K. Tangri

On the Hall-Petch relationship and substructural evolution in type 316L stainless steel

Tensile specimens of Type 316L stainless steel having grain sizes in the range 3.1--86.7 {micro}m were deformed to 34% strain at temperatures 24,400 and 700 C and strain rate 1 {times} 10{sup {minus}4}s{sup {minus}1} to investigate the Hall-Petch (H-P) relationship, the nature of stress-strain curves and the substructure development. Up to {approximately}5% strain the H-P relationship exhibits bi-linearity whereas the single Hall-Petch relation is exhibited at larger strains. The presence of bi-linearity is explained by the back stress associated with the difference in the dislocation densities in the vicinity of grain boundary and in the grain interior. The log stress ({sigma})-log strain ({epsilon}) plots depict three regimes and follow the relationship {sigma}=K{epsilon}{sup n} in each regime, but with varying magnitudes of the strength coefficient (K) and strain-hardening exponent (n).

Study on the substructure evolution and flow behaviour in type 316L stainless steel over the temperature range 21–900°C

Abstract Tensile specimens of type 316L stainless steel with a grain size of 5.0 μm have been deformed at a constant strain rate of 10−3 s−1 over the temperature range 21–900°C and by differential strain-rate test technique over strain rates from about 10−5 to 10−3 s−1 at temperatures in the range 750–900°C. The normalized yield and flow stresses against temperature plots exhibit three regions. While in regions I and III the stresses decrease with increasing temperature, they increase with increasing temperature in region II. Transmission electron microscopy studies on deformed specimens show that at small strains the dislocations generated at grain boundaries have characteristic distributions: in region I the dislocations are confined to the vicinity of the grain boundary, in region II the dislocations are spread into the grain interior, and in region III the dislocations rearrange to form walls. The evolution of substructure and the work-hardening behaviour are explained by considering both intragranula...

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part I. derivation of flow curve equations

A semi-mechanistic model for predicting the flow behavior of a typical commercial dual-phase steel containing 20 vol pct of ‘as quenched’ martensite and varying amounts of retained austenite has been developed in this paper. Assuming that up to 20 vol pct of austenite with different degrees of mechanical stability can be retained as a result of certain thermomechanical treatments in a steel of appropriate low carbon low alloy chemistry, expressions for composite flow stress and strain have been derived. The model takes into account the work hardening of the individual microconstituents(viz., ferrite-@#@ α, retained austenite- γr, and martensite -α′) and the extra hardening of ferrite caused by accommodation dislocations surrounding the ‘as quenched’ as well as the strain-induced(γr→ α′) martensite. Load transfer between the phases has been accounted for using an intermediate law of mixtures which also considers the relative hardness of the soft and the hard phases. From the derived expressions, the flow behavior of dual phase steels can be predicted if the properties of the individual microconstituents are known. Versatility of the model for application to other commercial steels containing a metastable phase is discussed.

On the grain boundary statistics in metals and alloys susceptible to annealing twinning

Abstract Grain boundary distribution which includes grain boundary character distribution (i.e. distribution of boundaries by the reciprocal density of coincidence sites Σ) as well as distributions of boundaries by misorientation angles and axes is an important parameter describing polycrystalline microstructure. Numerous experimental data on grain boundary distributions in low stacking fault energy f.c.c. materials that are susceptible to annealing twinning have been analyzed and it has been established that there is a certain stable grain boundary distribution characterized by the dominance of Σ3n boundaries in all statically recrystallized materials of this class. computer modelling based on the assumption that multiple twinning is the main process controlling structure formation has confirmed this conclusion. It has been also found that distribution of lengths of different types of grain boundaries is more sensitive to the stacking fault energy and treatment of the material. Relation between grain boundary distributions and grain orientation distribution has been studied both experimentally and by computer modelling. It has been established that the grain boundary distribution is not completely determined by texture but is only influenced by it, because the grain boundary spectrum is primarily dependent on the orientation correlations which may exist between various crystallites of a polycrystal. Control of grain boundary distributions by means of various treatments has been demonstrated and possibilities of grain boundary design for improving the bulk properties of polycrystalline materials are discussed.

Computer simulation study of grain boundary and triple junction distributions in microstructures formed by multiple twinning

Abstract Microstructures formed as a result of multiple twinning have been simulated by means of computer modelling. Grain boundary misorientation (character) and triple junction distributions have been studied with the emphasis on the effect of initial texture and multiple twinning process. Although grain boundary distributions are similar in all the microstructures modelled, sharp initial texture leads to a somewhat enhanced amount of Σ3 boundaries and to a considerable increase in the number of triple junctions containing two Σ3 boundaries. The impact of these parameters on the material susceptibility to intergranular crack propagation has been analysed and implications for grain boundary engineering has been discussed.

