Effect of Co Addition on the Microstructure, Martensitic Transformation and Shape Memory Behavior of Fe-Mn-Si Alloys

The effect of Co addition has been studied in Fe-30Mn-6Si-xCo (x = 0 to 9 wt pct) shape memory alloys in terms of their microstructure, martensitic transformation and shape recovery. Microstructural investigations reveal that in Fe-Mn-Si-Co alloys, the microstructure remains single-phase austenite (??) up to 5 pct Co and beyond that becomes two-phase comprising ?? and off-stoichiometric (Fe,Co)₅Mn₃Si₂ intermetallic ??-phases. The forward ??-?? martensite transformation start temperature (M _S) decreases with the addition of Co up to 5 pct, and alloys containing more than 5 pct Co, show slightly higher M _S possibly on account of two-phase microstructure. Unlike M _S, the ??-?? reverse transformation start temperature (A _S) has been found to remain almost unaltered by Co addition. In general, addition of Co to Fe-Mn-Si alloys deteriorates shape recovery due to decreasing resistance to plastic yielding concomitant with the formation of stress induced ?? martensite. However, there is an improvement in shape recovery beyond 5 pct Co addition, possibly due to the strengthening effect arising from the presence of (Fe,Co)₅Mn₃Si₂ precipitates within the two-phase microstructure and due to higher amount of stress induced ?? martensite.     

NiCoMnSn高温形状记忆合金的马氏体相变与显微组织

采用差示扫描量热分析(DSC)、光学显微镜、X射线衍射分析(XRD)、扫描电子显微分析(SEM)、压缩试验和物性测量系统(PPMS),系统研究了Ni50-xCoxMn41Sn9(x=0,1,3,5,7,9,10,12,14)合金的马氏体相变、微观结构、相组成、力学性能和磁性能。结果表明,Co含量≥10%时,Ni50-xCoxMn41Sn9合金中析出多呈枝节状的第二相。第二相为富Co贫Sn相,属于面心立方结构,多在晶界上析出,其体积分数随Co含量的增加而增多,热处理不能消除第二相。Co的加入不改变合金的相变顺序,即所有成分合金均发生一步马氏体相变和逆相变,但会导致相变温度下降,当Co含量由0增加到10%(原子分数),马氏体相变开始温度(Ms)从196.9℃降低到144.8℃,这是因为Co取代Ni使得合金基体的价电子浓度下降导致的。Co加入NiMnSn后,提高了合金的力学性能,特别是当合金中存在第二相时,合金的断裂强度和断裂应变量呈现跳跃性增长,两者分别提高了600 MPa和7%,主要原因是晶粒细化。磁学测试结果表明,随着Co含量的增加,合金磁性有所增强,室温下Co含量为12%的合金表现出典型的铁磁性特征,饱和磁化强度高达118 A·m2·kg-1。     

Ni-Mn-Ga磁控形状记忆合金中的马氏体相变

文章采用电弧熔炼法熔炼Ni48Mn31Ga21和Ni2MnGa两种合金,并对这两种合金进行了热处理,借助交流磁化率测定、金相显微镜观察、X射线以及磁化曲线等手段研究了Ni-Mn-Ga磁控形状记忆合金中的马氏体相变。结果表明：（1）Ni48Mn3lGa21合金在室温下发生了马氏体转变,而Ni2MnGa合金在室温下则未发生马氏体转变;（2）室温下Ni2MnGa合金的饱和磁化强度比Ni48Mn3lGa21合金高;对于Ni48Mn31Ga2l合金,其马氏体态的饱和磁化强度要高于奥氏体态。     

Effect of W Contents on Martensitic Transformation and Shape Memory Effect in Co-Al-W Alloys

To clarify the effect of W contents on the shape memory effect (SME) in the Co-Al alloys and its influencing mechanism, the SME, martensitic transformation, and deformation behavior were studied in the Co-7Al-xW (x = 0, 4, 6, 9 wt pct) alloys. The results showed that the additions of W all deteriorated the SME in Co-7Al alloy when deformed at room temperature. However, when deformed in liquid nitrogen, the SME in Co-7Al alloy could be remarkably improved from 43 to 78 pct after the addition of 4 pct W, above which the SME decreased rapidly with the increase of W content although the yield strength of the parent phase rose due to the solution strengthening of W. The deterioration in SME induced by the excessive addition of W could be ascribed to its resulting significant drop of the start temperature of martensitic transformation.     

Effect of Co-Doping on the Microstructure,Martensitic Transformation Behavior,and Magnetocaloric Effect of Ni-Mn-Sb-Si Ferromagnetic Shape Memory Alloys

Metallurgical and Materials Transactions A - We have studied the effect of Co-doping on the microstructure, martensitic transformation, and magnetocaloric effect (MCE) of a Ni50Mn38Sb9Si3 alloy....     

