EFFECT OF LASER INFLUENCE ON MARTENSITE TRANSFORMATION INTO THE NEARSURFACE LAYERS OF Fe-Al-C ALLOYS

V.A.Andrushchenko,E.N.Dzevin

Institute of Metal Physics Ukrainian National Academy of Sciences, Vernadsky blvd. 36, 252142 Kiev, Ukraine

The solid solution of Fe-Al-C alloys in the wide concentration and temperature intervals consists of austenite phase with f.c.c. lattice (g-phase), carbide FeAlC, tetragonal -martensite and aluminium ferrite with b.c.c. Coherency and mutual orientation of f.c.c. lattices of g- and K-phases interchangeably with injected into g-lattice carbon atoms determine not only parameters but OR being formed from austenite of a-martensite with b.c.t. lattice:

1) a-martensite formed from austenite with f.c.c. lattice which isn't coherent with K-phase f.c.c. lattice has tetragonality degree proportional to the quantity of dissolved in austenite carbon and 24 OR K-S;

2) a-martensite formed from austenite with f.c.c. lattice being coherent with K-phase f.c.c. lattice has anomalous high meaning of b.c.t. lattice elementary cell c/a and 48 twinned G-T OR. The reason of arising the anomalous high tetragonality and additional a-martensite twinning, apart from other factors, consists in a value of coherent stresses between f.c.c. and b.c.t. crystal lattices of K- and a-phases, respectively.

It was chosen the monocrystallic Fe-4%wt.Al-2%wt.C sample (after quenching the alloy from 1150oC into the water) the axis of which was coincided with the axis [100] of austenite f.c.c. lattice. This specimen was exposed to fivehold (8msec) laser impulse action (10J) in the end of it, i.e. along the axis [001]g. Then the sample was cooled into the liquid nitrogen (-196oC). The series of X-ray surveys was performed in the X-ray chambers of rotation RKV-86.

The obtained data allow to determine the character of martensite transition in different nearsurface layers of syngle crystal sample after laser exposure:

  1. surface layer (up to 0,2 mm) consists only of g-phase which doesn't undergo g->a martensite transformation up the liquid nitrogen temperature;
  2. in near-surface layers 0,25÷0,75 mm and deeper than 4,5 mm from the surface presence of a-martensite with anomalous high degree of tetragonality testifies that there are additional coherent stresses between the a-martensite b.c.c. lattice and f.c.c. lattice of K-phase submicrovolumes;
  3. formation of a-martensite with c/a=1,091 at a depth of 0,75÷1,0 mm and 4.0÷4.5 mm witnesses to the absence of additional coherent stresses between b.c.c and f.c.c lattices of a- and K-phases and corresponds to availability in crystal lattice 2% implanted atoms of carbon;
  4. formation of a-martensite with c/a=1,009 at a depth of 1,0÷3,0 mm from the end witnesses about a deficit of alloying elements in near-surface layers on the one hand, and an absence of no matter additional coherent stresses on the other hand.

Thus, with the help of X-ray structural investigations it has been established that in austenite single crystal samples Fe-4%Al-2%C subjected to fivehold impulse laser exposure (8 msec,10J) in the direction of crystal lattice [001]g , structure change of austenite reaches the depth of 4,5 mm from the surface. Analysis of martensite crystal lattice parameter and its orientation relations allowed to make a supposition that after laser exposure near-surface 4,5 mm layer consisted of six different in structure austenite interlayers. Martensite has different orientation in reference to residual austenite in these interlayers.