Saad, G

Youssef, T H

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In d ia n  J .  P h y t .  49, 212-217 (1076)On the fracture characteristics o f heated ironT. H. Y o u ssb f  a n d  G. S a a bMetal Physics Unit, National Research Centre, Cairo, Egypt (Received 5 August 1974)Th.0 fracture of polycryslallino iron wires was studied in the tem­perature range (R.T. 500°C). I t  was found tliat heating decreased the fracture surface energy. This was attributed to increase in cleavage crack distances caused by excess formation of minute cavities at the grain boundaries.1. IntbodtjctionThe fracture surface energy (y) is one of very important material characteristics that appear in the models of brittle fracture initiation (Dobbs et al 1973). In this work (y) was evaluated from'the results of stress-strain experiments done for wires liaving different grain diameters. The principle of this evaluation consists of the comparison of the Fetch (1953, 1958) and Cottrell (1959) relationso'v — <TQ-\-Kyl (1)1-* (2)whoro (Ty : yield stress.(Tg : frictional stress.Kp : Fetch slope.I ; average grain diameter.(Tp : fracture stress.B i a constant which is unity for tensile stress experiments.0 : rigidity modulus.From the evaluated fracture surface energy, the cleavage fracture distance (d) taking place in the fracture process could be calculated using Gilman’s (1960) relation.2d* , ^r  =  I - (<r„ep). (3)where (<Tp€p) is the energy absorbed during plastic flow. The object of this paper is to show the effect of heating up to SOO^ C on the fracture of polycrystalline iron wires.212Fracture characteristics of heated iron2. Expbeimbntal Peoobdure2 l aThe ineterial used was supplied by the British Steel Corporation in the form of wires 0.25 mm diameter. Precise chemical analysis is as follows (in wt. %) C-0.012, Si, 0.018, Mn, 0.92, Po, 0.014, S, 0.016, and Cu, 0.02. Wires pf different grain diameters wore prepared by annealing in vacuum (10“® mm Hg) for 4 hours at different temperatures. The attained average grain diameter as measured by the line intercept method for each of the annealing temperatures is given in table 1 .Table 1Annealing temperature (°C) 605 660 726 780Average grain diameter (mm) .065 .076 0.11 0.14X-ray photographs using GoKg^  radiation for these annealed wires showed that all these grains have fully recrystallized structure. The stress-strain experi­ments for these annealed wires were done using conventional type tensile testing machine at a constant rate of extension of 0,5 mm/min. The wires were heated i o the working temperature while they were clamped in the tensile testing machine.3. Expbeimbntal ResultsTypical stress-strain curves for the annealed wires at tho different tempera­tures R.T. (25®C), 100°C, 205°C, 310°C, 400°C, and 500'^C are given in figure 1 . I t is clear that there are no sharp yield points in agreement with similar previousG TRA I N lO’iOr-3Fig. 1. Stress-strain curves for tested iron wires. Numbers on curves refer to working temperature.214 T. H. Youssef and G. SaadobsorsratiojiH on iron (K(^ li I9f)2, You8«of at at 1974). Tlio Poicli-Cottrell rela- UonH (2) and (3) inovedto bo valid lor the studied wires. This was judged from the linear dependence of yield and fiacturc stresses on 1“  ^ shown in figures (2a), (26), fi’orn which ii. can lie deduced tliat (a) the friction stress ((Tq) is constant for all relaiifUiH independent on the oking temjjerature and (b) the Fetch slope {Ky) decreased by increasing working temperature. It was found that at all tempera- tures tlie wires fracture in a brittle manner as shown in figure 3, which shows intcrgranulaj' fracture ^vith little or no localized necking. A metallo- graphic study was done for fractured VAdros, and revealed the existence of many grain boundary ci acks as shown in figure 4 in which a triple point crack occurs behind a fracture area.2. Toinporatuio and p-ain diameter dependence of the yield and fracture stroBaeB-of iron.By taking ngidity modulus (C;) for iron as 7.9x 10i*jdyne/om* (Keh 1962), ind substituting in (/tdtrell s relation (2), the fracture surface energy for tested wires at tin. diffeient temperatures was calculated. The dependence of fracturo surface energy on working temperature is given in figure 5, which