Profuse Twinning Research Articles

Shock-induced deformation twinning in tantalum

Shock-wave deformation of tantalum to a pressure of 45 GPa and duration of 1.8 μs generates profuse twinning. The post-shock mechanical response is significantly affected, with shock hardening exceeding the expected hardening due to the transient shock strain ϵ s = ( 4 3 ) ln( V V 0 ) ; this enhanced hardening, and other alterations in response, are attributed to the barriers presented to plastic deformation by the deformation twins. A constitutive model is proposed that predicts the threshold shock stress for mechanical twinning; it is based on the application of the Swegle-Grady relationship between shock stress and strain rate to constitutive equations describing the critical stress for slip and twinning. This constitutive model incorporates grain-size effects and predicts a threshold twinning stress that is a function of temperature and grain size; predictions of the model are in qualitative agreement with experimental results.

Effects of Cold Work on Precipitation Hardening of Cu-4.5 mass%Ti Alloy

The structure and properties of a solution treated, cold worked and aged Cu-4.5%Ti alloy were studied by means of optical, scanning and transmission electron microscopy as well as hardness, tensile tests and electrical conductivity measurements. Prior cold work (rolling) increases the peak hardness from 340 to 425 VHN, the tensile strength from 890 to 1380 MPa and the conductivity from 10 to 25% IACS on aging the alloy in the temperature range from 400 to 450°C. Maximum strengthening of the alloy is attributed to the precipitation of a coherent, metastable β′ phase (Cu4Ti). The alloy is deformed through profuse twinning. The ductile mode of fracture of the aged alloy is not affected by the prior cold work.

Proprietés mécaniques de surface d'un acier inoxydable traité par explosif

A new surface treatment technique by primary explosive was used on a 316L type austenitic stainless steel. Two explosive compositions, inducing a pressure of 1 GPa and 4 GPa respectively, were deposited in a thin layer on plane samples. The detonation induced effects have been characterized by different techniques. The surface roughness increases with the applied pressure, but remains lower than that obtained after shot peening. The microhardness profiles along a direction perpendicular to the treated surface and the microstructural study exhibit a marked hardening on several hundred micrometers associated with a profuse twinning. Moreover, for the 4 GPa pressure a very thin surface layer of fine recrystallized grains appears. In both cases, tensile residual stresses are measured at the surface and compressive stresses beyond 10 μm. The microstructure in the surface layers results of a noticeable thermal effect. The wear resistance of this stainless steel is improved by a factor 10 to 200 depending on the treatment conditions

Twinning and multiaxial cyclic plasticity of a low stacking-fault-energy f.c.c. alloy

Out-of-phase cyclic tension and torsion tests followed by metallographic measurements are carried out on Co33Ni specimens. It appears that, even for small strain ranges, nonproportional loadings induce profuse mechanical twinning and a strong kinematic hardening in this low stacking-fault-energy f.c.c. alloy. These results are analysed and an attempt to incorporate twinning into micromechanical simulations of f.c.c. polycrystals elastoplastic behaviour is described.

Deformation behavior of CdTe and (Cd, Zn)Te single crystals between 200 and 600 °C

The deformation behavior of CdTe and (Cd,Zn)Te single crystals oriented for single slip has been investigated in the temperature range 200–600 °C. The addition of 4.5 at.% Zn increases the yield strength by about eight times over that of CdTe crystals. Further, in both cases the yield strengths are nearly temperature independent above 300 °C. Arguments have been developed to rationalize these observations. Deformed structures have been evaluated by transmission electron microscopy. Parallel sets of dislocations, dislocation dipoles, loops, tangles and networks are observed in the deformed samples. In addition, the (Cd,Zn)Te samples deformed at 200 °C show profuse twinning. The evolution of the deformed structures has been discussed in the light of earlier studies pertaining to the elemental and III–V compound semiconductors. Also, the formation of twins is rationalized.