Transmission electron microscopy characterization of interfacial boundaries in heat-treated Ti3Al+TiAl two-phase alloy

Abstract A Ti3Al(α2) + TiAl(γ) two-phase alloy made of Ti-44 at.% Al was heat treated to refine the microstructure for improving ductility. Quantitative transmission electron microscopy was carried out on the interfacial boundaries in this alloy which is characterized by lamellar structure with numerous Ti3Al-TiAl and TiAl-TiAlT interfaces, and with the antiphase domain boundaries (APBs) in Ti3Al lamellae. Two types of intrinsic interfacial dislocation which accommodate the interfacial misfits were seen on the Ti3Al-TiAl interfaces. Extrinsic interfacial dislocations were also present on most of the interfaces. Three different types of defect structure which depend upon the mechanism of twinning during heat treatment were observed on the twin boundaries in TiAl lamellae. The APBs in Ti3Al which appear as a maze pattern without preferential crystallographic orientation are of the type (a′0/6)〈1010〉 and (a′o/6)〈1120〉. The potential contribution of these interfaces to the mechanical properties is discussed.

Yielding and work hardening behaviour of titanium aluminides at different temperatures

Abstract Yielding and work hardening behaviour of a series of TiAl alloys containing 34, 44, 48, 50 and 52 at.% aluminium were investigated through compression tests at 293, 673 and 1073 K. It was found that a coarse-grained polycrystalline single-phase γ alloy showed yield strength anomaly while two-phase α2 + γ alloys showed a marked insensitivity of yield strength to temperature in the range 293–1073 K. The two-phase alloys and the single-phase γ alloy showed multistage deformation behaviour, while the single-phase α2 alloy did not. The yield strength and the work hardening tendency of the two-phase alloys increased with increasing volume per cent of the lamellar α2 + γ constituent.

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part II. calculation of flow curves

The role of metastable retained austenite(γR), its volume fraction, and mechanical stability on the flow characteristics of a dual phase steel containing 20 vol pct of ‘as quenched’ martensite in a ferrite matrix has been examined in this paper employing the flow curve expressions derived in Part I of this paper. It has been found that for a given volume fraction ofγR, its mechanical stability plays a crucial role in enhancing the ductility. Whereas highly stableγR does not contribute either to strength or ductility of the steel, highly unstableγR which causes an increase in the strength is detrimental to ductility. AγR which is moderately stable and undergoesγR → α′ transformation over a larger strain range is beneficial to enhanced ductility. Increasing amounts of moderately stableγR significantly increase both the strength and the ductility of dual-phase steels through a sustained work-hardening due toγR → α′ transformation. Load transfer which is determined by a parameterq has a significant contribution to work-hardening. A value of ∣|q∣|= 4500 MPa has been found to partition realistically the stress and strain in these steels.

The influence of starting microstructure on the retention and mechanical stability of austenite in an intercritically annealed-low alloy dual-phase steel

Retention of austenite during the intercritical annealing of a low carbon, low-alloy, dual-phase steel and the mechanical stability of retained austenite(γR) have been studied as a function of starting microstructure and annealing conditions. A quenched and tempered(QT) starting microstructure has been found to result in higher γR volume fractions compared to fully martensitic(Q) and ferrite plus pearlitic(F + P) starting structures for all annealing conditions employed in this work. The austenite formed by annealing up to 792 °C (where the kinetics are dominated by higher nucleation rates) is more prone to retention compared to that formed by annealing beyond 792 °C (where the kinetics are mainly dominated by higher growth rates). A smaller size of γR particles has a better mechanical stability against deformation-induced martensite transformation.

The effect of small plastic deformation and annealing on the properties of polycrystals: Part II. Theoretical model for the transformation of nonequilibrium grain boundaries

The nonequilibrium grain boundary state which has a high energy state is the result of absorption of a certain density of extrinsic grain boundary dislocations (EGBD’s). The equilibrium of such a boundary occurs by annealing at higher temperatures. A model has been proposed in this paper which assumes that the equilibrium of a nonequilibrium grain boundary involves the annihilation of EGBD’s by climbvia lattice diffusion of vacancies at the triple points. Due to the stress field of the EGBD’s, there is a vacancy concentration gradient around the triple points. The profile of the vacancy concentration gradient is derived by assuming a steady state flux of vacancies. Using this vacancy concentration profile, the expressions for the rate of climb of EGBD’s are derived. The proposed model predicts that the time required for the equilibration of nonequilibrium grain boundaries is dependent not only on the annealing temperature but also on the initial density of EGBD’s and the boundary length (which is related to the grain size). It has also been shown that the equilibrium behavior predicted by our model is in good agreement with the experimental results obtained for 316L stainless steel.