Martensitic Type Research Articles

Altered microstructure, phase transformation behaviors and shape memory response of CuAlMn shape memory alloys fabricated by selective laser melting

In this paper, nearly defect-free CuAlMn SMAs were successfully fabricated by selected laser melting (SLM). The result of microstructure show that only one type of thermal-induced martensite was detected in the prepared specimens, and the process parameters play a vital role in phase transformation behaviors. The alloy shows excellent ultimate tensile strength of 820 MPa and elongation of 11.5 %. Furthermore, the high shape memory response, i.e. shape recovery rate of 91.4 %–99.6 %, shape memory strain of 2.59 %–5.46 %, was presented under compressive strain not exceeding 8 %.

A point of view of the elastocaloric effect associated with 7M and 5M modulated martensite in Ni–Mn–Ga alloys

Solid-state refrigeration has emerged as the most promising alternative to conventional refrigeration technology. However, for this technology to be applicable, the caloric effects produced in the alloys must be highly reversible. In this context, we compare the elastocaloric effect of two Ni–Mn–Ga alloys with different types of modulated martensite. The elastocaloric effect, quantified as the isothermal entropy change (ΔSela), was investigated in Ni50Mn28Ga22 and Ni50Mn30Ga20 alloys with 5M and 7M modulated martensite, respectively. Maximum ΔSela values obtained were 1.91 J kg−1 K−1 during cooling and 1.83 J kg−1 K−1 during heating in martensite 5M and 0.19 J kg−1 K−1 during cooling and 0.26 J kg−1 K−1 during heating in martensite 7M, for a constant applied stress of 10 MPa. However, although the 7M modulated martensite exhibited a lower ΔSela, its reversibility was higher. Therefore, our results could be useful for selecting a good material to be used in solid-state refrigeration.

Effects of Preheating on the Microstructure of Laser-DED AISI 420

AISI 420 stainless steel (420 SS) is a martensitic type known for its high strength and abrasion resistance, and it is extensively used in hard-facing applications (Refs. 1, 2). However, Type 420 SS contains untempered martensite in the as-welded condition and is generally considered to have poor weldability. Successful arc welding of 420 SS requires proper preheating, post-weld heat treatment, and strict adherence to low hydrogen practices. Direct energy deposition (DED) is a fusion-based additive manufacturing (AM) process that melts metal powders to produce complex shapes and can be used for surfacing or hard-facing operations. However, as small-volume AM deposits undergo self-quenching, the cooling rate can be much faster than in welding. Consequently, the phase transformations in the AM deposit deviate further from equilibrium relative to traditional welding processes, resulting in heterogeneous microstructures and mechanical properties, potentially limiting the direct use of AM components without post-AM heat treatment (Ref. 3). Our prior work with laser-DED (L-DED) hard-facing 420 SS powder without preheat combined experiments, characterization, and kinetic modeling to rationalize the presence of remnant δ-ferrite and austenite quantitatively in room temperature microstructures. This study aimed to quantify the effects of preheating above the Ms temperature on the extent of solidification segregation and microstructures for a wall build for L-DED processing of the same heat of 420 powder. The results of the current work are contrasted with our prior study, which allows for a greater understanding of L-DED opportunities with 420 SS.

Calorimetric and Volumetric Studies of Dislocations During Martensitic Transformations in Shape Memory TiNi Alloy

