CYCLIC DEFORMATION OF FACE CENTERED CUBIC CRYSTALS AND ITS DISLOCATION INTERACTION MODEL——Ⅰ.CYCLIC DEFORMATION OF COPPER CRYSTALS

Cyclic stress-strain responses and dislocation structure of copper single crystals with varioustensile axes were systematically studied and compared with each other.Experimental resultsreveal that the evolution of microscopic dislocation configurations in a crystal and,accordingly,its macroscopic cyclic behaviours are closely related with its orientation.Re-markable secondary slip has been observed in some crystals with orientations well inside thecrystallographic triangle, which are usually considered as single-slip ones.Their cycliobehaviours and dislocation structures at saturation are similar to those of their neighbouringmulti-slip crystals.These results have constructed the experimental basis for the newly pro-posed dislocation interaction model Jor cyclic deformation of fee crystals,which will be des-cribed in the second part of this article.     

EFFECT OF SLIP ORIENTATION ON STRESS RESPONSE,INTERNAL FRICTION AND ULTRASONIC ATTENUATION DURING CYCLIC DEFORMATION IN Al SINGLE CRYSTALS

The variation of cyclic stress,internal friction and ultrasonic attenuation during cyclicdeformation and relations among them have been investigated with different slip orientationAl single crystals.The results indicate that the value of cyclic stress σ,internal frictionQ~(-1)and ultrasonic attenuation △α depend obviously on the slip orientation.There are largedifferences in above three parameters for different slip orientation Al crystals.In early stageof fatigue life,σ and △α increase and Q~(-1)decreases with cycles N,and △α reached maxi-mum before σ,while Q~(-1)and σ get the valley and the peak,respectively,at same cycles.     

BAUSCHINGER EFFECT AND DISLOCATION STRUCTURE IN STAINLESS STEEL DURING CYCLIC DEFORMATION

Cyclic deformation in symmetrical push-pull mode was carried out at room temperature in airusing a Schenck hydropuls machine.The total strain amplitude which was kept constant dur-ing the test ranged from ±0.004 to±0.012.The 0.2% offset yield stress σ_(0.2f) in tensionand σ_(0.2r) in compression and peak stress σ were measured from the stress-strain hysteresisloop at various cycles.The Bauschinger strenghth differential factor(BSDF)was then calcu-lated from σ_(0.2f) and σ_(0.2r).The energy loss △E of each cycle was determined fromhysteresis loop areas.These parameters,BSDF,σ and △E,appear to have two distinctivelydifferent stages.The dislocation structures were observed using TEM in specimens deformed cyclically,forvarious cycles.The observation shows that the dislocations pile-up mainly against grainboundaries and there exist large amount of deformation twins.The addition of 0.25 wt-% ni-trogen reduced the stacking fault energy of the alloy significantly.Cross-slip and climb aretherefore rather difficult to occur during the cyclic deformation at room temperature,andwell-defined dislocation cells and walls can only be seen at the final stage of fatigue.     

EFFECTS OF SLIP MODE ON CYCLIC BEHAVIOR OF PRESTRAINED MATERIALS

The cyclic behaviours of two prestrained materials with different slip modes have been com-pared.From the analysis of hysterisis loops,it is suggested that the decrease in back stressdominates the cyclic process of Cu-Zn alloy with planar slip mode,while the decrease infrictional stress is the main factor affecting the behaviour of Cu with wavy slip mode.The re-lation between the dislocation configuration and internal stress has been also discussed.Bymeans of the relation just mentioned,the variations of dislocation configuration before andafter fatigue of the two materials,the differences in their softening mechanism and mechani-cal behaviour could be interpreted.     

