Plastic deformation behaviour and deformation substructure in Al-rich TiAl single crystals deformed at high temperatures

Plastic deformation behaviour in Ti–54.7 at.%Al and Ti–58.0 at.%Al single crystals was examined around and above the anomalous strengthening peak temperature (T_p) focusing on the effect of Al₅Ti₃ superstructure. The Al₅Ti₃ superstructure developed in the L1₀ matrix of Ti–58.0at.%Al, and the size of the Al₅Ti₃ phase once increased during annealing at 8008C and then decreased withincreasing temperature, while no significant evidence of the Al₅Ti₃ particles was obtained in Ti–54.7 at.%Al from TEM observation although diffuse scattering corresponding to the spots for the Al₅Ti₃ superstructure was observed. The transition of slip plane for ½<110]; ordinary dislocations from {111} to {110) and/or (001) occurred at and above T_p due to anisotropy of anti-phase boundary energies on {111}, {110) and(001) in the Al₅Ti₃ superstructure. Anomalous strengthening is related to the development of this superstructure which may assist the cross-slip of some parts of ½<110] ordinary dislocations onto {110) and/or (001) resulting in the formation of dragging points to the motion of the dislocations.     

Nitrogen-doped TiAl alloys Part II Plastic deformation behavior

Effects of the nitrogen addition on high temperature tensile properties and creep resistance of the fully lamellar and the duplex Ti-48.5Al-1.5Mo (at.%) alloy were investigated. High temperature yield strength of the nitrogen-doped alloys increased due to solute hardening and precipitation hardening of Ti₃AlN. Nitrogen addition led to remarkable improvement of creep resistance in the duplex microstructure as well as in the fully lamellar microstructure. In particular, the primary creep deformation of the 1.0 at.% nitrogen-doped alloy with the duplex microstructure decreased definitely to a similar or superior level of the fully lamellar alloy. We consider that precipitate hardening of p-phase (Ti₃AlN) and solute hardening of nitrogen atoms may be responsible for such remarkable creep resistance of the nitrogen-doped alloys.     

Cyclic deformation behaviour of TiAl6V4 and TiAl6Nb7 in different quasi‐physiological media

In the present investigation the cyclic deformation behaviour of TiAl6V4 and TiAl6Nb7 was characterized in constant‐amplitude and load increase tests in laboratory air and quasi‐physiological media by cycle dependent mechanical hysteresis, temperature and/or corrosion potential measurements. Microstructural changes were evaluated by scanning electron microscopy for defined fatigue states and after specimen failure. In constant‐amplitude tests, the alloys show pronounced cyclic softening and/or hardening. In load increase tests with strain and temperature measurements, estimation values for the endurance limit were determined which were lower than the fatigue limits determined in conventional Woehler tests to a maximum cycle number of 2 × 10⁶. For loading in quasi‐physiological media, the corrosion potential reliably indicates microstructural damage on the specimen surface.     

Cyclic deformation and lattice strain distribution of high Nb containing TiAl alloy

Low cycle fatigue of lamellar TiAl with 8.5 at.-%Nb was studied with a total strain amplitude of 0.28% at three temperatures: room temperature, 750°C and 900°C. At room temperature, the material exhibited cyclic hardening and the fracture mode was mainly interlamellar. At 750°C and 900°C, the material showed cyclic softening and the fracture mode was translamellar. The lattice strain in γ phase was almost tensile and larger tensile lattice strain in γ phase seems detrimental. Besides, the opposite direction of {201}_γ and {100}_α2 lead to crack propagation along α₂/γ interfaces. B2/β_o phase always suffered compressive lattice strain in the tests. The destruction of lamellar microstructure was the reason for colony refinement at 750°C and 900°C.     

Effect of twins on deformation of graphite single crystals

A detailed investigation has been made of deformation and fracture in graphite single crystals, between 20 and 2400° C, under a tensile stress parallel to the basal plane. Crystals are shown to be inherently weak when twins are present and the low value of modulus recorded in twinned crystals is attributed to dislocation glide within these regions. A mechanism of fracture is proposed which is consistent with the low strength and the fracture characteristics of graphite.It has been shown that graphite single crystals exhibit anomalous behaviour in that the tensile fracture strength increases if tests are made at temperatures greater than 2000° C.This increase in strength is associated with the movement and annihilation of twin boundaries and subsequent reduction in stress concentration. Delamination is also shown to result from twin boundary movement.     

孪晶片层结构在室温轧制过程中的微观结构演变

研究了一种具有纳米孪晶片层结构的电解沉积铜的微观结构特征及其在室温轧制形变后的微观结构演变.结果表明,电解沉积制备的纯铜样品由柱状晶组成,柱状品内含有平行于样品沉积表面的纳米量级厚度的高密度孪晶片层结构,在孪晶界上缺陷很少,为共格孪晶界.形变后,孪晶片层的微观结构特征与片层厚度密切相关.粗大的孪品片层的形变行为以全位错运动为主,而细小的孪晶片层的形变行为以肖克莱(Shockley)位错在孪晶界上的滑移为主,从而导致几个纳米厚的超细孪晶片层消失.     