Based on the use of appropriate approaches, methods and results of the analysis of a number of topical problems of physical materials science, an in-depth analysis has been done of calorimetric and volumetric data for direct, reverse and deformation martensitic transformations in a nanostructured Ti49.3Ni50.7 shape memory alloy obtained by cold rolling with simultaneous action of pulsed high-density current. For the first time, by applying a new technique for processing calorimetric spectra (peaks), the staging and “kinetics” of changes in heat content, as well as thermal effects (enthalpies of individual stages) during direct and reverse martensitic transformations during cooling or heating of samples at a constant rate (3 × K/ min) in the range 170–370 K has been done. It is shown, by processing volumetric data, using theoretical values of the dislocation density and elements of the classical theory of dislocations, that in the Ti49.3Ni50.7 shape memory alloy subjected to cold deformation accompanied by the action of a pulsed current, a deformation martensitic transformation occurs, leading to a positive volume effect (∆V/V) ≈ 3 × 10–3), which can be largely due to dislocations. It is shown, by applying the theoretical values of the dislocation density and elements of the classical theory of dislocations, that the possible contributions of dislocations to the enthalpies of direct and reverse martensitic transformations (in the Ti49.3Ni50.7 alloy) can and should be significantly lower in absolute value, but opposite in sign to the observed enthalpies of direct and reverse martensitic transformation in a given alloy. It is shown that the physics of the processes under consideration is contained to a certain extent in scientific discoveries No. 239 “The phenomenon of thermoelastic equilibrium during phase transformations of the martensitic type – the Kurdyumov effect” and No. 339 “The phenomenon of the formation of non-equilibrium grain boundaries in polycrystals when they absorb lattice dislocations”.

The role of cell structures in hydrogen embrittlement of L-PBF 18Ni(300) maraging steel

This work analyzed the role of unique cell structures in hydrogen embrittlement of additively manufactured maraging steel. For comparison, some as built samples were solution treated to eliminate cell structures, producing the typical martensite where dislocations randomly distributed. By contrast, the cell structures constituted by dislocation tangles in as built samples introduced more hydrogen traps and delayed hydrogen diffusion, but caused much higher HE susceptibility, because the hydrogen accumulation around cell walls created locally hydrogen-enriched environment for the grain boundaries closely adjacent to cell walls, increasing the risk of intergranular cracking. Hydrogen susceptibility was markedly reduced after eliminating cell structures.

Effect of the duplex aging on the phase constitute and antibacterial properties of Ti-13 Nb-13Zr-7Ag

Biomedical titanium alloy Ti-13 Nb-13Zr-7Ag (TNZ-7Ag) has a low elastic modulus, but its antibacterial ability is still not satisfied. In this paper, a duplex aging treatment was applied to TNZ-7Ag alloy to improve the antibacterial properties, and the effects of the duplex aging on the microstructure, antibacterial properties and mechanical properties of TNZ-7Ag were studied, and the corrosion resistance and cytotoxicity were assessed. Microstructure observation indicated that the chemical component segregation appeared in the alloy matrix after the pre-aging at 350℃, and two types of martensite α" phases, namely primary α"M and secondary α"T, appeared in the solute-rich areas. During the subsequent aging at 600℃, α"T promoted the precipitation of Ti2Ag, which enhanced the antibacterial properties. In the solute-poor areas, the spherical α phase precipitated, which significantly improved the ratio of strength-to-modulus. Finally, TNZ-7Ag treated by a duplex aging treatment of 350 ℃/2 h+600 ℃/2 h exhibited an antibacterial rate of 99.9 % and the ratio of strength-to-modulus of 18.59.

Hydrogen promoted Ti3Al precipitation during Ti-6Al-4V martensite tempering

Temporary hydrogen alloying (THA) treatments are extensively used to refine coarse Ti-6Al-4V microstructures. The present study focuses on a THA treatment which promotes martensite decomposition; it highlights the effect of hydrogen on tempered martensite evolution by comparing martensite decomposition in hydrogenated and control samples. Microstructure evolution was characterised using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). Bulk XRD measurements indicate that quenching hydrogenated Ti-6Al-4V from the beta region causes a partial beta to hexagonal (α’) plus orthorhombic (α’’) martensite transformation. The EBSD, TKD and HRTEM analyses show that hydrogen saturated plate martensite decomposes to form a nanobanded structure which consists of nanosized, periodical Al-rich, and V-rich HCP regions that stabilise D019 Ti3Al within 3 h of tempering. In comparison, typical martensite decomposition is observed in the control experiment. Quenching non-hydrogenated Ti-6Al-4V from the beta region causes a full beta to hexagonal (α’) martensite transformation. Elemental partitioning occurs, but not to the extent that D019 Ti3Al is promoted and compositional banding is not observed. Formation of the nanobanded structure suggests that hydrogen altered the transformation pathway in hydrogenated martensite because of the presence of α’’possibly promoting spinodal decomposition.