CYCLIC DEFORMATION OF FCC CRYSTALS AND ITS DISLOCATION INTERACTION MODEL Ⅱ. DISLOCATION INTERACTION MODEL OF CYCLIC DEFORMATION

运用位错之间的交互作用理论,对各种轴向Cu晶体的循环形变行为进行了分析和比较,确认晶体循环形变的应力-应变响应以及形成的位错组态是由各滑移系统的位错反应的模式和强度决定的,根据晶体轴向在标准取向三角形中的位置,可以推断其循环形变行为.     

INTERFACIAL DEBONDING OF FRC UNDER CYCLIC LOADING

With the increasingly use of FRC (fiber-reinforced composite) in urban lifelines, mechanical properties investigation is very important for disaster resistance, especially the investigation of fatigue properties. Based on the shear-lag model, an usual composite model under cyclic loading is established. According to the Paris formula, the relationship between interracial fatigue parameters and the number of cycles is obtained under the cyclic loading. Interracial fatigue properties of this model and the growth of the interracial fatigue crack are analyzed. And the Poisson ratio is considered also.     

PLASTIC DEFORMATION AND DISLOCATION STRUCTURE IN γ'-PHASE OF A DIRECTIONALLY SOLIDIFIED Ni-BASE SUPERALLOY

The dislocation structure of directionally solidified(DS)Rene 80 superalloy during tensileand creep deformation has been studied by TEM.In tensile deformation,γ'shearing occurs byslip of APB-coupled(1/2)<110>dislocation pairs on{111}planes at temperatures below760℃.and by slip of SISF-seperated(1/3)<112>superpartials which have dissociated on{111}planes into a(1/2)<110>screw superpartial and a(1/6)<112>edgesuperpartial with an APB in between at temperatures equal to or above 760℃.In creepdeformation,γ'shearing occurs again by slip of SISF-seperated(1/3)<112>superpartialswhich have not dissociated on{111}planes and by diffusive slip of APB-coupled(1/2)<110>dislocation pairs at 760℃,618 MPa.However,no γ'shearing occurs at 980℃,190MPa.During secondary creep at 760℃,618 MPa or at 980℃,190 MPa,2-dimensional net-works have formed at γ/γ'interfaces,and the strain is mainly the result of diffusion control-led climb of dislocation networks at γ/γ'interfaces.The dependence of secondary creep rate εupon size a and volume fraction v_■ of cubic γ' has been found to be ■∝α/v_f~(2/3).     

A MICROMECHANICAL MODEL FOR γ-TiAl BASE PST CRYSTALS

An analytical micromechanical method is proposed to examine the dependence of plastic deformation on the microstructure for a PST crystal. The sub-domain microstructure of the γ phase and the effect of the α2 phase are taken into account by a proper micromechanical formulation, the dislocation slip and twinning deformation mechanisms are considered in the context of crystal plasticity. The model can well predict the dependence of stress-strain relations on loading angle with respect to the microstructure. The influence of the twinning and lamellar spacing on the deformation behavior and biaxial yield surfaces for PST crystals are also examined.     

CYCLIC DEFORMATION OF COARSE GRAINED POLYCRYSTALLINE PURE Ai——Ⅰ.SLIP CHARACTERISTICS

Comparing with ordinary ploycrystalline materials sized to μm grade,the slip morphology ofthe coarse grained polycrystalline pure Al is characterized by:(1)several slip domains occurin a grain,and in same domain,several slip systems operate at same time or one after anotherintensely,a beautiful and neat slip pattern is forming on the specimen surface;(2)for highΣ-value coincident and random grain boundaries,the grain boundary affecting zone(GBAZ),bout 50—120μm wide,is favourable site to form intergranular crack at early fa-tigue life easily,and migration or slide of the boundaries were often observed.While lowΣ-value near-coincident grain boundaries show a higher degree of slip continuity and straincompatibility than high Σ-value ones.Intergranular crack is not easily nucleated at lowΣ-value near-coincident boundaries;and(3)due to suppression of grain boundary slip attriple grain boundary node,the high Σ-value and random grain boundary among the threeboundaries of tricrystal crack easily during cyclic deformation.     

