Modelling Of Compressive Creep-fatigue And Life Prediction In TiB₂/Al Reinforced Composite

Under the cycling high temperature and explosion pressure, the piston suffers from bothcompressive creep and fatigue, which might create the failure of piston. Thus, it is essentialto predict the creep-fatigue life of piston accurately. TiB2particle reinforced aluminummatrix composites has potential applications in piston for its high specific strength, highelastic modulus, and well temperature performance. This paper studies TiB2particlereinforced aluminum matrix composites’ performance under compressive creep and creep-fatigue from the micro-scope, and modelling for those behaviours. Then, based on energymethod, makes a life predictive calculation on the piston throat under the interaction betweencreep and fatigue.(1) This paper compares the basic mechanical properties and the micro scope-structuresbetween cast aluminum and TiB2particle reinforced aluminum matrix composites, and findthat the reinforce mechanism of TiB2particle reinforced aluminum matrix composites is thatthe TiB2particles transforms the needle-like Eutectic silicon to short-rod-like.(2) Investigations are made on the effects of tension/compression asymmetry duringcreep deformation under different conditions. The asymmetry is found to be dependent onstress and temperature. At higher temperatures or higher levels of stress, the differencebetween the tensile and compressive creep is larger. Scanning electron microscopy (SEM)and transmission electron microscopy (TEM) are both used to visualize the micro-scopedefects. Cavity nucleation appears to be the cause of the asymmetry in creep behavior, whichis determined by temperature and stress. A new mathematical model for creep is constructedand validated considering the different asymmetric mechanisms of tensile and compressivecreep. And verification was made based on the experimental results. The model is validated.Comparing the enhanced intensity between TiB2particle reinforced aluminum matrixcomposites and cast aluminum, it is found that the TiB2particle reinforced aluminum matrixcomposites’ compressive creep performance is strengthened under medium or lowtemperature, but weakened under high temperature. The new model for tensile andcompressive creep’s applicability for cast aluminum matrix composite is also validated. (3) Combined with the elastic-plastic performance, TiB2particle reinforced aluminummatrix composites’ compressive creep performance is further studied. The interactionbetween compressive creep and cycling plasticity were analyzed carefully. Under medium orlow temperature (200℃), cycling plasticity affects little on the creep performance; underhigh temperature (300℃), cycling plasticity would reduce creep resistance. Compressivecreep caused further weaken on the cycling elastic-plastic performance. Negative yieldcenter’s influence by compressive creep is not obvious, while the positive yield center moveforwards the positive direction after coupled with compressive creep. Transmission electronmicroscopy is used for micro-scope observation, the mechanism of the interaction betweencompressive creep and cycling elasticity-plasticity is analyzed. While adding creep, thedislocations formed under cycling elasticity-plasticity rearranges. The higher the creep, themore obvious the dislocation wall and the crystal substructure in that material, which is thereason for the positive yield center moving. But under the high temperature, dislocationsrestore more while creep deformation increases, which contributes to the dislocation densitydecreases. Thus, the material becomes soft and yield radius decreases. Based on the macroand micro analysis, new mechanical model is made. Applying Matlab for verification, themodel displays the interaction between compressive creep and cycling elasticity-plasticity,and reflects the stress-strain relationship similar to experimental results.(4) Applying finite element method to simulate the piston’s working condition, thevulnerable spot is determined on the piston throat. Based on the results above, gather thecycling stress data into the new cycling compressive-creep-elasticity-plasticity model andcalculate. A stable hysteresis loop is maintained. Make up the defects that the FEM softwarecould not account the interaction between creep and elasticity-plasticity. Then, based onenergy method, makes a life predictive calculation. The result shows that, the new model’shysteresis loop predicted life agree with the experimental results.