Temperature induced strengthening in TiNiCuNb shape memory alloy

Mechanical behavior of two-phase (Ti54Ni34Cu12)90Nb10 alloy with B2–B19–B19’ martensitic transformation types was studied by tensile test in wide temperature range. A temperature-dependent stress diagram was obtained based on experimental data, from which the influence of temperature on the different critical stresses was determined. It was shown that the increasing in testing temperature leads to the increasing in dislocation yield of martensite and ultimate fracture stresses, while the dislocation yielding stresses of austenite are stabilized. It was proposed that the phase hardening caused by difference in thermal expansion coefficients between B19 and β-Nb phases may be a possible reason for the temperature induced strengthening. High dislocation density near B19/β-Nb interface supports this assumption.

Effect of nano-SiC on microstructure evolution and enhanced high wear resistance mechanism of laser surface alloyed M42 high-speed steel

M42 high-speed steel is one of the main materials for preparing bimetallic band saw parts. In this paper, nano-SiC particles of different densities (0–1.00 mg/mm2) were precoated on the surface of M42 high-speed steel samples, then, the samples were prepared by laser alloying. The matching relationship between the quantity, size, and type of martensite and MxCy carbides in laser alloyed samples play a decisive role in improving wear resistance. It is found that under the optimized particle density of 0.75 mg/mm2, compared with M42 high-speed steel, the hardness was increased by 50%, the wear rate decreased by 84.7%. The matching relationship between Martensite and MxCy carbides formed by laser alloying of nano-SiC particles on the improvement of wear resistance was elucidated.

К вопросу о влиянии температуры закалки и уровня одноосных напряжений растяжения на параметры гамма-альфа превращения в мартенситно-стареющей стали при переохлаждении в интервал минусовых температур

Based on the constructed diagrams of the isothermal decomposition of quenched retained austenite, the kinetics of martensitic gamma-alpha-transformation under mechanical load at sub-zero temperatures in maraging steel 08H15N5D2T (rus) was studied. An analysis was made of the changes in the retained austenite resistance to overcooling, isothermal decomposition and resistance to uniaxial tensile stresses in the temperature range from 0 to -100 °C. It has been shown that without load, the stability of retained austenite in the sub-zero temperature range depends on the quenching temperature. After quenching from 1050 °C residual austenite has the highest resistance to overcooling. The resistance of retained austenite to isothermal decomposition is more complex and depends on the quenching temperature. Applied elastic tensile stresses in the range from 20 to 450 MPa significantly and ambiguously changed the nature of the martensitic gamma-alpha-transformation. The instability of the pre-martensitic state and the low shear stability of the retained austenite in the surface layer predetermined the formation of two types of martensite in the structure of the steel under study and the wave-like (stage-by-stage) nature of the resistance of austenite to elastic loads.

Crystallography, twinning elements and interface structure of hierarchical lamellar γ type martensite in Ni-Mn-Ga alloys

Microstructural features of lamellar γ in as-cast Ni54Mn28+xGa18–x (0, 4, 7, 9, 11, and 15) alloys are investigated. With increases of Mn content, γ grain size increases from one to hundreds of micrometers. In small grain, {011} primary microtwins with tetragonal crystal structure are found. The twinning elements are K1 = (011), η1 = [011¯], K2 = (011¯), η2 = [011], and s = 0.169. In medium-sized grain, it comprises of six groups of self-accommodated variants, each of which contains {011} secondary nanotwins. Within a single group, there are four variants, designated as A, B, C and D. The A-C and A-B interfaces are nearly straight, the A-D interface is zigzag, and they have a few lattice dislocations. In the vicinity of γ grain, tertiary nanotwins of non-modulated (NM) martensite arise. In large grain, a series of {111} twin intersections occurs within γ micro-lamella, forming configurations resembling self-accommodated NM martensite. Orientation relationship between lamellar γ and NM martensite is determined. Owing to the microstructural similarity to NM martensite, the lamellar γ is defined as a new type of martensite. The variation in grain size affects the hierarchical twin microstructure of γ type and NM martensites. Occurrence of such phenomenon may mainly source from their different stresses required to activate higher-order twinning. Influence of γ on the mechanical property of Ni-Mn-Ga is discussed.

