Synopsis Structural change in f c-6 .CJ at"" )\10 allo), dl/.rillg ageillg treatment was examilled to kllow Ihe relalioll belweell the structure alld Ihe mechallical Ilrolierties. 1t was foulld Ihat Ihe lemlleralure dellCll{lellce of ) 'ield stress was less ill Ihe quenched allo.Y Ihall ill a-iron. Th e variatioll of )'ield slreJs wilh tem/Mralure was larger in the aged slale wilh Iwo-Ilhase slruclure Ihan in Ihe quenched slate. The reialiollshi/I belween Ihe aclivalioll /lammetersfor /JloJtic

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... eformation and the efrectil'e stress ill bolh the quenched and the aged stales was different from Ihat in a-iron. Activation IJam meters in Ihe aged state (aged to obtain the maximum hardness at Ih e ageillg lem/lerature of lOO ' C) were almost the same as those in. the quenched Fe-4.1 at 0 " )\/[ 0 allo.Y. which show intermedia te /la lueJ between Ihose ill a-iroll and the quenched Fe-6 .9 a tO" 1\110 01/0.)" .. lIlhough Ihe si.:e alld Ihe meall s/J{lcing qf /Jreci/litates became larger. the aclimtion /Jarameler did not challge b), the ageing 10llger than 200 mill. These facts suggest that the tem/lerature de/ICI/{Iellt /J{lrt of )'ield stress of the aI/a), is determined b), the dislocation motion in rt -/Jhase. It was also foulld. in the quenched state. that the mobile dislocation dens it)' calclliated from the results qf mechanicaltestillg was about 1/ 10 to 1/ 100 of the dellsity determilled b), tmnsmissioll electron microsco/ry. ( ,I. ,l.