Showing papers on "Deformation (engineering) published in 1969"

Strength and structure under hot-working conditions

Abstract: The main feature of hot working is that extremely large strains are applied to materials at high rates of strain at temperatures above ∼ 0.6Tm, where Tm is the melting temperature in degrees Kelvin. Strength and ductility under these conditions are markedly dependent on both temperature and rate of straining. Although this review is confined to strength and structure during hot working, ductility is intimately related to the deformation processes that govern plastic flow. This aspect has been recently reviewed by one of the authors. These large strains can be achieved with little or no strain-hardening, indicating that dynamic softening processes can operate sufficiently rapidly to balance the strain-hardening processes. In this situation, the structural changes involved can be used to obtain information on the mechanism of deformation. However, as emphasised later, care must he taken in the interpretation of such hot-worked structures, since significant structural changes can occur on holding at ...

Change of shape due to dislocation climb

Abstract: The geometry of the deformation produced by the climb of edge dislocations is defined and the conditions are determined under which dislocation climb, by itself or in conjunction with glide, can produce a general change of shape. The results are important in understanding the high-temperature deformation (T > 0.5Tm) of single crystals and polycrystals of materials with fewer than five independent slip systems. Some of these are discussed in detail. In particular the high-temperature creep of sapphire single crystals pulled parallel to (0001) is shown to be quantitatively consistent with Nabarro (Phil. Mag., 16, 231, 1967) creep by climbing dislocations.

An Experimental Investigation of the Influence of Speed and Scale on the Strain-Rate in a Zone of Intense Plastic Deformation

Abstract: Experiments are described in which an explosive quick-stop device and printed grids (0.002 in square) were used to measure the deformation (streamlines of flow) in the zone of intense shear in which the removed chip is formed in orthogonal machining. A method is given for calculating the strain-rate from the experimental streamlines and it is shown that both the maximum and mean values of the maximum shear strain-rate in this zone are directly proportional to speed (shear velocity) and inversely proportional to scale (depth of cut).

Dislocation Channeling in Neutron‐Irradiated Niobium

Abstract: Dislocation channels cleared of radiation‐produced defect clusters were observed by transmission electron microscopy in polycrystalline niobium irradiated to 4×1018 neutrons/cm2 (E>1 MeV) and then deformed in tension to 6.6% strain. From electron diffraction patterns and diffraction contrast (g·b) analysis, the {110} plane was determined to be the plane on which the dislocation channels formed in most cases. The strain due to the motion of slip dislocations in the channels was deduced from the offset at intersections between the channels and other microstructural features. The strain corresponded to the passage of 1–3 slip dislocations per slip plane. Several mechanisms for the clearing of defect clusters by slip dislocations are discussed. These include: annealing due to the heat of plastic deformation, chopping up or sweeping up of defect clusters by slip dislocations, and annihilation by antidefects.

Modulus and damping of polymers in relation to their structure

Abstract: For most applications of polymer products, the modulus or stiffness, i.e. resistance to deformation under load, is of prime importance. The mechanical losses or damping reveal molecular movements of small groups of atoms, which movements can be significant for the impact strength. This review illustrates the effect of crosslinking, polarity, and steric features of both side-groups and main molecular chain on both shear modulus and damping as functions of temperature. The examples are mainly taken from commercial or recently developed polymers. It is concluded that the most promising way of increasing the softening temperature of thermoplastics is to introduce rings into the main molecular chain.

Defect microstructure and mechanical properttes in shock-hardened metals - The relationship of defect microstructure and mechanical properties in shock-hardened metals and alloys and the comparison with simply compressed and cold-rolled metals was investigated by transmission electron microscopy

Abstract: This paper is concerned primarily with the direct observation of shock-induced defect microstructures in 70/30 brass, 304 stainless steel and copper; and the attempt to relate the microstructural features observed with experimentally determined mechanical properties. The method of direct observation utilizes both bright- and dark-field transmission electron microscopy. Some attention is also given to the relationship of shock-induced defect structures to those resulting from conventional modes of deformation such as simple compression and cold reduction by rolling. It is shown, through the use of the electron microscope, that structures in stainless steel vary greatly with the mode of deformation. Some consideration is also given to the apparent role of stacking-fault energy in determining residual-shock structures in fcc metals and alloys.

The plastic deformation of ferromagnetic face-centred cubic Fe-Ni-C alloys

Abstract: When face-centred cubic alloys with the physical properties typical of Invar are strained over a sufficient range of deformation variables, peculiar deformation characteristics become apparent. For example, there is an unusually large temperature-dependence of the flow stress, completely atypical of f.c.c. metals. Factors such as compositional departure from the Invar range, the intentional formation of martensite, arranged prohibition of martensite, and compositional control of Curie temperatures, were manipulated to determine the origin of the deformation behaviour. Only the Invars reacted as a class, and ferromagnetism appeared causal both to the onset of the special deformation and to its nature. It was observed that the increase of flow stress produced by cooling below the Curie temperature can be completely reversed upon heating, that the formation of martensite is irrelevant, and that the nucleation of martensite is inhibited by the spontaneous ferromagnetism. Observations such as these is...