The Influences of Nb Microalloying and Grain Refinement Thermal Cycling on Microstructure and Tribological Properties of Armox 500T

This study aims to investigate the combined effect of niobium (Nb) microalloying and austenite grain refinement, using a specific heat treatment cycle, on the microstructure and tribological properties of Armox 500T steel. In this work, Nb addition and thermal cycling were utilized for grain refinement and enhancement of the mechanical properties of Armox 500T alloy, to provide improved protection via lightweight armor steel components with a high strength-to-weight ratio. The kinetics of transformation of the developed Armox alloys were studied using JMATPro version 13.2. The samples were subjected to two austenitizing temperatures, 1000 °C and 1100 °C, followed by 4 min of holding time and three consecutive thermal and rapid-quenching processes from 900 °C to room temperature. Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) was employed to analyze the microstructure, which primarily consists of four types of martensite: short and long lath martensite, blocky martensite, and equiaxed martensite. Additionally, a small percentage (not exceeding 3%) of carbide precipitates was observed. The wear characteristics of the investigated alloys were evaluated using a pin-on-disc tribometer. The results demonstrate that alloying with Nb and grain refinement using a thermal cycle significantly reduce the wear rate.

Itinerant ferromagnetism and room temperature giant magnetocaloric effect in (Mn0.6Fe0.4) (Ni0.5Co0.5) Si shape memory alloy

In this article, the feasibility of obtaining a magneto-structural coupling over a wide working temperature range of about 127 K (from 206.5 K to 333.5 K) with negligible thermal hysteresis in combination with the structural, microstructural, magnetic, and magnetocaloric properties of a 50 % Co-doped (at the Ni site) polycrystalline (Mn0.6Fe0.4) (Ni0.5Co0.5) Si (MFNCS-0.5) magnetic shape memory alloy (MSMA) has been thoroughly investigated. Along with leaf-like microstructure, at room temperature, the system shows a mixed phase of TiNiSi type low-temperature high-magnetic (ferromagnetic) orthorhombic martensite and Ni2In type high-temperature low-magnetic (paramagnetic) hexagonal austenite, with the dominance of the second one. To estimate the magnetocaloric effect (MCE) across the phase transition, the isothermal magnetic measurement technique has been employed. The merging of heating and cooling iso-field thermo-magnetization (M-T) curves, the presence of negligible thermal hysteresis in the MH curves, universal magnetic entropy curves, a proposed phenomenological model (across the Curie temperature), and the positive slopes of the Arrott plot (Banerjee criterion) show that the first-order structural and magnetic transition are coupled along with a second-order ferromagnetic to paramagnetic (FM → PM) phase transition (SOPT). By extrapolating the Arrott plot to the H/M = 0 axis the spontaneous magnetization value very close to 0 K, Ms(0), is found to be 71.92 emu/g (∼1.83 µB/f.u.). The linear dependency of MS2T on T2 below SOPT and the RWR (Rhodes–Wolhfarth ratio) = 4.29 > 1 indicate the long-range itinerant ferromagnetism in this alloy. A giant magnetic entropy change (ΔSm) of 25.77 J kg−1 K−1 and a refrigerant capacity (RC) of 472.83 J kg−1 are obtained at 6 T field. This observed magnetic refrigerant's key characteristics, including a giant magnetocaloric effect (MCE) near room temperature (RT), negligible thermal hysteresis loss, a wide operating temperature range of 127 K, and the use of cost-effective, non-toxic constituent elements, make it a suitable and very promising member for applications in solid-state refrigeration technology near RT.

Influence of Manganese Content on Martensitic Transformation of Cu-Al-Mn-Ag Alloy.

