile strength of the material. In metallic materials creep occurs more easily at elevated temperatures; in general the temperature at which creep becomes pronounced is related to the liquidus temperature i.e. above 150-200oC for Al alloys and above 350oC for low alloy steel. Face centred cubic (FCC) matrix structures provide better creep resistance when alloyed compared to body centred cubic (BCC) structures hence the most creep resistant alloys, the “superalloys”, have matrices based on the higher m.pt. FCC metals nickel or cobalt. Together with microstructural stability and oxidation & high temperature corrosion resistance , the effects of creep must be considered in the design of plant and equipment which operates at elevated temperatures, e.g. in steam and chemical plants, turbines, jet engines, and furnace support parts, etc. . Creep mechanisms In a creep curve strain is plotted against time for fixed temperature and stress level as in Figure 1. This shows that creep typically occurs in 3 stages: primary (1st stage), secondary (2nd stage) and tertiary (3rd stage). Primary creep is often referred to as “Transient” creep and the combined effects of stages 2 and 3 are called “Continuous” creep. The rate of creep in each stage will depend on the applied stress and service temperature. Creep strain and the onset of stages 2 and 3 are accelerated by increasing stress level and/or temperature. Creep curves are derived from creep testing of test pieces which are similar in form to tensile test pieces. The test piece is enclosed in a specially designed electric tube furnace that can be maintained accurately at a fixed temperature over a prolonged time period. Permanent creep deformation leading to fracture results from a number of mechanisms including: • Glide of dislocations producing slip and hence plastic deformation • Climb of dislocations which enables locked (sessile) dislocations to become free (glissile) to move again by changing slip plane • Sliding of grain boundaries • Diffusion of site vacancies to grain boundaries producing voids. During primary creep dislocation movements producing slip occur but dislocation interaction results in work hardening thus reducing the creep rate. At lower temperatures where recovery is limited work hardening effectively stops creep - this is transient creep. However at higher temperatures work hardening is relieved by recovery processes and primary creep then develops into secondary (steady state) creep during which the rate of work hardening becomes balanced by the rate of recovery. Recovery involves the unlocking of immobile dislocations allowing further slip i.e. creep strain. Recovery is easier at higher temperatures since the generation and diffusion of site vacancies is more rapid. Vacancies can diffuse to or away from dislocations thus allowing the dislocations to climb up or down to parallel slip planes in the lattice. By this means the dislocations are able to regain their mobility and creep continues. During the latter stages of stage II the grain boundaries play an increasing part in creep mechanisms (Figure 2). At grain boundaries normal to the main creep stress vacancies can be created so rapidly that they do not all have time to diffuse away and hence can join together to produce voids on these grain boundaries. During stage 3 intergranular failures can then develop by tearing between these voids and such failure is also assisted by cracks which develop at grain boundary junctions since at elevated temperatures grains can slide relative to each other. Superalloys and steels for creep resistance at high temperatures In conventional alloys the most effective strengthening mechanisms to resist creep involve precipitation hardening of suitable FCC solid solution matrices based on Ni or Co with very stable “precipitates” of intermetallics which do not overage or significantly change during service at elevated temperatures. Due to the nature of the dislocations in the FCC structure, work hardening is more significant especially when alloy elements are present in substitutional solid solution. In FCC metals each dislocation tends to dissociate into 2 partial dislocations to create a stacking fault. The extended dislocation is then harder to move and gives greater work hardening. Certain alloy elements lower the energy required to produce stacking faults in the lattice, and hence encourage wider separation in each pair of partials which are separated by a stacking fault. Each dislocation is effectively increased in width thus increasing the degree of interaction and locking with other dislocations, and with any precipitates present, during plastic deformation, hence significantly increasing the rate of work hardening. The wider the separation of the partials the lower is the rate of recovery since each pair of partials must re-associate for a dislocation to be able to break free by climb and continue in the deformation process. Over the last 50 years or so the continual development of the Ni base superalloys for aero-engine and power generation gas turbines has involved solid solution strengthening with a variety of alloy elements including Cr, Mo, W, Co, Fe, Ti, Al, Ta, Re, etc. [5-8]. Both cast and wrought grades are subjected to solution and ageing heat treatments to provide microstructures containing suitable distributions of – prime “precipitate” zones which are coherent with the – FCC matrix. The – prime phase is based on Ni3Al and has very little miss-match in lattice parameter with the matrix such that it can remain stable and coherent with the matrix during prolonged service at elevated temperatures. Coherent precipitate zones are required for strengthening since they provide much higher resistance to the movement of dislocations than incoherent particles. The presence of around 10% Co in solid solution lowers stacking fault energy of the – matrix to widen extended dislocations and increase interaction. The balance of alloying elements is controlled to avoid the formation of undesirable hard intermetallic phases such as Laves and sigma whose presence reduces crack resistance and can cause brittle fracture at low temperatures. Improvements in casting technology have enabled highly controlled directional solidification of investment vacuum cast turbine blades with columnar or single crystal structures containing internal cooling air passages. Directional solidification improves creep ductility by 2-3 times by limiting void formation since there are no grain boundaries normal to the creep stress. Alloys for production of single crystal blades do not require the presence of carbide or boride precipitates which are needed for grain size control and for grain pinning in polycrystalline material. As summarized in Figure 3, progress in directional solidified and single crystal blades alloy composition together with alloy development has increased high temperature capability to around 1100oC . Current turbine blades are single crystal type which are electron beam/physical vapour deposition coated with a ceramic material such as yttria-stabilized ZrO2 to provide An outline of creep in cast metals: superalloys and steels By John Pearce TEcHNicAL fEATURE C Figure 1. Creep curve of strain against time at constant stress and temperature . Figure 2. development of intergranular failure during creep . Figure 3. progress in turbine blade capability due to improved casting technology . iN fcc METALs EAcH DisLOcATiON TENDs TO DissOciATE iNTO 2 PARTiAL DisLOcATiONs TO cREATE A sTAcKiNG fAULT.
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