The plastic deformation behaviour of long-range ordered iron-aluminium alloys

Abstract: The deformation scheme proposed in Part I to account for the multiple linear hardening stage behaviour of long-range ordered iron aluminium alloys has been confirmed by direct transmission electron microscopical observations of superlattice dislocations of the predicted type. Observations of the dislocation sub-structure produced by deformation of single crystals under various conditions are presented, together with a discussion of the properties of said sub-structure and their effect upon the hardening rates of the alloys.

Theory of Low Temperature Work-Hardening of Body-Centred Cubic Metals

Abstract: Stress-strain curves of bcc metals at low temperatures, where screw-dislocations predominate, are calculated for crystals with the [100] deformation axis. The main assumptions are (1) an increase of flow stress is proportional to the square root of forest dislocation density, (2) dislocations are multiplied by the cuttings of screw dislocations with repulsive elastic interaction, and (3) a screw dislocation annihilates with another dislocation with the opposite sign. The calculated stress-strain curve is compared with the observed stress-strain curves. It is concluded that the important property of screw dislocations in bcc metals is the easiness of cross-slip; this the origin of the variety of the work-hardening in bcc metals at low temperatures.

The morphology and growth of creep cavities in α-iron

Abstract: Electron fractography has been used to study the intergranular cavities formed in alpha-iron during slow tensile deformation at high temperatures. A minimum in ductility is observed at about 700° C: this coincides with conditions where grain-boundary sliding makes a maximum contribution to the overall deformation and where the morphology of the cavities tends to be dendritic and finely terraced. This is explained in terms of the gradient of chemical potential for vacancies which may develop at the cavity periphery during grain-boundary sliding. Under other testing conditions, planar growth is observed and the cavity surface is often faceted.

Effects of surface condition on the mechanical properties of ice crystals

Abstract: A comparative study of the deformation behaviour of mechanically and chemically polished ice crystals is described For single crystals, mechanical disturbance of the surface layer greatly reduces the maximum stress for basal glide in constant strain rate tests in compression Single crystals with sub-boundaries do not show any prominent surface effect, but exhibit quite a low maximum stress The results indicate that mechanically disturbed surface layers and sub-boundaries do not appear to act as strong barriers to dislocation movement in ice crystals, but rather as sources of dislocations Surface condition does not appear to affect the maximum stress for polycrystals The influence of strain rate (range 21 × 10−5 to 17 × 10−4 s−1) and temperature (range −10 to −30°C) upon the maximum stress was also investigated for both single crystals and polycrystals of ice

Deformation of Cube-Oriented MgO Crystals Under Pressure

Abstract: Plastic deformation in MgO crystals compressed parallel to a cube axis was studied under confining pressures up to 10 kbars at room temperature. The specimens were jacketed either in thick commercial rubber or in very thin latex. There were marked differences from behavior at atmospheric pressure, especially when thick rubber jackets were used at the higher pressures, at which they were in the glassy state; the stress-strain curves of the MgO then tended to be abnormally high, and there was more marked activity of obliquely oriented slip planes. These effects can be attributed to additional stresses arising from the constraint of the rubber jackets, which can also give rise to kink bands at the longitudinal edges of the specimens. Such effects demonstrate the more effective hardening that arises from the interaction and intersection of the dislocations on oblique {110} slip planes and have important implications for the deformation of grains in the constrained conditions within polycrystalline specimens.

Strain-hardening effects in bcc, fcc, and hcp metals loaded at very high rates

Abstract: Plastic deformation in typical bcc, fcc and hcp metals has been studied as a function of initial impact energy (up to about 4200 J). Materials included 1100-F aluminium, 2024-T3 aluminium, lead, OFHC copper, type-200 nickel, columbium, tantalum, vanadium, molybdenum, and cadmium. Macroscopic deformation resulting from projectile impacts at velocities up to 8 km/sec was recorded by sub-microsecond photography and flash X-radiography. A correlation was established between homologous test temperature (T/Tm) and size of craters resulting from impact, as was a relationship between the relative increase in microhardness in the impact affected region beneath the target crater and impact stress over the square root of compressibility. The microstructures of impulse-loaded regions were correlated with the stress levels effecting them. Strain profiles beneath the impact position for the type-200 nickel were determined metallographically and correlated with microhardness profiles.