Tensile impact behavior and deformation mechanism of duplex TiAl intermetallics at elevated temperatures

Investigations are made on the effects of strain rates on the tensile behavior and deformation modes of Duplex Ti–46.5Al–2Nb–2Cr (DP TiAl) at temperatures ranging from room temperature to 840 °C and under strain rates of 0.001, 320, 800, and 1350 s⁻¹. The dynamic strength is higher than quasi-static strength but does not change much over the high strain rate range. Yield stress anomaly is not found. Brittle-to-ductile transition temperature (BDTT) increases with the increased strain rates. A Zerilli–Armstrong constitutive model with appropriate coefficients is chosen to describe the high strain rate flowing behavior. TEM analysis indicates that both ordinary dislocations and superdislocations are found and dislocation pile-up only appears in samples deformed under quasi-static loadings at elevated temperatures. The deformation twins are common in equiaxed grains and the proportion of twinned grains increases with the increased strain rate from 46–72% under quasi-static loadings to 69–95% under high strain rate loadings. No deformation twins are found in lamellar colonies.     

The stability of deformation twins in aluminum enhanced by alloying elements

Introducing and stabilizing twins in aluminum is a challenge for metals research due to their high formation energy.Employing first-principles calculations,we investigated the twin boundary segregation of alloying elements and their impact on the twin boundary energy in aluminum.Alloying elements with small solubilities but strong interaction with twin boundary would significantly reduce twin boundary energies in aluminum at low temperatures.With increasing temperature,their segregation near twin boundary weakens,leading to their influence on twin boundary energies reduced.Some elements with large solubilities may greatly reduce the twin energies not only at low temperatures but also at high temperatures.Based on careful analysis of charge density and atomic radius,it has been found that chemical difference has little influence on twin boundary energy whereas the atomic size effect plays a leading role in causing the change of twin boundary energy.     

Tensile properties of Cu with deformation twins induced by SMAT

High density nano-scale deformation twins were introduced in the surface layer of Cu sample by means of surface mechanical attrition treatment （SMAT） at room temperature. The Cu sample with deformation twins shows a yield strength of about 470 MPa in tension tests. The significant strengthening may be attributed to the effective inhibition of slip dislocations by abundant twin boundaries.     

A quantitative understanding on deformation twins generation in single-phase austenitic stainless steels

Single-phase austenitic stainless steels (316L) have attracted widespread attention from scientists because of their duplex microstructure. In this paper, to have a quantitative understanding on the microstructure deformation of 316L, a physical model based on dislocation theory and strain gradient theory is established to find out the critical conditions when deformation twins generate. The twinning stress and the stress caused by strain gradients are two factors affecting the deformation twinning process. Numerical simulation results reveal that the twinning stress decreases with the increase of twin spacing and the decreases of volume fraction of twins and the orientation of external shear stress; the stress caused by strain gradients increases with the decrease of matrix grain size.     

Numerical simulation of plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl

Plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl have been simulated by using periodic unit cells representing the relaxed-constraint model recently proposed by Lebensohn et al. [Acta Mater. 46 (1998) 4701–4709] for the co-deformation of the lamellar compound of PST-TiAl. The unit cells contain both intermetallic phases, ₂-(Ti₃Al) and γ-(TiAl). Furthermore, the six orientation variants of the γ-phase are also considered. The constitutive behaviour of both phases is described by crystal plasticity, and the damage behaviour has been implemented by means of cohesive elements. The unit cells have been used as submodels for multi-scale finite element simulations of compression tests and fracture mechanics tests of notched micro-bend specimens. It is shown that the anisotropy of plastic deformation and damage in PST-TiAl can be well represented.     

Twins and twin-related domains in a grain boundary-engineered 304 stainless steel

ABSTRACT

The grain boundary (GB) network of a GB-engineered 304 stainless steel was investigated in three dimensions. The GB-engineered sample had a high proportion of twin-related boundaries (～70%). However, these boundaries were not distributed uniformly, but rather in large-sized twin-related domains (TRDs). All grains within a TRD can be connected by a tree-shaped twin-chain. Any two grains within a TRD have ∑3ⁿ-type misorientation, where the n-value could be determined according to the twin-chain. These results show that the formation of large-sized TRDs is correlated with extensively progressed multiple-twinning processes. A quadruple-junction has three twin-boundaries at most. Furthermore, all observed quadruple-junctions within TRDs have at least one twin-boundary. The common spatial morphologies of twin-boundaries are plane-shape, tunnel-shape and semi-closed tunnel-shape.

This paper is part of a thematic issue on Nuclear Materials.     

Analysis of high temperature deformation behavior of a high Nb containing TiAl based alloy

The effects of deformation temperature and strain rate on the hot deformation behaviors of as-cast Ti-45Al-8.5Nb-(W,B,Y) alloy were investigated. The results indicated that when deformation temperature is below 1250 °C, the flow stress decreases with the increase of deformation temperature and decrease of strain rate, once deformation temperature reaches 1250 °C, the flow stress is not sensitive to strain rate any more. A neural network model was established to predict the flow stress of this high Nb containing TiAl based alloy during hot deformation. The predicted flow stress curves are in good agreement with experimental results.