indicates that (a) the Iracture surface energy at room temperature (25“C) equals 3.6x10* org/cm- m reasonable agreement with that previously calculated (Vreeland et al 9o3) and (b) the tempi-raturo dependence of fracture surface energy yielded negative temperature rate of 46 erg/cm*/' C.Fracture characteristics of heated iron 215H’ig. H. Croaa-soction on tensile toatod wire showing intergranular fracture. Magnification X 100.Fig. 4. Triplet point crack in fraetm-ed wire. Magni-fication x600.216 T. H. Youssef and G. Saad5. Dppendonce of frtiotupR Hurfoce enorgy on working ttimperatur©.working Temperaiurc ( °C )Pig. (1. Dopendcnce of tho oJoavago fracturo cliRtanco on working tfimporatiire for iron wires of different grain diameters.4. D isc u ssio nIt was argued that three meehauisms wore considered for fracture in poly­crystalline metals, these were (a) boundary strength, (b) unpinning of Frank- Read sources, (c) goneiat-iou of dislocations from within grain boundaries* Since neither lU’ank-Read sources nor dislocation pile-ups were observed in iron (Roberts e.t al 1970), the only possible mechanism for fracture in iron is the genera­tion of dislocations from witliin grain bouiidarjes. However, the fact that all data shown in figure (2a) extrapolate back to the yield stress of a single crytstal at Z  ^ 0 would indicate that matrix dislocations are playing a minor role inFracture characteristics of heated iron 217the fracture process, and th at dislocations in grain boundaries offer a much stronger barrier to  slip dislocations than th e dislocations in the m atrix, and it is tl^e grain boundary dislocations th at control the yield  stress. This m ight explain  the intergranular fracture seen in photom icrograph 3, and th e triple point crack shown in photom icrograph 4 which is often caused by grain-boundary sliding (Garafalo 1967).The different Fetch slope [ K y )  values observed in figure (2a) are no doubt due to the varying character o f the grain boundaries in the different woi king tem pera­tures. I t  is generally believed th at (Ky) value is related to difficulty of propagat­ing slip across grain boundaries. Such propagation is assisted by  the stress con- (!entration ahead o f a slip band in th e yielded grain in itiating slip in a neigh­bouring grain, and consequently cleavage cracks would be nucleated at the point of sliding grain boundaries as had been previously observed (Golland et al 1967), Calculating th e cleavage fracture distance (d) for tested  vHres by substituting in Gilman’s relation (3), and putting (y) as taken  from results o f figure 4. The dependence of cleavage fracture distance on working tem perature is given in figure 6, which indicates th at raising tem perature increased cleavage fracture distance, and the rate o f such increase rises with grain diam eter. On the basis of these data the negative dependence o f fracture surface energy observed in  figure 5 m ight be attributed to  the therm al activation  o f the fracture mechanism, becjause while straining at elevated tem peratures m inute cavities were nucleated  in the gi’ain boundaries, and weaken the boundary more than in the bulk of the  grain, and decrease the stress concentration in th e boundaries, consequently  less energy would be supplied for the creation o f cleavage cracks in the grain boundaries, and as a result decrease the corresponding fracture surface energy.R e f e r e n o b sCottrell A. H. 1959 C onference on  Fracture^ Swarnpscoit.Dobbs H. S., Field J. E. & Maitland A. H. 1973 P h i l  M a q . 28. 33.Garafalo F. 1967 D u c til i ty  ASM, pp. 87.Gilman J. J. 1960 A c ta  M e t, 8, 665.Golland D. I. & James P. L, 1967 A c ta  M e t. 15, 1889.Koh A. S. 1962 A I M E  Im p e rfec tio n s  in  C rysta ls, p. 213.Fetch N. J. 1963 J J . S . I .  174, 26.Petch N. J. 1968 P h i l  M a g . 3, 1098.Roberts M. J. & Jolley W. 1970 M e t. T ra n s . 1, 5, 1389.Vreeland T., Wood D. S. & Clark D. S. 1953 A c ta  M e t. 1, 44.Yoiissof T. H. & Saad G. 1974 Proc. o f  the M a th , a n d  P h ^s , TJ .A .R . 38, 46.

On the Fracture Characteristics of Heated Iron

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On the Fracture Characteristics of Heated Iron

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