EFFECT OF DYNAMIC STRAIN AGING ON LOW CYCLE FATIGUE BEHAVIOUR OF 18-8 AUSTENITIC STAINLESS STEEL

Studies were made of the symmetric tensile-compressive low cycle fatigue behaviour and theinfluence of dynamic strain aging(DSA)pre-treatment of 18-8 austenitic stainless steel.Within the testing amplitude range of strain.±0.5 % to±1.5 %,the three processes ofcyclic hardening,cyclic saturation and cyclic softening were observed.In the same amplitudeof strain,the peak stress of the samples pre-treated by DSA is higher than that of solid-solu-tion and cold working pre-treatment,but no remarkable differences of the fatigue lives ofthem were found.TEM observation shows that the uniform and stable dislocation networkswith high density form after DSA pre-treatment,which increases the cyclic peak stress.Thecyclic softening results from the low dislocation density and elongated cell structure with lowenergy.     

RELATIONSHIP BETWEEN FRACTAL DIMENSION AND CYCLIC CREEP PROPERTIES OF ROTOR STEEL 34CrMoA

Vertical section method has been used to measure the fractal dimension of cyclic creep frac-ture surfaces of rotor steel 34CrMoA at 550℃.The relationship between fractal dimensionand cyclic creep properties has been analyzed.The fractal dimension is dependent upon thepower product of minimum creep rate and rupture time,and satisfies the linear function withrupture strain.     

MODEL FOR INFILTRATION KINETICS OF LIQUID METALS AND ITS APPLICATION

On the basis of fluid kinetics,a model for infiltration kinetics of liquid metals into fibre pre-fabricated composite has been proposed.The infiltration processing,together with themeasurements of infiltrating velocity and permeability,for the C fibre-reinforced Al matrixcomposites have been studied.The model proposed seems to be in good agreement with the ex-perimental results.Two different infiltration modes,uniform and non-uniform,were foundand a new technique was suggested.Thus,the quality composites of C fibre-reinforcedAl may be made under low pressure regardless of non-wettability of liquid metal to the fibres.     

CYCLC DEFORMATION OF Nb SINGLE CRYSTALS

The Nb single crystals of both[321]and[110]orientations have been cyclicly deformed intension-compression at constant strain rate 8×10~(-4)s~(-1)over a range of plastic strainamplitudes between 10~(-3)and 10~(-4).The cyclic hardening,the changes in shape of crystals andthe asymmetry of stress have been studied.The hardening curve can be divided into threestages,i.e.first,rapid-hardening and saturated stage.In the first stage of cyclic hardening curve dominant features of dislocation configura-tions are high density networks and debris loops.In the rapid-hardening stage the main fea-ture is the formation of dislocation bundles.In the saturated a well defined bundle structurefully develops and between them it is filled with only screw dislocations and the imposed strainis accommodated mainly by the motion of screw dislocations travelling to and from betweenthe bundles.Three-dimension cell,two-dimension cell or bundle structures are summarized asthe saturated structures of bcc metals.     

IN-SITU OBSERVATION OF CRACK TIP DEFORMATION IN A DUPLEX STAINLESS STEEL

The development of dislocation structures in the plastic zone ahead of a crack tip has been in-vestigated in a duplex stainless steel during in-situ deformation experiments in a scanningtransmission electron microscope.It was found that the dislocation distribution wassignificantly different in the ferrite and in the austenite.In the ferrite grains,the dislocationsemitted by the crack tip may cross-slip out of the original slip planes and form a broad plasticzone.However,in the austenite,the dislocation free zone is small and the dislocations emittedby the crack pile up in its slip plane.The selection of slip systems at the crack tip depends onthe crack tip Schmid factors in both phases.But after large deformation,the selection of thesecond slip systems at the craek tip in austenite does not depend on the Schmid factors.     

