PROPERTY AND THERMOSTABLITY STUDY ON TC6 TITANIUM ALLOY NANOSTRUCTURE PROCESSED BY LSP

TC6 titanium alloy samples are processed by laser shock peening （LSP）. Then, some samples are vacu- um annealed at 623 K for 10 h for the study on the thermost.ablity of the nanostructure produced by LSP. The characteristics of the strengthened layer and nanostructure are studied by atomic force microscopy（AFM）, scan- ning electron microscope （SEM）, electron backscatter diffraction（EBSD）, X-ray diffraction（XRD）, and transmis- sion electron microscopy（TEM） appliances, meanwhile the enhanced microhardness is tested at cross section. AFM of the processed surface indicates that the deformation is approximately uniform, and LSP slightly increases the roughness. SEM and EBSD of the strengthened cross section show that a phases are compressed to strip- shaped, a proportion of a and ~ phases is shattered to smaller phases from surface to 200 ttm in depth. The sur- face XRD shows that although there is no new produced phase during LSP, the grain size refinement and the in- troduction of lattice micro-strains lead to the broadened peak. The TEM photographs and diffraction patterns in- dicate that the shock wave provides high strain rate deformation and leads to the formation of nanocrystal. Com- pared with the samples before annealing, the dislocation density is lower and the grain-boundary is more distinct in the annealed samples, but the nanocrystal size does not grow bigger after annealing. The microhardness measurement indicates that LSP improves the microhardness of TC6 for about 12.2% on the surface, and the layer affected by LSP is about 500/~m in depth. The microhardness after annealing is 10 HVo.5 lower, but the affected depth does not change. The thermostable study shows that the strengthened layer of TC6 processed by LSP is stable at 623 K. The strengthened thermostable layer can significantly improve the fatigue resistance, wear resis- tance and stress corrosion resistance of the titanium alloy. The study results break the USA standard AMS2546 that titanium parts after LSP are subjected in subsequent processing within 589 K.

Expression and Characterization of a Thermostable Acyl-peptide Releasing Enzyme ST0779 from Sulfolobus tokodaii

Acyl-peptide releasing enzyme（AARE） belongs to a serine peptidase family and catalyzes the NH2-terminal hydrolysis of Nα-acylpeptides to release Nα-acylated amino acids. ORF0779（ORF=open reading frame） from thermophilic archaea Sulfolobus tokodaii（ST0779） was cloned and expressed in E. coli BL21 and the expressed protein was identified as a thermostable AARE. The target protein could be optimally overexpressed in E. coli at 30 °C for 8 h with 0.1 mmol/L isopropyl β-dthiogalactoside（IPTG）. The crude enzyme was heated at 70 °C for 30 min, and then the target protein could account for above 40% of the total protein. The purification fold was 27 and the enzyme showed both esterase activity and peptidase activity. The optimal temperature and pH for ST0779 were 70 °C and 8.0 when Ac-Ala3 was used as substrate. The half-life of the enzyme（0.2 mg/mL） at 90 °C was about 16 h, indicating that the enzyme exhibits a favorable thermostability. The activity of ST0779 could still remain over 85% after being treated at 25 °C in different buffers with pH range from 6.0 to10.0 for 24 h, which indicates ST0779 is stable in neutral or slight alkali environment. Under neutral or slightly alkali conditions, the enzyme exhibits really high catalytic efficiency against acyl-peptide, and the optimal substrate is Ac-Ala3. Most metal ions have no inhibition effect on the activity of ST0779, while 4% activity of ST0779 is inhibited in the presence of K＋. This enzyme was supposed to be applied in the analysis of protein sequencing and the synthesis of small peptides.