Microstructural evolution, nanoprecipitation behavior and mechanical properties of selective laser melted high-performance grade 300 maraging steel

The shape-memory effect is a solid state phenomenon which exploits a reversible phase transformation to repeatedly achieve an initial shape, even after some deformation of the material. Numerous metal alloys exhibit this effect. One of the most widely used shape-memory alloys is TiNi, due to its large range of recoverable deformations and its relative ease of processing. In bulk and wire form, TiNi has been applied to a number of applications, and as a thin film, TiNi is an excellent material for use as a microactuator in microelectromechanical systems (MEMS), due to its large recovery forces and high recoverable strains. Several TiNi-actuated MEMS devices have already been reported.

https://iopscience.iop.org/article/10.1088/0960-1317/8/3/007/pdf

The TiNi shape-memory alloy and its applications for MEMS

Nanoprecipitate-hardened 1.5 GPa steels with unexpected high ductility

Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe–18Ni–9Co–3.4Mo–1.2Ti) produced by SLM. We found that despite the “intrinsic heat treatment” and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (μ phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291414002040

Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting

Shape Memory Alloys (SMA's) are novel materials which have the ability to return to a predetermined shape when heated. When an SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape--which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure which causes it to return to its original shape. If the SMA encounters any resistance during this transformation, it can generate extremely large forces. This phenomenon provides a unique mechanism for remote actuation.

Introduction to Shape Memory Alloys

The crystallography, structure, and composition of the strengthening precipitates in maraging steels C-250 and T-250 have been studied utilizing analytical electron microscopy and computersimulated electron diffraction patterns. The kinetics of precipitation were studied by electrical resistivity and microhardness measurements and could be described adequately by the Johnson-Mehl-Avarami equation, with precipitate nucleation occurring on dislocations and growth proceeding by a mechanism in which the dislocations serve as collector lines for solute from the matrix along which pipe diffusion occurs. The strengthening of the Co-free, higher Ti T-250 steel is caused by a refined distribution of Ni3Ti precipitates. High strength is maintained at longer times from the combined effect of a high resistance of these precipitates to coarsening and a small volume fraction of reverted austenite. In the case of the Co-containing, lower Ti C-250 steel, strengthening results from the combined presence of Ni3Ti (initially) and Fe2Mo precipitates (at longer times). Loss of strength at longer times is associated, in part, with overaging and mainly from the larger volume fraction of reverted austenite. The resistance to austenite reversion is dependent on the manner in which the relative nickel content of the martensite matrix is affected by the precipitating phases, and the difference in the reversion tendency between the two steels can be explained on this basis.

Precipitation reactions and strengthening behavior in 18 wt pct nickel maraging steels

The microstructural changes and precipitation reactions in FeNiMnTi martensitic alloys aged at 400–550°C for up to 600 h have been studied utilizing electron microscopy and diffraction, and X-ray microanalysis. Lath martensite with a high density of dislocations was observed in previously annealed specimens. Upon aging from 400 to 550°C, the alloys showed classical age-hardening behavior and the activation energies for precipitation were calculated as 38–46 kcal mol −1 , which supports precipitate nucleation occurring on dislocations and growth proceeding by a mechanism of pipe diffusion. Aging results in a decrease in dislocation density and a refined distribution of needle-like precipitates of η-Ni 3 Ti. The orientation relationship between the matrix and Ni 3 Ti was found to be (0001) η ‖(011) M and [1120] η ‖[111] M . Widmanstatten austenite appeared in the lath martensite at longer aging times along with lath-like reverted austenite and globular austenite. Loss of hardness at longer aging times is associated in part with overaging, but mainly from a large fraction of reverted austenite. For 550°C aging, recrystallized austenite appears at lath packet boundaries after long times and some transforms to lath martensite on cooling.

Precipitation behavior and microstructural changes in maraging FeNiMnTi alloys

By using TEM and X-ray diffraction, the domain structure in ceramic LaNbO{sub 4} has been studied. It has been found that the domains have two different orientations, separated by a boundary with irrational Miller indexes between (20{bar 4}){sub I}/(402){sub II} and (20{bar 6}){sub I}/(602){sub II} lattice planes, which is in good agreement with the boundary indexes calculated from elasticity theory and spontaneous strain tensor. The orientation relationship between the domains has been determined as a rotation of 95.6{degree} about the [010] axis. A model for domain boundary configuration, a diffuse boundary with a transition zone, has been proposed according to TEM diffraction patterns and HRTEM images. Based on this domain boundary model, a domain switching mechanism is suggested, and the corresponding driving force for domain switching is considered.

C. Marvin Wayman

Papers

Precipitation reactions and strengthening behavior in 18 wt pct nickel maraging steels

Precipitation behavior and microstructural changes in maraging FeNiMnTi alloys

Domain Boundary and Domain Switching in a Ceramic Rare-Earth Orthoniobate LaNbO4

The origins of the shape memory effect

An Electrical Resistance Study of Phase Transformations in Quenched and Annealed CsCl Au-Cd Alloys