CYCLIC DEFORMATION OF COARSE GRAINED POLYCRYSTALLINE PURE Al——Ⅱ.FRACTURE SURFACE MORPHOLOGY

SEM photos for very coarse grained and high ductile polycrystalline pure Al show followingcharacteristics:A few fracture sources are concentrated,the crack propagates in fan-shapenear them,and the fatigue striations are cyclic cleavage facets.Each striation consists of awide cleavage facet and a narrow cleavage stage.Those stages often blunt due to plasticdeformation.The fatigue striations are clear and continual,and distribute over whole zone ofstage Ⅱ crack propagation.Two kinds of fatigue striations form in different levels,areconjoined by a“twisted baked piece of pastry”band.The secondary crack along the fatiguestriations was observed frequently.Some of them have already developed into secondarymacrocracks and secondary fatigue striations were found clearly on secondary fracture sur-face.The fracture surface can be divided into two parts,initiation and propagation zone of thecrack,however,no statical-fracture zone was observed.The mechanism to form striationswas preliminarily discussed.     

EFFECT OF ORIENTATION ON STRESS RESPONSE,FRICTION STRESS AND BACK STRESS DURING CYCLIC DEFORMATION IN Al SINGLE CRYSTALS

The symmetrical push-pull fatigue tests at strain amplitude of 2×10~3 with different sliporientation of Al single crystals,were carried out at room temperature in air.The peakstresses of various cycles were measured and the stress-strain hysteresis loops for selected cy-cles were recorded.The energy losses.friction stresses,back stresses and shape     

CYCLIC DEFORMATION OF FCC CRYSTALS AND ITS DISLOCATION INTERACTION MODEL Ⅰ. CYCLIC DEFORMATION OF COPPER SINGIE CRYSTALS

系统地观察了各种不同轴向Cu晶体的循环应力-应变响应及其饱和位错结构.实验结果表明,晶体的宏观循环形变行为和微观位错组态的演变都与晶体的轴向紧密相关.在一些轴向位于标准取向三角形内的单滑移晶体中观察到明显的多滑移现象;其循环形变行为和饱和位错结构都与相邻的双滑移或多滑移晶体有许多相似之处.     

CYCLIC OXIDATION BEHAVIOUR OF MICROCRYSTALLIZED CoCrAl COATING

An investigation was carried out of the resistance to cyclic oxidation of the as-castnormal-grained Co-30Cr-5Al alloy substrate at 1000℃ in air together with its sputteredcoating of microcrystallized CoCrAl alloy grains sized less than 0.5μm.Acoustie emissi oncounts showed that the initially formed Al_2O_3 scale on the bare alloy substrate cracked andspalled severely off after only one or two cycles on cooling and then the worse protective layerof Cr_2O_3 and CoCr_2O_4 spinel occurred on surface.While such substrate coated withsputtered microcrystallized CoCrAl alloy film was not damaged under oxidation even through100 cycles at 1000℃.In comparison with the normal-grained alloy,the microcrystallized al-loy,coating of similar constitution exhibite excellent resistance to oxide spalling on surface.     

FATIGUE BEHAVIOUR OF SiC_p/6061Al COMPOSITE

The fatigue of SiC_p/6061Al composite containing 15 v.-% SiC particles has been compared with 6061Al alloy.Dislocation structure and microprocess of fatigue crack initiation and propagation in the composite have been investigated by using SEM and TEM.The results in- dicate that the fatigue strength at 10~7 cycles of the composite is 196 MPa,i.e.about 25% higher than matrix alloy.The voids and microcracks initiated at and near the interface be- tween SiC_p and matrix,where has a higher density of dislocations,will propagate and link up to form the fatigue crack.It is an important evidence to note that the dislocation channels where screw dislocation can travel are formed near interface and corner region of SiC_o in the composite subjected to fatigue stress(σ_(max)=274 MPa N=2.4×10~5 cycles),demonstrating the relationship between fatigue crack initiation and dislocation movement in the SiC particles reinforced 6061 Al alloy composite.