Deformation twinning in metals and ordered intermetallics-Ti and Ti-aluminides

The role of deformation twinning in the strength and ductility of metals and ordered interrnetallic alloys is examined on the basis of crystallography, energetics and kinetics of deformation twinning. A systematic analysis is made by taking Ti, T13Al, TiAl, and A13Ti as four model systems. In comparison with profuse twinning in Ti, the intrinsic difficulty of twinning in T13Al is rationalized in terms of the interchange shuffling mechanism. A fault (SISF) dragging mechanism based on the interaction torque explains the physical source for the low mobility of screw superdislocations in TiAI, which may lead to (1Ii) [1Ii] twin nucleation. In TiAI and A13Ti alloys, the twin-slip (ordinary) conjugate relationship makes an important contribution to the strain compatibility for high,temperature plasticity. Potentially beneficial alloying additions to promote twinning are discussed. (I) Research sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences, U-SDepartment of Energy, under contract DE-ACo5-840R21400 with Martin Marietta Energy Systems, Inc. (2) Permanent address.Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931, U-S-Aloss JOURNAL DE PHYSIQUE III M 6

A metallographic method for determining plastic strains at the tip of a fatigue crack

A metallographic method, applicable to all materials which exhibit deformation twinning under cyclic loading, is proposed for estimating plastic strains at the tip of fatigue cracks. This method is based on the principles of quantitative metallography and the use of an appropriate etching procedure to reveal the microstructure for observation in an optical microscope; calibration by means of low cycle fatigue specimens is necessary. The metallographic technique was evaluated for a binary CoNi alloy which exhibits profuse twinning and was compared with the more classical microhardness measurements. Good agreement was found between the two techniques for observation of stage II fatigue crack growth. The results obtained and the applicability of this method are briefly discussed.

Microstructure of Austenitic stainless Steel Explosively Bonded to low Carbon-Steel

An austenitic stainless steel plate was explosively bonded to low carbonsteel plate, and tbe microstructure of the bonded composite has been investigated by transmission electron microscopy. The bond zone of several μm width consists of a mixture of stainless steel and low carbonsteel, and each component metal is composed of small crystal grains of several thousand Å size. The dislocation density in the small grains is of the order of 109/cm2. The regions, of about 10 μm width, immediately adjacent to the bond zone consist of either small crystal grains or elongated grains of several thousand Å width. The dislocation density in these regions is 109–1010/cm2. Outside these regions crystal grains in the stainless steel as well as the low carbon-steel are much larger, and contain the high dislocation density of 1010–1010 /cm2Tangled dislocations, dislocation loops, and undeveloped cell structure are observed. In addition to the dislocations, in the stainless steel profuse deformation twins of 50–230 A width are seen except the regions near the bond zone, while no significant hep e-phase is detected. The formation of the small gralns in the bond zone and in its neighbouring regions are considered.

Twinning, slip and catastrophic flow in niobium

Mechanical tests were carried out in tension and compression on single and poylcrystalline niobium. The orientation dependence of the mechanical properties at 77°K was studied, and the behaviour was found to be highly anisotropic. Crystals with axes near [011] deformed by profuse twinning in compression, while under tensile loading these crystals yielded by slip at much higher stresses. Similar behaviour was observed at 78°K in polycrystalline niobium containing a strong [011] recrystallisation texture. Crystals oriented near [1̄11] underwent catastrophic, localised flow when tested in tension; the plane of shear was found to be near (011) and the shear direction near [1̄00]. It is deduced that this behaviour was caused by the motion of dislocations with an a[1̄00] Burgers vector, and is a consequence of the elastic properties of niobium. Crystals with axes near [001 ] deformed entirely by slip. Observations of slip lines on the crystals are described, and it appears that both {110} and {112} are preferred slip planes in niobium at 80°K. There is evidence that slip on {112} planes in the sense of the direction appropriate for twin formation occurs at lower stresses than slip in the opposite sense.

Grain-size effects on the deformation of tantalum at low temperatures

The dependence of yield stress on grain size in high-purity powder metallurgy tantalum has been investigated by tensile tests on specimens having a wide range of grain sizes at 298°K, 203°K, and 77°K. The variation of lower yield stress, σLY, with grain diameter, d, has been found to obey the Petch equation, σ LY = σ 0 + k yd −11 2 ; values determined for k y are 2.3 · 10 7 dynes/cm 3 2 at 298°K, 3.8·10 7 dynes/cm 3 2 at 203°K, and 5.2·10 7 dynes/cm 3 2 at 77°K. The effective surface energy for cleavage crack propagation, γ, has been estimated to be > 1.8·10 5 erg/cm 2. Comparisons of the values of k y and γ for tantalum with those determined for other b.c.c. metals suggest that the exceptionally high ductility of tantalum at low temperatures is mainly due to the large effective surface energy required for cleavage crack propagation. A strong dependence of deformation twinning on grain size at 77°K has been observed. Profuse twinning occurs in all coarse-grain specimens even at low strain rates, but not in fine-grain specimens. The failure to induce twinning by high strain rates in fine-grain specimens cannot be attributed to the heat generated during rapid loading.