Plastic Deformation of Alkali Halide Crystals at High Pressure: Work‐Hardening Effects

Abstract: The influence of hydrostatic pressure on the plastic deformation of alkali halide single crystals at large strain has been observed. The rates of work hardening in stages I and II are approximately independent of pressure in LiF, NaCl, and KCl, but both are doubled at 4.3 kbar in KI. In stage II the fractional increase of plastic flow stress at high pressure in an interrupted compression test, δσ/σ, is accounted for by the pressure‐induced increase in elastic constants in all the alkali halides examined. Evidently, the strength in stage II is controlled by elastic interactions. The stress at the onset of stage III, σIII, in the deformation of NaCl at 4.3 kbar is about half that at 1 atm; it is reduced slightly in KCl and is unchanged in KI. Deformation proceeds at lower stresses in stage III in NaCl and KCl under 4.3 kbar, and if compression at 1 atm in this stage is interrupted and reinitiated at 4.3 kbar, pronounced work softening is observed. The dependence of flow stress on pressure in stage III at co...

The friction and deformation of clean metals at very low temperatures

Abstract: A study has been carried out on the friction and surface damage between hemispheres and flats of the same metal at low sliding speeds at temperatures ranging from 25 K to room temperature in a vacuum of the order of 10 -10 Torr. All the metal specimens were polycrystalline and included gold, silver, copper and nickel (f .c. c.), tantalum, iron, molybdenum and tungsten (b. c. c.), zirconium, titanium, beryllium and cobalt (hexagonal). There are three main types of behavior. With f. c. c. metals at room temperature the friction is high ( μ ≂ 2.5) and the slider produces grooving in the lower surface. At very low temperatures a transition is observed: the friction falls to μ ≂ 1.5 and there is a change in the type of surface damage. This transition is associated with a marked rise in the work-hardening rate of the metal. With some of the b. c. c. metals there is a similar change at low temperatures but this is associated with a ductilebrittle transition. Under brittle conditions the friction does not exceed about μ = 1.0. With hexagonal metals the behaviour depends on the ductility of the metal. Zirconium and titanium behave like cubic metals. With beryllium and cobalt, which possess a limited number of slip systems, interfacial deformation is restricted and the friction and surface damage are small. The lowest friction ( μ = 0.5) and the least surface damage are observed between cobalt specimens in agreement with the recent work of Buckley & Johnson (1968). With the relatively clean surfaces used in the present study the sliding process involves two main mechanisms. The first is junction-growth (Courtney-Pratt & Eisner 1958). Because interfacial adhesion is strong and the slider deformable the action of a tangential stress is to enlarge the true area of metallic contact. The second is a ploughing action. The front of the slider ploughs out a groove in the lower surface displacing material ahead of and around it. These two processes interact and it is shown that, if junction-growth is appreciable, tensile stresses may develop in the trailing portion of the contact region. The way in which the metal can withstand these tensile stresses plays a crucial part in determining the friction and type of surface damage produced. The present work shows that under conditions of strong adhesion the most important material property is the ductility of the metal. This analysis represents an advance on the simple junction-growth model proposed by Tabor in 1959.

Temperature dependence of yield and fracture in polymethylmethacrylate

Abstract: The effect of temperature on the deformation and fracture of polymethylmethacrylate has been determined by constant strain rate compressive and tensile tests from 78°K to 350°K. In compression, the yield stress increased continuously with decreasing temperature and, except at 78°K, substantial macroscopic plastic flow occurred at all temperatures. At 78°K, only a small amount of plastic flow occurred before fracture. A similar type of temperature dependence was found in tension between 230° and 350°K for both the yield and fracture stresses, but the tensile yield stress was always lower than the compressive yield stress by about 1500 p.s.i. In this temperature range, fracture is controlled by macroscopic yielding in that defects (crazes) produced during the yielding process serve as crack nuclei. It is deduced that the temperature dependence of fracture in this temperature range is a reflection of the temperature dependence of the yield stress. Below about 200°K, PMMA fractures in tension with li...

Strength, deformation modes and fracture in titanium-aluminum alloys,

Abstract: : The variation of yield strength, deformation modes and fracture characteristics of titanium:aluminum alloys containing up to 25% aluminum were studied. This composition range includes three phase fields based on hexagonal phases, a solid solution, and ordered phase based on the composition Ti3Al and a two phase region. An attempt is made to account for the factors that control the strength and fracture characteristics of these alloys. (Author)

Deformation of polypropylene single crystals

Abstract: The plastic deformation modes of single crystals of polypropylene have been studied as a function of applied uniaxial strain and as a function of orientation of the lathlike crystals relative to the draw direction. The crystals were drawn on a Mylar substrate and the morphology of the drawn crystals was observed by electron microscopy for various imposed uniaxial strains varying from nominal values of 3 to 100%. For crystals oriented in, or close to, the direction of draw, microcracks developed at low strains and grew progressively larger as the draw ratio increased. Fibrils, about 50 A in diameter, were found to extend across the apparent cracks. The fibril diameter appeared to be independent of the imposed strain but the fibril length was found to be proportional to the strain. For crystals oriented at an appreciable angle to the strain, additional modes of plastic deformation were found to occur. These included simple and multiple shear, slip, and buckling. Crystallographic twinning also appea...