Pulsed laser induced deformation in an Fe-3 Wt Pct Si alloy
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power pulsed l a s e r s generate s t r e s s or shock waves in a t a r g e t sample by subjecting it t o very s h o r t pulses of intense l a s e r light. The resultant disturbance in the sample is primarily mechanical s i n c e the t h e r m a l effects are s m a l l and confined t o a r e g i o n within a few micrometers from the l a s e r irradiated surface. The shock wave is created by the momentum impulse imparted t o the specimen's s u r f a c e when a thin surface l a y e r is rapidly vaporized by the intense l a s e r light. Since this process depends on effectively coupling the l a s e r energy into the specimen's surface, the magnitude of the pressure is affected by such factors as the s u r f a c e condition, its reflectivity, and the t a r g e t material's sublimation and ionization energy. Also, phenomena occurring within the blowoff m a t e r i a l which absorb, reflect, or r e r a d i a t e energy, such as the formation of a plasma, l i m i t the amount of energy r e a c h i n g the surface of the specimen. For t h e s e reasons, it was anticipated that the modification of surface conditions through the use of various overlay materials, and blowoff confinement by transparent overlays would affect the peak p r e s s u r e s . In addition, modifications of the pulse length and shape could also influence the magnitude and length of the p r e s s u r e pulse. Various recent studies have examined methods of modifying the m a t e r i a l surface conditions t o enhance the magnitude of l a s e r induced shock waves.l"~ It has been shown that a 7075 aluminum alloy was shock hardened a f t e r irradiation with a high power, Q switched l a s e r through a transparent quartz overlay covering the t a r g e t specimen,s Another study showed that very short (~1 × 10-9 s), high fluence pulses could A. H. CLAUER is Associate Manager, Metal Science Section, and B. P. FAIRAND is Principal Physicist, Physical Sciences,Battelle, Columbus Laboratories, Columbus, OH43201. B. A. WILCOX, formerly affiliatedwithBattelle, Columbus Laboratories,is now with National Science Foundation, Division of Materials Research, Washington, D.C. 20550. Manuscript submitted April 26, 1976. METALLURGICAL TRANSACTIONS A
c a u s e back s u r f a c e spalling in thin aluminum d i s c s irradiated in vacuum. 6 This paper reports on a s e r i e s of experiments which r e l a t e the magnitude and extent of shock induced p l a s tic deformation produced in an Fe-3 wt pct Si alloy t o the l a s e r pulse length and power density for s e v e r a l different types of surface overlays. The overlays studied include a transparent overlay t o confine the blowoff m a t e r i a l t o the sample surface and an overlay of lead, which has a low sublimation energy. The etch pit patterns on the Fe-3 pct Si specimens a f t e r shocking and sectioning are qualitatively correlated with the m e a s u r e d and predicted s h o c k pressures. A previous paper7 presents the computer code calculations, l a s e r physics, and p r e s s u r e environments for the
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