Hu Zhengfei

High resolution electron microscopy of precipitates in high Co-Ni alloy steel.

In the secondary hardening reaction in ultrahigh strength steel of high Co-Ni alloy, the needle-shaped precipitating carbide is the main strengthening phase. Particular attention has been paid to the nucleation, growth and coarsening of the precipitate and its interface with the ferrite matrix. High resolution transmission electron microscopy observations show that when tempered at a low temperature condition of 454 degrees C for 5h, the precipitating carbides begin to nucleate in the form of clusters; at peak hardening tempered at 482 degrees C, the growing carbides are fully coherent with the matrix, exhibit black-white contrast in bright-field images and have their own hexagonal crystalline structure as M(2)C. Coarsening carbides tempered at high temperature lost their coherence and exhibited Moiré fringes. Lattice images directly indicate the crystallographic relationship of M(2)C carbides and ferrite, [100](alpha)//[1101](beta), [100](alpha)//[1120](beta).

EFFECT OF MICROSTRUCTURES ON STRAIN HARDENING EXPONENT PREDICTION OF CAST ALUMINUM ALLOY

Strain hardening exponent(n) of a material is an important parameter reflecting its hardening property whose determination is of great importance.It has a widely application in material scientific research and engineering fields such as fatigue life prediction,stress-concentration factor calculation,etc..The value of strain hardening exponent varies with their microstructures in cast aluminum alloys,but a few theoretical and experimental investigations have been reported to understand the effects of the microstructural parameters on the strain hardening exponent in these alloys till now. In the present study,the influence of secondary dendrite arm spacing(SADS),aspect ratio and volume fraction of particles on internal stress in aluminum alloys were discussed,and a quantitative prediction of strain hardening exponent was established on the basis of Hollomon and internal stress equations.It shows that the strain hardening exponent represents their hardening ability in relatively large plastic deformation(larger than the upper limit for the no plastic relaxation regime).A new microstructural hardening parameter relatively to strain hardening exponent was defined.Besides,a group of A319 cast aluminum alloys with microstructural heterogeneities were tested.The calculated strain hardening exponents are in agreement with the experimental ones in A319 alloy as well as in some commonly used cast aluminum alloys.For the same grade of alloys,the microstructural hardening parameter and strain hardening exponent are quite sensitive to SDAS and particle aspect ratio while the influence of volume fraction of particles is relatively little.As the values of SDAS and aspect ratio of particles increase,the value of the strain hardening exponent decreases.A liner relationship between microstructural hardening parameter and strain hardening exponent was proposed.For A319 and A356/357 alloys,the optimum correction coefficients are 0.17 and 0.11,respectively,and the mean prediction error of n is only about 10%.

Carbides Evolution in 12Cr Martensitic Heat-resistant Steel with Life Depletion for Long-term Service

Evaluating the residual life of exposed components in power industry is a very important procedure in routine examination. The microstructures of a series of X20CrMoV12.1 martensitic superheater tube samples in a boiler in different service periods were investigated extensively to extract a quantitative relationship. During long-term service from start to rupture, hardness decreased monotonically with life depletion, and the decrease of hardness in prior austenite grain boundary was steeper than that in the matrix. Microstructure observation showed obvious damage characteristics, including carbide coarsening and martensite decomposing, and the martensite structure decomposed completely in rupture state. The morphology, distribution and composition of the main precipitates M23C6 varied distinctly. The aspect ratio of coarsened carbides along grain boundary increased several fold with respect to their original size. The composition of coarsened M23C6 carbide shows the most regular trend of Cr enrichment and the statistical result of Cr enrichment in M23C6 shows a linear correlation between the ratio of Cr to Fe and service time to the power of 3/2, which may be considered as an index of material degradation due to long-term service exposure.