The influence of manganese content on the formation of martensite structure and the final properties of a quaternary Cu-Al-Mn-Ag shape memory alloy (SMA) was investigated. Two alloys with designed compositions, Cu- 9%wt. Al- 16%wt. Mn- 2%wt. Ag and Cu- 9%wt. Al- 7%wt. Mn- 2%wt. Ag, were prepared in an electric arc furnace by melting of high-purity metals. As-cast and quenched microstructures were determined by optical microscopy and scanning electron microscopy equipped with EDS. Phases were confirmed by high-energy synchrotron radiation and electron backscatter diffractions. Austenite and martensite transformations were followed by differential scanning calorimetry and hardness was determined using the Vickers hardness test. It was found that the addition of silver contributes to the formation of the martensite structure in the Cu-Al-Mn-SMA. In the alloy with 7%wt. of manganese, stable martensite is formed even in the as-cast state without additional heat treatment, while the alloy with 16%wt. of manganese martensite transforms only after thermal stabilization and quenching. Two types of martensite, β1' and γ1', are confirmed in the Cu-9Al-7Mn-2Ag specimen. The as-cast SMA with 7%wt. Mn showed significantly lower martensite transformation temperatures, Ms and Mf, in relation to the quenched alloy. With increasing manganese content, the Ms and Mf temperatures are shifted to higher values and the microhardness is lower.

Ballistic Efficacy of Carbide Free High Strength Nano Structured Bainitic Armour Steels

Carbide free nano-structured bainitic steels typically have strength more than 2.0 GPa and impact toughness of 7 J or less. Most of this class of steels have sluggish kinetics and takes 16-72 h for complete bainitic transformation. The present work discusses key perspectives in developing carbide free nano-structured bainitic steel having strength more than 2.0 GPa and toughness more than 15 J. Further, ballistic evaluation of newly developed carbide free nano-structured bainitic steels having strength more than 2.0 GPa and high toughness of 20 J was carried out to understand the adaptability of these steels in combat vehicle applications. A comparison is made between newly developed high strength and tough carbide free nano-structured bainitic steels with typical martensite based ARMOX 500 class of armour steels. The developed nano-structured bainite showed ballistic performance much superior to ARMOX 500 steel. Monolithic plates of bainite provide complete protection against 7.62 AP projectiles at an areal density of 120 kgm-2. The ballistic efficiency of monolithic plates was further enhanced by using perforated geometrical configurations.

High strength-ductility combination by quenching and partitioning of a low carbon microalloyed dual-phase steel

This work investigates the quenching and partitioning of hot and cold rolled (HR & CR) low carbon micro-alloyed (Fe-0.05C-1.3Mn-0.4Si-0.6Cr) dual-phase (DP) steel. The quenching (Q) step involved intercritical annealing (IA) at 840°C for 60min and water quenching (200°Cs−1). This is followed by partitioning (P) treatment comprising of below critical annealing at 600°C for 15min and water quenching. The final microstructure after Q and Q&P contains ∼33% martensite within fine ferrite grains with lath for HR and island type martensite morphology for CR sheets. A high strength, ductility and isotropy combination of ∼544±4MPa UTS; 44%(n=0.30) &49% (n=0.32) and Lankford parameter (r‾)=∼1, respectively were obtained for the HR and CR conditions. The strength could be attributed to the equiaxed fine ferrite grain structure (4.3−6.5μm avg. grain size), secondary martensite phase, and dislocation density. The ductility could be a combined effect of the fine ferrite grain size, low dislocation density, geometrically necessary boundaries networking inside the grains, increased co-incidence site lattice boundaries (CSLB), TRIP effect, and texture weakening. ‘P’ treatment successfully stabilized the retained austenite in this lean alloy and induced the TRIP effect. The martensitic transformation during quenching led to ∑3, ∑11, ∑19b, ∑25b, ∑31b and ∑33c CSLB formation (predominantly ∑3), and partitioning treatment could further increase the CSLB density. The presence of CSLB led to the development of weak and partial α-, β-, γ- and θ- fiber texture in the final microstructure. The weak texture led to isotropy (r‾∼1).

