Microstructural evolution and stress state related to mechanical properties of electron beam melted Ti-6Al-4V alloy modified by laser shock peening

In this study, the tensile properties of a tempered martensitic 9Cr oxide dispersion strengthened (ODS) steel are investigated. The tensile tests were performed in the temperature range of 25 °C to 800 °C at a nominal strain rate of 10−3 s−1. At room temperature, the material exhibited high yield strength and ultimate tensile strength of 929 and 1052 MPa, respectively. A decrease in strength was observed with increase in temperature down up to 156 MPa of ultimate tensile strength at 800 °C. The total elongation-to-failure that was 7.6% at 25 °C increased sharply with increase in temperature and reached a maximum of 38.4% at 700 °C. In order to compare the influence of heat treatment the tensile tests were also performed on specimens when in a ferritic state. Interestingly, at elevated temperatures both states presented a similar trend of strength and elongation. Transmission electron microscopy after deformation revealed a modification of the deformation mechanism with the temperature. The dislocation activity that was homogeneously distributed at room temperature was localized close to grain boundaries at elevated temperatures. A strong particle–dislocation interaction was observed at all testing temperatures. Orowan mechanism is supposed to govern particle–dislocation interaction at moderate temperatures. At elevated temperatures, an attractive particle–dislocation interaction phenomenon called interfacial pinning was identified. The additional microstructural evolution that includes reduced dislocation density, the transformation of lath structure into coarse equiaxed grains and the M23C6 carbide coarsening, resulted in a loss of strength at elevated temperatures. Fracture surface investigation at room temperature revealed intragranular fracture with dimples. As the temperature increased, the fracture surface formed with a shear-lip zone at the outer periphery and dimples that were larger and deeper in comparison to the ones formed at lower temperatures. These observations are associated with the increased ductility. A change in fracture mechanism from intragranular to intergranular fracture was observed at 650 °C. This change became fully apparent at 800 °C where it is associated with a reduced ductility. The change in damage mechanism is due to the modification of the deformation mechanism. In comparison to other commercial, as well as experimental, ODS steels, the material offers an excellent compromise between strength and ductility.

Study of the deformation and damage mechanisms of a 9Cr-ODS steel: Microstructure evolution and fracture characteristics

The dynamic tensile test with a strain rate of 500 s −1 and the quasi-static tensile test with a strain rate of 5.6×10 −4  s −1 were performed for a novel Fe-0.20C-1.49Mn-1.52Si-0.58Cr-0.05 Nb (wt%) quenching-partitioning-tempering (Q-P-T) martensitic steel with high amount of retained austenite, respectively. This low carbon steel was also treated by the traditional quenching and tempering (Q&T) process, and the same experimental tests were performed for the low carbon Q&T martensitic steel with little retained austenite to understand the effect of the retained austenite on the dynamic tensile behavior. The results indicate that compared with the quasi-static tensile test, the high strain rate in the dynamic tensile test raises the strength of the Q-P-T steel. However, the elongation slightly decreases. These results differ from the enhancement in both the strength and elongation of the Q&T steel in the dynamic tensile test. The increase in the strength of the Q-P-T steel in the dynamic tensile test is attributed to the strain rate hardening effect. The slight decrease in the elongation stems mainly from that the suppression of the dislocation absorption of the retained austenite (DARA) effect existing in the quasi-static tensile test, moreover, such a suppression is not effectively complemented by the adiabatic softening of the martensitic matrix in dynamic tensile test. The marked increase in the elongation of the Q&T steel in the dynamic tensile test is only attributed to the adiabatic softening of the martensite matrix because there is no DARA effect in the Q&T steel with little retrained austenite.

Effect of retained austenite on the dynamic tensile behavior of a novel quenching-partitioning-tempering martensitic steel

Investigation of polyimide–mica hybrid films for cryogenic applications

Characteristics of the formation, the interfacial structure, and the defects of M23C6 carbides in 100Mn13 steel after 1323 K (1050 °C) solution treatment and subsequent aging treatment were investigated by transmission electron microscope (TEM). The results show that M23C6 carbides precipitated on both sides of the grain boundaries. Each M23C6 carbide particle possesses a coherent interface and the cube–cube orientation relationship with the one austenite grain which M23C6 precipitated from. The orientation relationships between the M23C6 carbides and the austenite matrix were determined as follows: (1) [211]γ
 $$ \parallel $$
 [211]M23C6, (
 $$ 1\bar{1}\bar{1} $$
 )γ
 $$ \parallel $$
 (
 $$ 1\bar{1}\bar{1} $$
 )M23C6, (02
 $$ \bar{2} $$
 )γ
 $$ \parallel $$
 (02
 $$ \bar{2} $$
 )M23C6, (2) [110]γ
 $$ \parallel $$
 [110]M23C6, (
 $$ \bar{1}1\bar{1} $$
 )γ
 $$ \parallel $$
 (
 $$ \bar{1}1\bar{1} $$
 )M23C6, (
 $$ \bar{1} $$
 11)γ
 $$ \parallel $$
 (
 $$ \bar{1} $$
 11)M23C6. The M23C6 carbide also possesses an incoherent interface with the other austenite grain which the M23C6 carbide grew into by ledge mechanism. The precipitation of the M23C6 carbides along the boundaries can improve the hardness of the high-manganese steel to some extent. Besides, twinning and stacking faults were observed in M23C6 carbides. The reports about twinning and stacking faults in the M23C6 carbides have not been found.