Stress-Induced Martensitic Transformation Cycling and Two-Way Shape Memory Training in Cu-Zn-Al Alloys

The character and mechanism of two-way shape memory in Cu-Zn-Al alloys is investigated by means of closely controlled thermomechanical cycling and careful measurement of the progressive effect of the particular “training” routine, as well as by correlary studies of submicrostructural evolution as training proceeds. The results establish the quantitative relationship between the cyclic training routine and the ability of the sample to exhibit two-way shape memory. The variation of numerous training parameters with cycling is presented and interpreted. Microscopic studies indicate that as two-way shape memory training proceeds, specific physical features develop in the parent phase submicrostructure, particularly dislocation tangles and “vestigial” martensite markings; these assist in the nucleation and growth of a preferred martensite plate arrangement during cooling.     

TiNi Shape Memory Alloys with High Sintered Densities and Well-Defined Martensitic Transformation Behavior

This study demonstrates that a high density and a high transformation heat can be attained for PM TiNi. With the use of fine elemental powders, a composition of Ti₅₁Ni₄₉, two-step heating, and persistent liquid-phase sintering at 1553 K (1280 °C), a 95.3 pct sintered density was attained for compacts with a green density of 66 pct. A transformation heat, ΔH, of 31.9 J/g was also achieved, which is much higher than reported previously for sintered TiNi and is approaching the highest ΔH reported to date, 35 J/g, for wrought TiNi with low C, O, and N contents. The main reason for having these properties in powder metal TiNi with higher amounts of C, O, and N is that the extra Ti, that over the equiatomic portion in the Ti-rich Ti₅₁Ni₄₉, forms Ti₂Ni compound, which traps most of the C, O, and N. This results in low interstitial contents and a high Ti/Ni ratio of 50.5/49.5 in the TiNi matrix. The tensile strength, elongation, and shape recovery rate after five training cycles were 638 MPa, 14.6, and 99.1 pct, respectively, despite the presence of Ti₂Ni compounds at grain boundaries.     

Martensitic and Magnetic Transformation Behaviors in Heusler-Type NiMnIn and NiCoMnIn Metamagnetic Shape Memory Alloys

Martensitic and magnetic transformation behaviors of Ni₅₀MnIn, Ni₄₅Co₅MnIn, and Ni_42.5Co_7.5MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and transmission electron microscopy (TEM). The martensitic transformation starting temperature (M _s ) decreases with increasing In composition, while the Curie temperatures (T _c ) of the parent phase are almost independent in each alloy series. On the other hand, the addition of Co resulted in a decrease of the M _s and an increase of the T _c , and the degree of the decline of M _s was accelerated by magnetic transformation of the parent phase. The M _s temperature change induced by the magnetic field was also confirmed. It was found that the degree of M _s change is strongly related to the entropy change by the martensitic transformation, which shows a correlation with T _c -M _s . These behaviors can be qualitatively explained on the basis of thermodynamic considerations. This article is based on a presentation made in the symposium entitled “Phase Transformations in Magnetic Materials,” which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS-MPMD and ASMI-MSCTS Phase Transformations Committee.     

稀土多晶Ni-Mn-Ga铁磁记忆合金的马氏体孪晶与磁感生应变

铁磁性形状记忆合金Ni-Mn-Ga是一种新型功能材料,它在磁场作用下能产生大的应变.研究了Ni50Mn29Ga21Tb1.2室温下的晶体结构和磁感生应变,并用O'Handley的解析模型分析合金产生大磁感生应变的原因是马氏体孪晶在磁场下的迁动.     

Influence of Zn Addition on Microstructures and Martensitic Transformation in CuZr-based Alloys

Compositional dependences on microstructures and martensitic transformation behaviors in(Cu_(0.5)Zr_(0.5))_(100-x)Zn_x(x=1.5,2.5,4.5,7.0,10.0,and 14.0at.%)alloys were investigated.It was found that CuZr martensites were present in the present alloys.With increasing Zn content,the volume fractions of CuZr martensitic crystals and B2 CuZr phase gradually decrease and increase,respectively.With the addition of high Zn contents(i.e.,7.0,10.0,and 14.0at.%),the matrix proves to be eutectic.Thermal analysis results show that the initial martensitic transformation temperature(M_s)decreases from(412±5)K to(329±5)K as the Zn content increases from 1.5at.% to14.0at.%.The values of Msof Cu-Zr-Zn shape memory alloys are inversely proportional to the number and concentrations of valence electrons(i.e.,e_v/a and c_v),respectively,implying that the martensitic transformation in CuZrZn alloys could be of electronic nature.     

Microstructure and Shape Memory Characteristics of Powder-Metallurgical-Processed Ti-Ni-Cu Alloys