Process-microstructure-properties of CuAlNi shape memory alloys fabricated by laser powder bed fusion

Cu -based shape memory alloys (SMAs) show great potential in the application of high temperature, but their performance improvement combining with additive manufacturing is still challenging. In this paper, nearly defect-free (99.7% relative density) CuAlNi SMAs with two types of thermal-induced martensites, 18R and 2H, were successfully fabricated by laser powder bed fusion (LPBF). The remelting printing strategy with systematic optimization framework including densification and printability analysis was used to get optimal process window. The twin-related self-accommodation structure with long period stacking order substructure was formed during additive manufacturing process. The as-build samples show excellent mechanical properties (ultimate compressive strength of 1593 ± 10 MPa and large fracture strain of 23.0% ± 0.1%) with double-yielding, and outstanding shape memory effect, with maximum recoverable strain of 3.39% and nearly 100% recovery rate under compressive strain not exceeding 6%. Furthermore, the microstructural evolution in LPBF involving remelting, martensitic transformation, and 2H martensite was discussed. This result not only builds the microstructure framework involving high reflectivity alloys and martensitic transformation but also provides insight into the applicability of LPBF process in producing Cu-based SMAs.

Effect of Subcritical Treatment (Tempering) on Hardness and Wear of High Chromium White Cast Iron

High chromium white cast iron (HCWCI) is used in mining, crushing plants, as mill liners, and in the manufacture of grinding balls. The grinding ball is the essential element in fine fragmentation. It must have an excellent life to counter extreme wear and impact conditions that it undergoes. Hence it is important to improve its mechanical properties. This study aims to find the effect of tempering temperature on microstructure, hardness, and abrasive wear of High Chromium White Cast Iron used in the manufacture of grinding balls. In this study, the balls in high chromium white cast iron with 12 to 17% Cr, of 50mm and 70mm in diameter were austenitized at 950°C for 45 min and 55 min for the balls of 50 and 70mm then were quenched in forced air. The balls were tempered at 250°C, 400°C, and 600°C for 120 min and 180 min for both diameters 50 and 70mm respectively, and cooled in the furnace. The results showed that tempering practiced at 250°C and 400°C gives an excellent hardness than balls austenitized at 950°C. It is about 60 HRC and it drops to less than 50 HRC for the tempering at 600°C. This tempering causes a significant mass loss (short life) but tempering at 250°C and 400°C decreases this loss by up to 1.6%. The results of XRD showed the presence of the martensite and carbides type M7C3 after tempering at 250°C and a ferritic matrix after tempering at 600°C.

Analysis of Wear Behaviour on AISI 431 Martensitic Stainless Steel by Vacuum Annealing Process

In the research work, AISI 431 grade stainless steel were chosen to investigate the effects of vacuum annealing processes on reducing hardness and enhancing ductility. Although stainless steel of martensitic type are well known for their high hardness, wear resistance and they have limited applications due to high hardness properties. Chosen for this research work, vacuum annealing were carried out to temperatures of 740°C, 840°C and 940°C. The samples were prepared cylindrically to a dimension of 40 mm length and 10 mm diameter. An untreated material was kept aside for comparison.Vacuum annealing involves heating the material in a controlled environment to eliminate impurities and relieve internal stresses, resulting in reduced hardness and improved ductility. To evaluate the effect of wear on annealed material, a pin-on-disc wear test apparatus were used. The wear behavior of the annealed specimens were compared to the untreated material. It includes both the treated and untreated samples, were examined by using a scanning electron microscope (SEM) to obtain images of microstructure. The results of this research work contribute to the understanding of the annealing effects on martensitic stainless steels and provide insights into optimizing their properties for specific applications.

Дослідження напружено-деформованого стану циліндра з мікроструктурними перетвореннями за умов імпульсного навантаження

Axisymmetric dynamic problem of thermomechanical loading of a steel cylinder is considered. Volume strain caused by the microstructural transformations of the martensitic type under cooling solids as well as into consideration and dependence inelastic characteristics of material for multiphase state are taken. The thermomechanical nonlinear behavior of an isotropic material is described by unified flow model generalized for the case of multiphase material state. The problem is solved numerically by the implicit step-by-step time integration method, by the iterative method and by the finite element method. The investigation of the stress-strain state of an inelastic material with regard for the dependence of parameters of the flow model on the phase composition of a material is carried out by using of numerical simulation. We established that microstructural transformations significantly reduce residual inelastic strain and promote the appearance of compressive stresses. The results obtained in the work can be used in calculations of parameters of surface hardening technologies.