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Creep in Powder Metallurgy Products 211 moving extended distances in the lattice. This requires 1) generation of dislocations from discontinuities in the crystal lattice called dislocation sources. These may be segments of dislocations which are pinned at their ends, generally called Frank-Reed sources or grain-boundaries or free surfaces. 2) movement of the dislocations in the crystal lattice What happens when we apply a stress to the crystal? Under its influence dislocations are generated from the dislocation sources in the crystal. In single phase material this stress which causes generation of dislocation from sources is the stress associated with yielding.

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A dislocation mono! is prtiserttod in order to aiwount for the yield behavior of alloys with a finely dispersed second-phase. Tho criteria for yielding sod in thc model, is that appreciable yielding occurs in these alloys when the shear stress duo to piled-up groups of dislocations is sufficient to fracture or plastically deform the ii Fu3rsed second.phtise particles, relieving the back stress on tho dislocation sotirecs.EJliations tieriveil on the basis of this model, predict that the ield stress of the alloys varies as the reciprocal squal-root of the miienn free path botween dispersed particles. Experimental data is presented for seveini S-\J'.Type alloys. precipitilt RIO -hardened alloys and steels which are in good agreement with the 3-icM strength iiriiitioTr u-s a function of dispersion spacing prado-ted by this t,hieoretiem1 treatment.Cl1Tl'ERE DE lhw"rliruE 1'()Ul UN Al,lJ,\GE i)UIIUI 1'AR T)ISPERS]ON 1 I modt-k' tIe d ish,rtm lion ii. at r 11Ti'S(1> 1' C)I1 I CX 1)1 ((I1"r le >-onp tiP emnen I. de la ru pt iire (Ics nil inges ?L pluise speinilil ire fineitituit dispeise. Je cmitre (IC rip lure miiipluvi dans co tnttdle est-qu one to,> npltre(ial)le so 1 riiduit clam (,es ill itiges quit id lit elisi to tie eisa illeitti,nt iltit' a des ettipileincitis de rIishi.meat-i1i-t. c-st sulh-iaiilc >0111 s'tiilsc-r 11110 ritpl.iini ito one d(loriiint 1)11 plmist iui-des pmLrtictmlos (Ic Ia l u 11fl5e tNttltilt.t>Ie diaper's.-i'ltiihiiiit-lii. t-eiiiitiin mrrri'le ant irs sourca tie 1ii-dislocation. I )r's rjutit ions tlri'ivisw stir ii has>-d>' tt 101(1010 print isnt pitt-Iii Ii torte ilastirpite des ailuirges vane em> rtl inn inerse (Ill carte dc-lthres i coors riovnmis cot IC It's purtieriles dispers'es. Des vn-lr'itrs exl(riTI1ci!ttlles son teinut' pt' priciqImes rltr:igi-s thu typr-A]'. tIes rIling's pt'ctlt-ttt trIll strtictrti'ti.to nt des icier's. (pui sutti iii Iron ac-cord ave> itt. vtrrilt l-r.ir iii, Iii 1 irtrttn ('ltlStI(pile en iomtr'tion do in distance eat-re 1tnt cribs ilispi-rsc's preclit.m-s Iii r i-es tunsidirat tIltS I lieoricpiiri.

A technique was developed to produce ice containing a fine uniform dispersion of amorphous silica particles. Rates of creep in tension of dispersions with ½- and 1-volume % silica in ice were measured in the temperature range of −22 to −2°C and in the stress range of 4.64–17.8 bar and compared with those of pure ice. The silica dispersion strongly decreases creep in ice, resulting in steady-state creep rates in a 1-volume % dispersion 10–30 times slower than in pure ice. The activation energy for steady-state creep in dispersion-strengthened ice calculated from the temperature dependence of the creep rate is stress dependent with an average value of 97 kcal/mole, 6–7 times that for self-diffusion in pure ice. The steady-state creep rate increases exponentially with stress. This behavior differs from that of metallic coarse-grained dispersion-strengthened materials, in which the creep rate is proportional to a power of the stress and the activation energy is stress independent and equal to that for the self-diffusivity of the matrix metal. It is suggested that this unexpected behavior is due to particle-matrix decohesion in the ice-silica system, which interferes with the normal blockage of dislocation motion by second-phase particles during creep.