Abstract
The distribution of residual lattice strain as a function of depth were carefully investigated by synchrotron-based high energy X-ray diffraction (HEXRD) in TC11 titanium alloy after laser shock peening (LSP). The results presented big compressive residual lattice strains at surface and subsurface, then tensile residual lattice strains in deeper region, and finally close to zero lattice strains in further deep interior with no plastic deformation thereafter. These evolutions in residual lattice strains were attributed to the balance of direct load effect from laser shock wave and the derivative restriction force effect from surrounding material. Significant intergranular stress was evidenced in the processed sample. The intergranular stress exhibited the largest value at surface, and rapidly decreased with depth increase. The magnitude of intergranular stress was proportional to the severity of the plastic deformation caused by LSP. Two shocks generated larger intergranular stress than one shock.
Highlights
Mechanical surface treatments, such as shot peening (SP) and laser shock peening (LSP), are widely used in engineering components especially compressor discs and blades of jet engines to improve their fatigue performance.[1,2,3,4] Tensile stress is usually observed near material surface during its fatigue performance,[5] and generally fatigue cracks initiate at surface or near surface and propagate toward the material interior causing final failure
Synchrotron-based high energy X-ray diffraction (HEXRD) in transmission geometry has been used to measure the residual lattice strains in bulk TC11 titanium alloy through LSP-treatment direction for OS and TS regimes
The following results are reached: 1) The compressive residual stress was generated in LSP-processed sample at surface and subsurface, and the stress changed to tensile state because of the accommodation effect from the material in deeper region to counter the compressive stress in the above upper region
Summary
Mechanical surface treatments, such as shot peening (SP) and laser shock peening (LSP), are widely used in engineering components especially compressor discs and blades of jet engines to improve their fatigue performance.[1,2,3,4] Tensile stress is usually observed near material surface during its fatigue performance,[5] and generally fatigue cracks initiate at surface or near surface and propagate toward the material interior causing final failure. SP or LSP can delay the initiation and propagation of fatigue cracks by introducing a compressive residual stress at and near surface regions.[6] LSP has attracted more attentions than conventional SP recently since it introduced significant compressive stress distributed even more deeply inside a metal alloy and can greatly increase the fatigue life of components with high reliability applications.[7,8,9,10]. Synchrotron-based HEXRD was applied on the TC11 titanium alloy with two laser spots with 50% overlapping rate in LSP process for micro stress/strain characterization. Synchrotron high energy X-ray can penetrate thicker samples (mm in thickness) with good angular resolution,[12,16] which make it especially suitable for characterizing the microstructures and various stresses in dense material. Residual microstrains and FWHMs of different hkl planes were carefully measured through depth direction for one- and two-shock regions, respectively
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