Characterization of M 23 C 6 Carbides Precipitating at Grain Boundaries in 100Mn13 Steel

Characterization of M23C6 carbide precipitated at grain boundaries in a superalloy

The quenching-partitioning-tempering (Q-P-T) process was developed from quench- ing & partitioning (Q&P) process and their difference are first described in this paper. The development of Q-P-T steels from ultrahigh strength to high strength-ductility is summarized. Novel Q-P-T pro- cess and conventional quenching-tempering (Q-T) process and their different effects on mechanical properties are compared. The micro-mechanism of the ductility enhancement from retained austenite in unltrahigh strength steels during deformation is emphasized in discussion, which provides guidance for microstructural design and control on the further development of advanced high strength steels.

先进超高强度-高塑性q-p-t钢

Near 50 years ago,transformation induced plasticity(TRIP) effect was proposed and TRIP steels as an advanced high strength one are widely investigated.However,the mechanism of TRIP effect can be only qualitatively explained,and has not been experimentally and theoretically verified so far.In this work,a strain equivalent model for strain-induced martensitic transformation was built in a microstructure-based finite element model of novel quenching-partitioning-tempering(Q-P-T) steel.With the model,the TRIP effect under the condition of uniaxial tension was simulated,from which the micro-mechanism of TRIP effect is revealed.Stress relaxation from TRIP relieves the stresses within untransformed retained austenite and its adjacent martensite and blocks the formation of cracks,meanwhile,a considerable retained austenite still exists at higher strain level,which is the origin of TRIP effect.Compared with original(thermal-induced) martensite,fresh(strain-induced) martensite bears higher stress.Therefore,it could be predicted that cracks form at first in fresh martensite or its boundaries.Moreover,stress relaxation makes strain-induced martensite formed in intermittent and slow way,and this is consistent with experimental results.However,in stress-free relaxation state fresh martensite appears in successive and quick way,not consistent with experiments,and thus this verifies in opposite way that TRIP effect inevitably produces stress relaxation.

Finite element simulation of the effect of stress relaxation on strain-induced martensitic transformation

Development of high strength and high ductility rebar with obvious yield plateau is a tendency for anti-seismic requirement in building steels.In this work,the basic relationship between ferrite grain size and mechanical property,especially the elongation of yield plateau,was investigated by the combination of cold-rolling and heat treatment.After several passes of cold rolling followed by recrystallization annealing for 5~300 min at 600 ℃,ferrite with grain size between 5.2~40.4 μm was obtained in plain steel plates with a carbon content of 0.1%(mass fraction).Mechanical properties were studied by tensile test.With the decrease of ferrite grain size,not only the strength but also the elongation of yield plateau increases.Base on the scanning electron microscopy observation,the deformation becomes more homogeneous with the decrease of grain size,resulting in a higher elongation of yield plateau.A quantitative relationship between grain size da and the elongation of yield plateau dLwas derived from the combination of Hollomon and Hall-Petch equations.The application range of dα-dLequation was also determined,which is well consistent with the experimental results.The narrow-slit spraying equipment was used to treat the HRB400 rebar,and refined ferrite and a considerable non-equilibrium pearlite were obtained due to high cooling rate in treatment.A novel rebar with yield strength of 600 MPa grade was manufactured successfully in commerce,which also verified the above theoretical analysis.

600 MPa GRADE REBAR WITH HIGH DUCTILITY DEVELOPED BASED ON THEORY OF YIELDPLATEAU

The edge-to-edge matching crystallographic model was used to predict all the possible crystallographic relationships between crystals that have hexagonal close packed(HCP) and face-center cubic(FCC) structures.Taking a_H/a_F and c_H/a_H as variables,while a_H/a_F varies from 0.7～1.2 and c_H/a_H 1.5,1.6 and 1.7 respectively,eight groups of possible orientation relationship were obtained under the critical values(10% for interatomic spacing misfit and 6% for interplanar distance).Finally,based on the crystal structures and lattice parameters only,the model predicted the most possible orientation relationship of([11 ■0]_H/_F(0002)_H/(1 ■1)_F) between face-center cubic(FCC) austenite and hexagonal close packed(HCP) martensite in Fe-Mn-Si alloy.The predicted orientation relationship agrees well with the experimental results observed by transmission electronic microscopy,which confirms and shows the preciseness,practicability and convenience of the model.

Rong Yong-hua

Papers

Characterization of M23C6 carbide precipitated at grain boundaries in a superalloy

先进超高强度-高塑性q-p-t钢

Finite element simulation of the effect of stress relaxation on strain-induced martensitic transformation

600 MPa GRADE REBAR WITH HIGH DUCTILITY DEVELOPED BASED ON THEORY OF YIELDPLATEAU

An Edge-to-Edge Matching Model and Its Application to the HCP/FCC System