Even though Ti-Ni-Cu alloys have attracted a lot of attention because of their high performance in shape memory effect and decrease in thermal and stress hysteresis compared with Ti-Ni binary alloys, their poor workability restrains the practical applications of Ti-Ni-Cu shape memory alloys. Consolidation of Ti-Ni-Cu alloy powders is useful for the fabrication of bulk near-net-shape shape memory alloy. Ti₅₀Ni₃₀Cu₂₀ shape memory alloy powders were prepared by gas atomization, and the sieved powders with the specific size range of 25 to 150???m were chosen for this study. The evaluation of powder microstructures was based on a scanning electron microscope (SEM) examination of the surface and the polished and etched powder cross sections. The typical images showed cellular/dendrite morphology and high population of small shrinkage cavities at intercellular regions. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis showed that a B2-B19 one-step martensitic transformation occurred in the as-atomized powders. The martensitic transformation start temperature (M_s) of powders ranging between 25 and 50???m was 304.5?K (31.5?°C). The M_s increased with increasing powder size. However, the difference of M_s in the as-atomized powders ranging between 25 and 150???m was only 274?K (1?°C). A dense cylindrical specimen of 10?mm diameter and 15?mm length were fabricated by spark plasma sintering (SPS) at 1073?K (800?°C) and 10?MPa for 20?minutes. Then, this bulk specimen was heat treated for 60?minutes at 1123?K (850?°C) and quenched in ice water. The M_s of the SPS specimen was 310.5?K (37.5?°C) whereas the M_s of conventionally cast ingot is found to be as high as 352.7?K (79.7?°C). It is considered that the depression of the M_s in rapidly solidified powders is ascribed to the density of dislocations and the stored energy produced by rapid solidification.     

Ti-Ni-Ce合金的相变行为及其形状记忆效应

通过示差扫描量热仪(DSC)等试验手段,系统分析了添加稀土元素Ce后所形成的Ti-Ni-Cex(x＝2 at%、3 at%、4 at%)系合金的相变行为和形状记忆效应,以及相变温度和相变潜热与稀土含量之间的关系.研究结果表明,相变温度(Ms,Mf,As,Af)受稀土含量的影响较大,总的趋势是先升高后下降,然后有稍微升高;而对相变潜热(马氏体相变潜热ΔHB2M、逆马氏体相变潜热ΔHMB2)的影响,则是随着稀土元素Ce含量的升高呈现上升的趋势;稀土的添加使Ti-Ni二元合金的相变滞后明显变窄,但与稀土的含量没有明显关系.研究发现,添加稀土后Ti-Ni形状记忆合金优良的形状记忆效应没有改变,而相变温度却有明显的提高.     

钛合金的马氏体相变

本文简要阐述了钛合金马氏体相变的原理、特点和应用, 并对钛合金热处理工艺作了叙述。     

Role of Si in Improving the Shape Recovery of FeMnSiCrNi Shape Memory Alloys

The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (γ) up to 6 pct Si and, beyond that, becomes two-phase γ + δ ferrite. The Fe₅Ni₃Si₂ type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition does not affect the transformation temperature and mechanical properties of the γ phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct Si show poor recovery due to the formation of δ-ferrite. The shape memory effect (SME) in these alloys is essentially due to the γ to stress-induced ε martensite transformation, and the extent of recovery is proportional to the amount of stress-induced ε martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced α′ martensite through γ-ε-α′ transformation and the large volume fraction of δ-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and results in easy nucleation of stress-induced ε martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal ε martensite formed during cooling is also observed to decrease with the increase in Si.     

Interfacial Modulus Mapping during Structural Transformation in Shape Memory Alloys

Through the modified phase-field model the local soft mode mechanism of nucleation during martensitic transformation was confirmed in shape memory alloys. It was discovered that the modulus loss (8 pct) depended on the martensitic nucleation exceeding the loss (1 pct) during the martensitic growth. The elastic modulus and the stress across the martensite/parent interface differed from those across the martensitic twin boundary. The modulus losses in systems with three variants, two variants, and one variant were compared.     

Ni48Mn33Ga18Tb1合金的马氏体相变和磁性形状记忆效应

本文研究了 Ni48Mn3 3 Ga1 9合金添加微量的稀土元素 Tb后 ,对合金的马氏体相变、磁致应变性能及抗弯性能的影响 ,发现合金的马氏体相变温度、磁致应变值有所下降 ,而机械抗弯强度有显著提高。     

Phase-Field Study of Microstructure and Plasticity in Polycrystalline MnNi Shape Memory Alloys

The evolution of microstructure and plasticity in polycrystalline MnNi shape memory alloys was simulated by a phase-field method considering plastic deformation. Plastic strain was found to occur around grain boundaries and intersections between martensitic bands. In addition, the accumulation of plastic strain would keep on continuously occurring during the thermal or stress cycling process. The occurring of plastic strain could be caused by the lattice incompatibility between austenite and martensite near the tip of martensite plate.     

Intrinsic Micromechanism of Multi-step Structural Transformation in MnNi Shape Memory Alloys

Simulation of the multi-step transformation of cubic matrix → multi-variant tetragonal domain → orthorhombic domain was realized by phase-field method. The intrinsic micromechanism of the second-step transformation in MnNi alloys was studied. It was found that the orthorhombic variant originated from the tetragonal variant with similar orientation, and bar-shaped orthorhombic phase firstly occurred around the interface of twinning bands. The second-step transformation resulted in localized variation of internal stress.     

微量元素对Mg-Al-Zn系合金铸态组织及性能的影响

综述了Mg-Al-Zn系合金的化学成分，几种合金元素在镁合金中的作用及添加微量合金元素对Mg-Al-Zn系镁合金铸态显微组织及性能的影响。