Effect Of Surface Nanocrystallization On Fatigue Property Of Commercially Pure Titanium And Titanium Alloys

Titanium and its alloys are widely used in aerospace, biomedical and chemical industries and other fields due to the excellent properties as high specific strength, strong corrosion resistance and superior biocompatibility. However, they also have some disadvantages as low fatigue strength and poor wear resistance etc., which greatly restricted their further development and application. Therefore, it is very important to improve the fatigue property of titanium and its alloys.Since prof. K. Lu advanced the new concept of surface nanocrystallization (SNC) which induced by surface severe plastic deformation (SPD) in 1999, the research interests of the present authors currently focus on the mechanism, technique and the industrial application. This technique has been supposed that has vast development potential because its simple process and the nanostructured surface layer achieved by means of which continues to the substrate gradually.In this paper, the SNC of titanium and its alloys was carried out by high energy shot peening (HESP), then the effect of SNC on the fatigue of titanium and its alloys coupled with the intrinsic mechanism are studied systematically. Furthermore, a new approach that improves the fatigue strength of titanium and its alloys by SNC was proposed. Above mentioned study provides theoretical guidance for the engineering application of SNC.Surface anocrystallizationg on commercial pure titanium (TA2) and its alloy (TC4) were carried out by high energy shot peening (HESP) under the various shot time using crawler shot peening equipment. The microstructures on the surface teated by HESP have been characterized by X-ray diffraction analysis, transmission and scanning electron microscopy. Based the considering synthetically SNC degree, surface damage and roughness, both TA2 and TC4 achieved optimum peening result under the condition that the diameter of the shot is 1 mm, the peening velocity of the shot is 50 m/s and duration time is 2h. The average grain size in the as-treated surface layer is about 30 nm.It was found by observation of microstructure morphology using SEM that there is non-uniform deformation mechanism in the lamellar-structured titanium alloy (TC4) during HESP, i.e., the deformations are accomplished mainly through the extrusion plastic flowing and shear plastic flowing happened on a phase along the interface of a andβ, while theβlamellar deform hardly. Thus the strain concentration in unstable shear band is more exacerbated. The non-uniform deformation mechanism is the main reason that causes mechanical damage in lamellar-structured titanium alloy during HESP.The fatigue strength of original and as-peened samples was examined by rotary bending fatigue test. The results show that the fatigue limit of TA2 has been improved about 34% after SNC, namely improved from 220 MPa of fatigue limit of annealed specemen to 295 MPa. While the fatigue strength of equiaxial structured TC4 has been improved 20%, namely improved from 485 MPa (annealed fatigue strength) to 580 MPa. Meanwhile, HESP cause surface mechanical damage and surface roughness coarsening, which restrains the positive effect of surface nanostructure on the enhancement of fatigue resistance dramatically. Therefore, how to repair or reduce the surface damage so that let the nanocrystalline structure play full role on improvement of fatigue limit becomes a key factor.Based on the above mentioned results, a combined shot peening for surface nanocrystallization (CSPN) was presented as novel process of SNC, i.e. use smaller shots (0.5 mm in diameter) to peen the surface for 20 min after HESP, aiming at repairing the surface micro-damage and lowering the surface roughness. After CSPN, the fatigue limit of TA2 has improvee further to 335 MPa,52% higher than that of the anealled spcemen. While the fatigue limit of equiaxial structured TC4 increases further to 650 MPa,43% higher than the anealled spcemen. This result indicates that CSPN has marked effect on the fatigue limit enhancement, which is the most effective method that improves the fatigue limit of titanium and its alloy by means of SNC reported so far. The reason is due to CSPN may improve the surface quality, which inclose slowing down the inhomogeneity of local micro-deformation, reduction of the internal stress concentration in the crystal, and repairing crystal defects and damage in grain boundaries and phase boundaries as well as micro-damage caused by HESP.