The effect of heat treatment on mechanical properties and corrosion behavior of AISI420 martensitic stainless steel

Elastic interactions arising from a difference of lattice spacing between two coherent phases can have a strong influence on the phase separation (coarsening) behaviour of alloys. If the elastic moduli are different in the two phases, the elastic interactions may accelerate, slow down or even stop the phase separation process. If the material is elastically anisotropic, the precipitates can be shaped like plates or needles instead of spheres and can \sps arrange themselves into highly correlated patterns \cs. Tensions or compressions applied externally to the specimen may have a strong effect on the shapes and arrangement of the precipitates. In this paper, we review the main theoretical approaches that have been used to model these effects and we relate them to experimental observations. The theoretical approaches considered are (i) `macroscopic'' models treating the two phases as elastic media separated by a sharp interface (ii) `mesoscopic'' models in which the concentration varies continuously across the interface (iii) `microscopic'' models which use the positions of individual atoms.

Modelling of Phase Separation in Alloys with Coherent Elastic Misfit

A 17-4 precipitation-hardening stainless steel produced by selective laser melting contains 72% metastable and heavily faulted austenite and 28% highly dislocated and twinned martensite. The mechanical behavior is characterized by exceptional work hardening and a two-step plastic field, which are due to the strain-induced transformation of austenite and to the structural characteristics of the deformed austenite and martensite. Both phases tend to accumulate stacking faults in the first step and twins in the second. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

Metastable Austenite in 17–4 Precipitation-Hardening Stainless Steel Produced by Selective Laser Melting

Direct laser deposition of alloys from elemental powder blends

Modelling the evolution of phase boundaries in solids at the meso- and nano-scales

The effect of retained austenite (γ) on the microstructure and mechanical properties of a martensitic precipitation hardening stainless steel was experimentally investigated, whose chemical composition was Fe-1.8Cu-15.9Cr-7.3Ni-1.2Mo-0.08Nb-low C, N (mass %). The microstructures of all specimens consist of a typical lath martensite with interlath films of the retained γ, which is not reverted with aging. Cu-rich precipitates which may contribute to precipitation hardening can not clearly be observed. The tensile properties and Charpy absorbed energy are linearly approximated to the amount of retained γ as follows: 0.2% Y.S. (MPa) = 1192.3 − 13.6 × γ%, T.S. (MPa) = 1250.1 − 9.3 × γ%, El. (%) = 12.16 + 0.43 × γ%, R.A. (%) = 64.25 + 0.14 × γ%, and A.E. (J) = 72.5 + 0.8 × γ%. The introduction of retained γ is not beneficial to the fatigue limit. An excellent combinations of strength, ductility and toughness obtained in the present work is attributed to the introduction of retained γ and also to the chemical composition of the specimen used.

Effect of retained austenite on the microstructure and mechanical properties of martensitic precipitation hardening stainless steel

In the present paper, we newly propose a comprehensive non-linear diffusion equation where atomic interaction parameters, elasticity, and mobility are assumed to depend on local composition to calculate the phase decomposition in actual alloy systems. The proposed non-linear diffusion equation is called Discrete type non-linear diffusion equation in the article. The phase decomposition in the Al-Zn and Fe-Mo binary systems are simulated in a two-dimensional medium by using their thermodynamic data consistent with the equilibrium phase diagrams. The microstructures theoretically obtained are well coincident with experimental results of these alloys. Especially, various morphological transformations such as the mottled and modulated structures by the spinodal decomposition, the particle shape change including splitting, directional alignment of precipitates, particle size uniformity, etc. are demonstrated.

Computer simulation of phase decomposition in two dimensions based on a discrete type non-linear diffusion equation

The critical minimum size of stable precipitate in the vicinity of the edge of miscibility gap is experimentally determined for Ni3Al precipitate particles in the Ni-Al and Cu4Ti particles in the Cu-Ti binary alloy systems using the macroscopic composition gradient method recently proposed. The results obtained are as follows: the critical nucleus size shows a rapid increase to several tens of nanometers in a very narrow composition region less than 0.3 at. pct from the phase boundary. Such a big critical size of the nucleus is statistically rationalized by the conventional nucleation theories, but remains kinetically unreasonable.

Determination of the critical nucleus size of precipitates using the macroscopic composition gradient method

https://www.jstage.jst.go.jp/article/materia1994/38/8/38_8_624/_pdf

Computer Simulation of the Microstructure Changes Based on the Phase-Field Method.

The spinodal decomposition of the β Ti-Cr binary alloys system is still questionable, because there are only rare experimental and thermodynamics data; moreover, simulation results for a real alloy system have not been found. Transmission electron microscopy (TEM) and quantitative computer simulations based on Khachaturyan’s diffusion equation have been employed to study the microstructural evolution occurring in the metastable β Ti-Cr alloys. Our study results reveal that the metastable β Ti-Cr undergoes a phase separation reaction β → titanium rich (β
 1)+chromium rich (β
 2) through a spinodal decomposition. The coherent two-phase fields show extremely fine platelike precipitates distributed homogenously through the bcc matrix, which are parallel to the {100} plane. Those precipitates are highly elastic induced from the first step of the phase separation. There is good agreement between the observed microstructure and the simulation results. The mode of formation of α phase is also discussed.

Toru Miyazaki

Papers

Effect of retained austenite on the microstructure and mechanical properties of martensitic precipitation hardening stainless steel

Computer simulation of phase decomposition in two dimensions based on a discrete type non-linear diffusion equation

Determination of the critical nucleus size of precipitates using the macroscopic composition gradient method

Computer Simulation of the Microstructure Changes Based on the Phase-Field Method.

Computer simulation and experimental investigation of the spinodal decomposition in the β Ti-Cr binary alloy system